ASE L1

L1ADVANCEDENGINEPERFORMANCESPECIALIST

CHAPTER ONE

Basic Powertrain Diagnosis

CHAPTER TWO

Computerized Powertrain Controls Diagnosis IncludingOBD II

CHAPTER THREE

Ignition System Diagnosis and Repair

CHAPTER FOUR

Fuel and Air Induction SystemDiagnosis and Repair

CHAPTER FIVE

Emission Control System Failures

CHAPTER SIX

I/M Failure Diagnosis

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TEST BACKGROUND INFORMATIONThe purpose of the L1 test is to evaluate your knowledge of di-agnosing powertrain driveability problems and emission fail-ures on electronically controlled systems. The ASE changedsome of the test questions and updated the composite vehicle.

Test ContentDiagnostic area Number of

questionsGeneral Powertrain Diagnosis 5Computerized Powertrain Control Diagnosis 13

(Including OBD II)Ignition System Diagnosis 7Fuel System and Air Induction Systems Diagnosis 7Emission Control Systems Diagnosis 10IM Failure Diagnosis 8

Total 50

Note:The test may contain up to 15 additional questions for ASE re-search purposes. Your answers to these questions do not affectyour score. However, since you do not know which questionsthey are, you must answer all to the best of your ability andplan time for up to 65 questions. At press time, according toASE, the L1 certification and re-certification tests have thesame content.

Summary of the ASE L1 TestYou are expected to be certified in A8 engine performance andhave skill diagnosing problems or failures in the followingareas:

• General powertrain• Computerized powertrain controls• Ignition systems• Fuel systems and air induction systems• Emission control systems• State emission inspection and maintenance programs

To test your ability to read and understand shop manuals, theASE designed a composite vehicle reference book that youmust reference for some test questions. For those techniciansthat are re-certifying, note the following:

The new type 3 composite vehicle has a generic four cycleV6 engine. The engine has four chain driven overheadcamshafts and 24 valves. The sequential multi-port fuel

injection system uses a mass airflow sensor. The ignitionsystem is distributorless and uses one coil over each sparkplug. The system uses no spark plug wires.

The major additions from the previous composite ve-hicle engine include VVT (Variable Valve Timing), TAC(Electronic Throttle Control Actuator), data communica-tions bus, anti-theft immobilizer system, electronicallycontrolled EGR (Exhaust Gas Recirculation) and ORVR(Onboard Refueling Vapor Recovery) evaporative emis-sion control system components.

Scan tool data includes on board diagnostic (OBD) IIsystem monitors, and readiness status.

The test may include engine cooling and exhaust system prob-lems. The use of the word “powertrain” means the technicianmust expect questions on electronic control of the transmis-sion, and the effect of modifications on electronically con-trolled systems. Diagnosis includes scope waveform analysis ofcrankshaft and camshaft sensors.

Fuel system diagnosis is strictly of fuel injection systems,and the subject of fuel quality has been added. Emission fail-ure diagnostic questions include: State emission inspectionand maintenance (IM) 240, acceleration simulation mode(ASM), and two speed idle (TSI) emissions tests results.

ASE L1 TASK LISTCarefully read the Task List, noting the areas in which your skillsare strong or weak. You can do this by checking off each task thatyou do not perform often or do not understand completely.

A. General Powertrain Diagnosis (5 questions)1. Inspect and test for missing, modified, inoperative, or

tampered powertrain mechanical components.2. Locate relevant service information.3. Research system operation using technical information

to determine diagnostic procedure.4. Use appropriate diagnostic procedures based on available

vehicle data and service information; determine if availableinformation is adequate to proceed with effective diagnosis.

5. Establish relative importance of observed vehicle data.6. Differentiate between powertrain mechanical and electrical/

electronic problems, including variable valve timing (VVT)systems.

7. Diagnose engine mechanical condition using an exhaustgas analyzer.

8. Diagnose driveability problems and emission failurescaused by cooling system problems.

ADVANCED ENGINE PERFORMANCE SPECIALIST


Advanced Engine Performance Specialist 601

L1

9. Diagnose driveability problems and emission failurescaused by engine mechanical problems.

10. Diagnose driveability problems and emission failurescaused by problems or modifications in the transmissionand final drive, or by incorrect tire size.

11. Diagnose driveability problems and emission failurescaused by exhaust system problems or modifications.

12. Determine root cause of failures.13. Determine root cause of multiple component failures.14. Determine root cause of repeated component failures.

B. Computerized Powertrain Controls DiagnosisIncluding OBD II (13 questions)

1. Inspect and test for missing, modified, inoperative, ortampered computerized powertrain control components.

2. Locate relevant service information.3. Research system operation using technical information


vehicle data and service information; determine if avail-able information is adequate to proceed with effective diagnosis.

5. Determine current version of computerized powertraincontrol system software and updates; perform repro-gramming procedures.

6. Research OBD II system operation to determine the enablecriteria for setting and clearing diagnostic trouble codes(DTCs) and malfunction indicator lamp (MIL) operation.

7. Interpret OBD II scan tool data stream, diagnostic troublecodes (DTCs), freeze frame data, system monitors, monitorreadiness indicators, and trip and drive cycle information todetermine system condition and verify repair effectiveness.

8. Establish relative importance of displayed scan tool data.9. Differentiate between electronic powertrain control

problems and mechanical problems.10. Diagnose no-starting, hard starting, stalling, engine mis-

fire, poor driveability, incorrect idle speed, poor idle, hes-itation, surging, spark knock, power loss, poor mileage,illuminated MIL, and emission problems caused by fail-ures of computerized powertrain controls.

11. Diagnose failures in the data communications bus net-work; determine needed repairs.

12. Diagnose failures in the anti-theft/immobilizer system;determine needed repairs.

13. Perform voltage drop tests on power circuits and groundcircuits.

14. Perform current flow tests on system circuits.15. Perform continuity/resistance tests on system circuits

and components.16. Test input sensor/sensor circuit using scan tool data

and/or waveform analysis.17. Test output actuator/output circuit using scan tool, scan

tool data, and /or waveform analysis.18. Confirm the accuracy of observed scan tool data by di-

rectly measuring a system, circuit, or component for theactual value.

19. Test and confirm operation of electrical/electronic cir-cuits not displayed in scan tool data.

20. Determine root cause of failures.21. Determine root cause of multiple component failures.22. Determine root cause of repeated component failures.23. Verify effectiveness of repairs.

C. Ignition System Diagnosis (7 questions)1. Inspect and test for missing, modified, inoperative, or

tampered components.2. Locate relevant service information.3. Research system operation using technical information

to determine diagnostic procedure.4. Use appropriate diagnostic procedures based on avail-

able vehicle data and service information; determine ifavailable information is adequate to proceed with effec-tive diagnosis.

5. Establish relative importance of displayed scan tool data.6. Differentiate between ignition electrical/electronic and

ignition mechanical problems.7. Diagnose no-starting, hard starting, stalling, engine

misfire, poor driveability, spark knock, power loss, poormileage, illuminated MIL, and emission problems on vehicles equipped with distributorless ignition (DI) systems; determine needed repairs.

8. Diagnose no-starting, hard starting, stalling, engine mis-fire, poor driveability, spark knock, power loss, poormileage, illuminated MIL, and emission problems on vehicles equipped with distributor ignition (DI) systems;determine needed repairs.

9. Test for ignition system failures under various engineload conditions.

10. Test ignition system component operation using wave-form analysis.

11. Confirm base ignition timing and/or spark timing control.12. Determine root cause of failures.13. Determine root cause of multiple component failures.14. Determine root cause of repeated component failures.

D. Fuel Systems and Air Induction SystemsDiagnosis (7 questions)

1. Inspect and test for missing, modified, inoperative, ortampered components.

2. Locate relevant service information.3. Research system operation using technical to determine

diagnostic procedure.4. Evaluate the relationships between fuel trim values, oxy-

gen sensor readings, and other sensor data to determinefuel system control performance.

5. Use appropriate diagnostic procedures based on availablevehicle data and service information; determine if availableinformation is adequate to proceed with effective diagnosis.

6. Establish relative importance of displayed scan tool data.7. Differentiate between fuel system mechanical and fuel

system/electronic problems.8. Differentiate between air induction system mechanical and

air induction system electrical/electronic problems, includ-ing electronic throttle actuator control (TAC) systems.

9. Diagnose hot or cold no-starting, hard starting, stalling,engine misfire, poor driveability, spark knock, incorrect


602 Advanced Engine Performance Specialist

idle speed, poor idle, flooding, hesitation, surging powerloss, poor mileage, dieseling, illuminated MIL, and emis-sion problems on vehicles equipped with fuel injectionfuel systems; determine needed action.

10. Verify fuel quality, fuel system pressure, and fuel systemvolume.

11. Evaluate fuel injector and fuel pump performance (me-chanical and electrical operation).

12. Determine root cause of failures.13. Determine root cause of multiple component failures.14. Determine root cause of repeated component failures.

E. Emission Control Systems Diagnosis (10 questions)

1. Inspect and test for missing, modified, inoperative, ortampered components.

2. Locate relevant service information.3. Research system operation using technical information


vehicle data and service information; determine if availableinformation is adequate to proceed with effective diagnosis.

5. Establish relative importance of displayed scan tool data.6. Differentiate between emission control systems mechanical

and electrical/electronic problems. Note: Tasks 7 though 11 refer to the following emissioncontrol subsystems: Positive crankcase ventilation, ignitiontiming control, idle and deceleration speed control, exhaustgas recirculation, catalytic converter system, secondary airinjection system, intake air temperature control, early fuelevaporation control, and evaporative emission control (in-cluding ORVR).

7. Determine need to diagnose emission control subsystems.8. Perform functional tests on emission control subsystems;

determine needed repairs.9. Determine the effect on exhaust emissions caused by a

failure of an emission control component or subsystem.10. Use exhaust gas analyzer readings to diagnose the failure

of an emission control component or subsystem.11. Diagnose hot or cold no-starting, hard starting, stalling,

engine misfire, poor driveability, spark knock, incorrectidle speed, poor idle, flooding, hesitation, surging, powerloss, poor mileage, dieseling, illuminated MIL, and emis-sion problems caused by a failure of emission controlcomponents or subsystems.

12. Determine root cause of failures.13. Determine root cause of multiple component failures.14. Determine root cause of repeated component failures.15. Verify effectiveness of repairs.

F. I/M Failure Diagnosis (8 questions)1. Inspect and test for missing, modified, inoperative, or

tampered components.2. Locate relevant service information.3. Evaluate emission readings obtained during an I/M test

to assist in emission failure diagnosis and repair.4. Evaluate HC, CO, NOx, CO2, and O2 gas readings; de-

termine the failure relationships.

5. Use test instruments to observe, recognize and interpretelectrical/electronic signals.

6. Analyze HC, CO, NOx, CO2, and O2 readings; deter-mine diagnostic test sequence.

7. Diagnose the cause of no-load I/M test HC emission failures,

8. Diagnose the cause of no-load I/M test CO emission failures.

9. Diagnose the cause of loaded-mode I/M test HC emission failures.

10. Diagnose the cause of loaded-mode I/M test CO emission failures.

11. Diagnose the cause of loaded-mode I/M test NOx emission failures.

12. Evaluate the MIL operation for onboard diagnostic I/Mtesting.

13. Evaluate monitor readiness status for onboard diagnos-tic I/M testing.

14. Diagnose communication failures with the vehicle dur-ing onboard diagnostic I/M testing.

15. Perform functional I/M tests (including fuel cap tests).16. Verify effectiveness of repairs.

ABOUT THIS STUDY GUIDEThis study guide does not attempt to instruct you in ASE A8 levelsubjects. If you need a review of those subjects, we recommendthe Chek-Chart ASE A8 Study Guide. The Chek-Chart ASE A6Study Guide should also be helpful if you need brushing up inthe electrical area. The Chek-Chart Scan Tool and Lab ScopeGuide would make an excellent companion to this study guide.

This guide begins by presenting a diagnostic path andthought process. This path describes a slightly different diag-nostic approach for driveability problems than it does foremission failure problems. The guide gives a review of diag-nostic tests and values used in testing basic engine systems.Emission control systems in the ASE task list are discussed bycomparing symptoms to problems. In addition, there arechapters on the ASE composite vehicle and OBD II system di-agnosis, and the I/M failure diagnosis, ignition systems, andfuel and air induction systems. In the back of the guide, youwill find a helpful glossary and sample test and discussion.

RECOMMENDED TEST PREPARATIONStudy and review the diagnosis of defects in the following sub-ject areas:

• Engine: mechanical, air intake, cylinder sealing, valvetrain, cooling, and exhaust systems.

• Transmission: torque converter lock up and electronicshift control.

• Ignition system: distributor and distributorless types.• Fuel injection system: fuel quality, fuel delivery, and fuel

control.• Emission systems: PCV, electronic timing control, deceler-

ation emission controls, idle speed controls, EGR, exhaustcatalysts, secondary air injection, intake air temperaturecontrols, early fuel evaporation systems, and evaporativesystems.

• IM: visual, functional, and tailpipe test failures


CHAPTER OBJECTIVES• The technician will complete the ASE task list on Basic Pow-

ertrain Diagnosis.• The technician will be able to answer 5 questions dealing with

the Basic Powertrain Diagnosis section of the L1 ASE Test.

DIAGNOSTIC PATH AND THOUGHT PROCESSTo diagnose powertrain driveability or emission problems anddetermine the root cause of a symptom, you must use yourknowledge of the following systems:

• Engine mechanical: Air intake, cylinder sealing, valve-train, and exhaust

• Ignition: Triggering, primary, and secondary• Fuel injection: Pump, regulator, lines, hoses, injectors, idle

air control, cranking, and open loop fuel control• Emission controls: Positive crankcase ventilation (PCV),

ignition timing control, deceleration enleanment, ex-haust gas re-circulation (EGR), catalyst, closed loop fuelcontrol, secondary air injection, intake air temperaturecontrol, and evaporative emission control (EVAP)

• Transmission and final drive: Electronic control of torqueconverter lock-up and shift control

Keep the driveability or tailpipe emission symptom in mind.Begin with a visual inspection, looking for obvious flaws suchas missing, modified, disconnected, or defective components.The term visual inspection may be misleading; moving thingsout of your way, flexing and wiggling wire and vacuum con-nections, and tapping on components are an important part ofvisual inspections. Also take the time to verify that all ECM andsensor grounds are clean and tight. Perform a ground circuitvoltage drop test if necessary.

Don’t waste time looking for the solution to a problem forwhich the original equipment manufacturer (OEM) already

has a proven cure. Check the technical service bulletins (TSBs)for the vehicle in your shop’s reference library or electronic files.

In some cases, you will want to look up OEM system op-eration to know correct OEM system operation prior to test-ing it. Before performing a test on a device or system, note thespecifications and any OEM special pre-test requirements orprocedures.

Locating electrical parts can be difficult and time con-suming. An electrical component locator manual can some-times indicate where to begin the search.

To find this or other information, you will need to prop-erly identify the vehicle application. You will need to use thefollowing information:

• Vehicle year• Make• Model• Production date• VIN• Engine size• Emissions certification type

Since the symptom has to do with engine performance ortailpipe emission, you should check the engine’s mechanicalcondition first, then make the customer aware of any expen-sive problems. At this point, it is the customer’s choice whetheror not to proceed.

The right approach to the diagnosis depends on the symp-tom and the amount of preliminary information available toyou. Think of the problem as existing somewhere in a pyramidof systems, figure 1-1.

Depending on the information already at hand, you couldstart the search for the cause of the symptom using a tailpipegas analysis. Check any emissions-related trouble code. Thenwork from the top of the pyramid down.

CHAPTER ONE

BASIC POWERTRAIN DIAGNOSIS

Catalyst: A substance that speeds or aids in a chemical reaction.Cylinder Sealing Parts: Engine parts that contain compression or combustion in the cylinder, piston rings, valves, and headgasket.Emissions Certification Type: A reference to whether the vehicle has a Federal or California emissions system configuration.Enleanment: To make leaner, as in adding less fuel to the mixture.Final Drive: Usually refers to the driveshaft, differential gears, and drive axles.Fuel Control: A statement of whether or not the PCM is able to deliver the correct, and quickly varying, fuel mixture to satisfy the needsof a three-way catalytic converter.Original Equipment Manufacturer (OEM): The manufacturer that made the component for its original assembly when new.Voltage Drop: The measurement of the loss of voltage caused by unwanted resistance in a circuit connection, conductor, or device.

L1


604 Chapter One: Basic Powertrain Diagnosis

In Chapter 5, Emission Control System Failures, you willbe guided to the next step of this diagnostic path, to uncoverthe causes of emission failures.

Your actual diagnostic path may be guided by:

• State emissions test failure flow charts• Original equipment manufacturer (OEM) no-code drive-

ability flow charts• OEM trouble code diagnostic flow charts• Company policies

InsightRemember, some flow charts do not tell you to check the basics, or the obvious; they take it for granted it has been done!

Verifying the Driveability SymptomKnowing the symptom helps organize your approach. The oper-ator of the vehicle is a good source for this information. Get thedriver to describe the symptom in as much detail as possible. Itis important to know what the complaint is, the conditions underwhich it occurs, and the severity of the symptoms. Typically, youwant to know:

• Whether it occurs regularly or at random, and if it is hap-pening now

• If certain conditions, such as accelerating or climbinghills, cause or contribute to the symptom

• If the problem exists all the time or some of the time

• If the problem occurs at certain temperatures, such as oncold starts or after a hot soak

• What all the symptoms are, noises, vibrations, smells, per-formance, or any combination

• Whether the problem has occurred before, and what wasdone to repair it

• When the vehicle was last serviced and what work wasperformed

The actual symptom may be different from the customer’s de-scription of it, or your understanding of it may be differentfrom the customer’s. Having the customer accompany you ona test drive to pinpoint symptoms as they occur would be ideal.There is nothing better than your own observation of thesymptom. Just make sure you are working on the right one.

Verifying the Emission SymptomThe customer’s description of a symptom may, or may not, berelevant to the emission failure. In this case, the customer’s de-scription of an unrelated symptom may mislead you from thetrue cause of the emissions test failure. Use your best judgment.

Begin with the vehicle inspection report (VIR) or per-form your own inspection.

Verify the type of failure first:

• Visual• Functional• Tailpipe emission

Then compare your inspection results to the VIR.

ELECTRONIC CONTROLS

EMISSION CONTROL SUB-SYSTEM

FUEL DELIVERY SYSTEM

IGNITION SYSTEM

ENGINE MECHANICAL

TRANSMISSION

FINAL DRIVE

Fig. 1-1. The pyramid of powertrain systems is made up of vehicle systems that can cause engine performance or emissions symptoms.

Hot Soak: A period of time after shutting down a warm engine where heat saturates the combustion chambers, valvetrain, intake, andresidual fuel.Vehicle Inspection Report (VIR): Reports the results of a state emissions inspection.

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Chapter One: Basic Powertrain Diagnosis 605

Intuitive DiagnosisAt this point, you may be tempted to use an intuitive approach.This approach relies on working knowledge and experience,based on past successes fixing problems. You are counting onthe high likelihood that the cause is the same as previous ex-perience has shown.

Use this method to guide you only to systems that needtesting. Do not let this method lead you to replace parts untilthe proper testing is done. Make sure the success of this repairis verified with care. The intuitive method can be a valuable ad-dition to your diagnostic skill.

ELECTRICAL REVIEWTo help you make better diagnoses, this section begins with abrief refresher on electrical behavior and explains measuring thedifferent aspects of an electrical charge using common test equip-ment. It is important to understand the fundamental behavior ofelectricity before you attempt to troubleshoot an electrical orelectronic problem. There is no mystery to electricity, and how itbehaves under any given circumstance is entirely predictable. Thesection concludes with a short discussion of Ohm’s Law and howto apply it to diagnostic situations.

Electrical CurrentElectricity is a form of energy that results when electrons,which are negatively charged atomic particles, transfer fromone atom to another. This electron transfer occurs most read-ily in materials known as conductors and can be activated byan external force, such as heat, friction, or a magnetic field.Electrons tend to move at random but can be organized anddirected. Electric current is the controlled flow of electronsfrom atom to atom within a conductor.

To control the flow of electrical power a path must be pro-vided for the current to follow. These pathways, or circuits, routethe electrical charge to various points, where it is used to per-form work. In order to function, a circuit must form a completeloop so that electron transfer remains uninterrupted, figure 1-2.Automotive circuitry begins at one battery terminal, travelsthrough the wiring harnesses, and returns to the other batteryterminal. If there is a break, or open, in the circuit, current can-not flow since the electrons have nowhere to go, and no workcan be performed, figure 1-3.

AmperageThe amount of current flowing through a circuit, conductor,or electrical device is rated in amperage or amps. Amperage isdetermined by counting the number of electrons that movepast a certain point in the circuit in a given amount of time.

The ampere is the unit that indicates the rate of electric cur-rent flowing through a circuit.

AmmeterAn ammeter is a gauge that is used to measure the current flowin a circuit. Typical ammeters are connected in series with thecircuit or component to be tested. The meter bridges the gap inan open circuit so that all the current flows through the meter,figure 1-4. The second type of ammeter uses an inductive pick-up clamp around the circuit being tested. The meter reads thestrength of the electromagnetic field created by the currentpassing through the inductive clamp. Digital ammeters havehigh input impedance that results in an extremely low amountof current being drawn off the circuit when connected in series.Since all ammeters have low resistance, they will act as a jumperwire to short a circuit if connected in parallel.

Observing correct polarity is important when using anammeter with an inductive pickup. Most inductive clamps aremarked with an arrow, which points in the direction of currentflow when properly connected, figure 1-5.

LAMP

CONDUCTOR

BATTERY

Fig. 1-3. Any break or open in a circuit prevents current flow.

Ampere (AMP): The unit of measure for electric current.Ammeter: A test instrument which measures current flow in a circuit.Conductor: A material that readily allows current flow.Current: The flow of electrons through a conductor.Electron: Negatively charged atomic particles.Impedance: Resistance to current flow often used in rating test meters.Jumper Wire: A length of wire with probes or clips at each end used to bypass a portion of a circuit.Ohm’s Law: A series of formulas that are used to determine the values in an electrical circuit. Any two of the values can be multiplied ordivided to determine the third unknown value.

L1

LAMP

CURRENT FLOW

BATTERY

CONDUCTOR

Fig. 1-2. No matter how simple or complex, an electrical circuitmust form a complete loop in order for current to flow.



Once you get an accurate reading on the ammeter, comparethe reading to the current specifications provided by the vehiclemanufacturer. Current specifications are not always available, soyou may need to use Ohm’s law and calculate the proper amountof current flow for a particular circuit. In general:

• If the ammeter shows no current flow, the circuit is openat some point. This indicates no continuity

• If the ammeter shows less current flow than is normal, thecircuit is complete but contains too much resistance. Thiscan be caused by improper or defective components or byloose or corroded connections

• If the current flow is greater than normal, some of the nor-mal circuit resistance is being bypassed by a short. This canbe caused by faulty components or defective wire insulation

Electromotive ForceTo flow current through a circuit requires an action that organizesall of the randomly drifting electrons and pushes them in one di-rection. This action is known as electromotive force (EMF), orvoltage. Voltage can be measured as the potential difference thatexists between two points in a circuit, such as the two terminals ofa battery. One of these points must have a negative charge, and theother must have a positive charge. The strength of the force de-pends upon the strength of the charges at each point.

VoltageVoltage is a force that is applied to a circuit and can exist evenwhen there is no current flowing. In automotive applications,voltage is supplied by the battery and the generator. Chemicalreaction creates a difference in electromotive force between thepositive and negative terminals of a battery, while mechanicalenergy is converted to electrical energy in a generator to keepthe battery charged. A voltmeter is used to measure voltageand results are recorded in units called volts. The actual valueof a volt is the amount of energy required to move one ampfrom the point of lower potential to the point of higher poten-tial. In practical terms, one volt is the amount of force requiredto move one ampere of current through one ohm of resistance.

VoltmeterA voltmeter can be either a digital or analog instrument. It isnormally connected in parallel with a circuit or across a voltagesource. As with ammeters, digital voltmeters have high imped-ance, which prevents high current from damaging the meter andlimits the load the meter places on the circuit. An internal resis-tor protects an analog voltmeter from too much current flow.Digital voltmeters also have an internal resistor that is in paral-lel to the circuit being tested. This resistor must be at least 1 megaohm, and a good digital meter will use a 10 megaohm resistor. Meters used on electronic circuits should have a mini-mum impedance of 10 megaohms. The high internal resistanceof a digital voltmeter draws very little current from a circuit and,when connected in parallel, the effect of the voltmeter on circuitvoltage drop is insignificant.

Testing with a VoltmeterTypically, a voltmeter is used to:

• Measure the source voltage of a circuit • Measure the voltage drop caused by a load • Check for circuit continuity • Measure voltage at any point in a circuit

Electromotive Force (EMF): The force that causes the electrons to move from atom to another atom. More commonly known asvoltage.Generator: A device that produces electrical energy by passing a magnetic field through a coil of wire. Known for many years as analternator due to the fact that alternating current is produced in the stator assembly; J1930 (OBD II) term for alternator (generatingdevice that uses a diode rectifier).Ohm: The unit of measure for resistance to current flow.Volt: The unit of measure for electrical pressure or electromotive force.Voltmeter: An electrical test meter that measures electrical pressure (EMF).

Fig. 1-5. Observing correct polarity is critical for ammetertesting; inductive pickup clamps usually have an arrow toindicate their polarity.

IGNITIONSWITCH

BALLASTRESISTOR IGNITION

COIL DISTRIBUTOR

V Hz~~

10A

RPM+

COM

A °C °F

RPM

V

°%

OFFV Hz

Fig. 1-4. A traditional ammeter is always connected in series tomeasure the current flow of a circuit.



To measure voltage or voltage drop, connect the voltmeter inparallel. To perform a continuity check, connect the voltmeterin series with the portion of the circuit being tested.

Checking Source VoltageThe source, or available, voltage within a circuit can be mea-sured with or without current flowing through the circuit. Thebattery is the voltage source for all DC automotive circuits. Itis checked by connecting the positive lead of the voltmeter tothe positive battery (B�) terminal and the negative lead to thenegative battery (ground) terminal, figure 1-6. This measures no-load, or open-circuit, battery voltage, which should beabout 12.2 volts with the engine not running.

Source voltage can be checked in a similar fashion, at anypoint along a circuit, by grounding the negative meter lead andprobing the supply wire with the positive meter lead, fig-ure 1-7. Low source voltage in a circuit is the result of highresistance, and loose or corroded connections are often at fault. A loss of source voltage indicates an open in the circuit.

Checking Voltage DropVoltage drop is the amount of voltage that an electrical devicenormally consumes to perform its task. However, excessive

voltage drop can be the result of a high-resistance connectionor failed component.

Checking voltage drop is one of the most important testsyou can perform on a circuit. Voltage drops can cause majordriveability symptoms in onboard computer systems. A voltagedrop on an engine control module (ECM) power ground cancause sensor voltage references to be higher than normal, throw-ing off the overall sensor calibration of the entire control system.

To check voltage drop, the circuit must be powered up andhave current flowing. The circuit must also have the maximumamount of current flowing under normal conditions for whichthe circuit was designed. The amount of voltage drop that is con-sidered acceptable will vary by circuit. Low-current circuits thatdraw milliamps will be affected by very small voltage drops, whilethe same amount of voltage drop will have a negligible affect ona high-current circuit. In general, voltage drop on a powerground circuit should be less than 0.1 volt.

To measure voltage drop, connect the meter in the samefashion used to take system voltage readings. Leave the negativemeter lead attached to the negative battery terminal, and use thepositive meter lead to probe at various points in the circuit tocheck a power ground, figure 1-8. You can compute voltage dropby checking available voltage on both sides of a load, then sub-tracting the voltage reading of the ground side from the readingon the positive side of the load. You can take direct voltage dropreadings by connecting the positive meter lead to the power sideof a load and connecting the negative meter lead to the groundside of the component. Check electronic sensor voltage drop ina similar way. Connect the digital multimeter (DMM) negativelead to the sensor ground terminal and probe the signal line withthe positive meter lead. Remember, the sum of all the voltagedrops in a circuit will equal the source voltage.

Checking ContinuityContinuity testing is similar to no-load voltage testing, since bothprocedures tell you if system voltage is being applied to a part ofthe circuit. However, for a continuity check, the voltmeter is con-nected in series with the circuit rather than in parallel.

M

IGN.SWITCH MOTOR SWITCH

VOLTMETER #1 VOLTMETER #2

VOLTMETER #3

MOTOR

V Hz~~

10A

RPM+

COM

A °C °F

RPM

V

°%

OFFV Hz

V Hz~~

10A

RPM+

COM

A °C °F

RPM

V

°%

OFFV Hz

V Hz~~

10A

RPM+

COM

A °C °F

RPM

V

°%

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Fig. 1-7. Source voltage can also be checked anywhere along acircuit by grounding the negative meter lead and probing thecircuit with the positive meter lead.

ENGINEGROUND

BATTERY

+ –

VHz~~

10A

RPM+

COM

A CF

RPM

V

%

OFFVHz

Fig. 1-8. Voltage drop testing is one of the best ways to checkthe integrity of a circuit or electrical device. This meter isdisplaying 0.18 voltage drop across the engine ground and the battery.

BATTERY

+ –

VHz~~

10A

RPM+

COM

A C F

RPM

V

%

OFF

VHz

Fig. 1-6. A voltmeter is connected in parallel across a voltagesource. This meter is displaying open-circuit battery voltage.

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To check continuity with a voltmeter, open the circuit at thetest point by disconnecting the power wire. Then, attach thepositive voltmeter lead to the source voltage side of the open cir-cuit and connect the negative voltmeter lead to the ground sideof the test point. Next, energize the circuit and note the volt-meter reading, figure 1-9. If the voltmeter reads system voltage,the circuit is complete. If the voltmeter reads near zero voltage,the circuit is open. Due to the high resistance of the DMM, thecircuit cannot carry current so the meter reads source voltage.

ResistanceVoltage forces current through a conductor, but all conductivematerials oppose current flow to some extent. This opposition,known as resistance, exists in some degree in all electrical de-vices. If you know how much resistance a circuit should have,you can quickly determine the overall condition of the circuitby measuring its resistance. There are five factors, or charac-teristics, that determine how much resistance is present in anelectrical circuit. These are:

1. The atomic structure of the material. All conductors havesome resistance, but the low resistance in a good conduc-tor will flow current when a fraction of a volt is induced

2. The length of the conductor. The longer a piece of wire orcable, the higher its resistance

3. The cross-sectional area of the conductor. The thinner apiece of wire or cable, the higher its resistance

4. The temperature of the conductor. In most cases, thehigher the temperature of the conducting material, thehigher its resistance. However, some sensors are designedto operate exactly the opposite

5. The condition of the conductor. Broken strands of a cableor a partially cut wire reduces the cross-sectional area ofthe conductor and raises resistance. Loose, dirty, or cor-roded connections have the same effect and are a majorcause of electrical problems

OhmsAn ohm is the unit established to measure electrical resistance.One ohm is equal to the amount of resistance present whenone volt of electromotive force pushes one ampere of currentthrough a circuit. The resistance of any electrical device or cir-cuit can be measured two ways:

• Directly with an ohmmeter measuring the resistance of-fered by the device or circuit in ohms

• Indirectly with a voltmeter, measuring the voltage dropacross the device or circuit

Since every electrical device, or load, in a circuit offers some re-sistance, voltage is reduced as it pushes current through eachload. Voltage drop testing was detailed earlier in this chapter.

Tips for Using an OhmmeterAn ohmmeter is a self-powered test instrument that can onlybe used when there is no voltage applied to the circuit devicebeing tested. Any current flow from an outside source willdamage the meter. Before testing with an ohmmeter, make surethe circuit is not under power, or remove the component to betested from the circuit, figure 1-10.

Ohmmeters, whether analog or digital, operate on thevoltage drop principle. When you connect the leads of an ohm-meter to a device for testing, the meter directs a low-voltagecurrent from its power source through the device. Since thesource voltage and the internal resistance of the meter areknown, the resistance of the test device can be determined bythe amount of voltage dropped as current flows through it. Theohmmeter makes this calculation and directly displays the re-sistance of the test device in ohms.

Be aware, ohmmeter testing may not always be conclusive.Resistance faults in wiring and connections often generate heat,which further increases the resistance of an operating circuit. Inthese cases, the fault may not be apparent unless the circuit isunder power. The device may be able to relay the low-voltage sig-nal of an ohmmeter, but not be able to carry the signal when sys-tem voltage is applied to it. Another consideration is the fact thatmost ohmmeters will only read as low as 0.1 ohm, yet smalleramounts of resistance can cause problems, especially on elec-tronic circuits. These low-resistance faults can only be determinedthrough voltage drop testing. However, an ohmmeter has definiteadvantages for many test situations and is particularly useful to:

• Measure the resistance of parts that have specific resis-tance values that fall within the usable range of the meter

• Measure high-resistance items, such as secondary ignitioncables and electronic pickup coils

• Test internal parts of components that require disassem-bly to reach the test points

• Bench test parts such as switches, circuit breakers, and re-lays before assembly or installation

• Check circuit continuity of components

Fig. 1-9. Check continuity with a voltmeter by connecting it inseries with the device being tested and energizing the circuit.



Ohm’s LawThe relationship between current flow, electromotive force,and resistance is predictable for any electrical, or electronic,circuit or device. This relationship was first stated as a theoryby George Ohm in 1827 and has since become known asOhm’s Law. Ohm determined that there are three characteris-tics at work in an electrical device: voltage, amperage, and resistance. If you know two of them you can always calculatethe third, since the relationship of these three never changes,figure 1-11.

According to Ohm’s Law, when a force of one volt pushesone ampere of current through a circuit, the resistance presentis 1 ohm. This establishes and gives a value to the ohm, the unitwith which resistance is measured. Now, you can use one ofthree simple mathematic equations to calculate the missingfactor:

• To calculate voltage, multiply amperes by resistance V � A � R

• To calculate amperage, divide voltage by resistance A � V � R

• To calculate resistance, divide voltage by amperage R � V � A

Even though you may never need to use one of these equa-tions to figure out the missing characteristic, it is important tounderstand the logic behind them.

Suppose you are dealing with a fused circuit operating onsystem voltage that keeps blowing the fuse after a short period oftime. A quick check tells you 12 volts are available on either endof the circuit, and you know the fuse is rated at 10 amps. There-fore, Ohm’s Law tells you there is low resistance in the circuit be-cause amperage is equal to voltage divided by resistance. So, ifvoltage is constant, a drop in resistance is the only condition thatwill allow enough current to flow through the circuit to overloadthe fuse.

Very few, if any, automotive problems will require you toactually calculate Ohm’s Law equations to repair them. How-ever, you will find it much easier to locate faults in electric andelectronic circuitry once you understand the relationships ofcurrent, voltage, and resistance expressed in Ohm’s Law.

In an automotive electrical system, DC voltage originatesat the battery, and the open-circuit, or no-load, voltage of agood battery will be about 12.6 volts. With the engine running,a typical charging system regulates output between 13.5 and 14 volts. This is the source, or system, voltage that providespower to all of the circuits on the vehicle. Therefore, voltageshould remain fairly stable, unless there is an unexpectedchange in resistance. Low voltage in a vehicle electrical systemis often the result of either a charging system problem or a badbattery. If resistance is unchanged, a drop in system voltage re-sults in less current flow, and a rise in system voltage will in-crease amperage, or current flow, as well. Ohm’s Law says:

• Voltage and amperage are directly proportional to eachother as long as resistance remains the same. Both mustmove in the same direction, figure 1-12

Resistance in an electrical circuit should only be that of the loaddevices specified by the engineer. This includes all switches, re-lays, motors, solenoids, lamps, and other parts that create resis-tance to perform usable work. The resistance of all the loads de-termines the circuit amperage. Remember, system voltageshould remain stable and within its designed range unless thereis a battery or charging system problem. Therefore, the circuitwith the greatest total resistive load will flow the least amount of

VVOLTAGE

AAMPERAGE

RRESISTANCE

Fig. 1-11. This diagram is an easy way to remember therelationship of the three elements of Ohm’s Law; when one ismissing you can calculate it based on the other two.

Fig. 1-10. Using an ohmmeter to measure the resistance of anelectronic fuel injector. Note, the multi-plug has beendisconnected to prevent current flow through the injector.

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current, and the circuit with the least resistance will allow thegreatest amperage flow. According to Ohm’s Law:

• Amperage and resistance are inversely proportional toeach other as long as voltage remains the same. They movein opposite directions, figure 1-13

Under normal circumstances, you will not see a situationwhere amperage is held constant while voltage and resistancechange. Amperage is the strength of the electrical charge mov-ing through a conductor, and it responds to changes in voltageor resistance. Although high amperage is the cause of manyblown fuses, it is most often the effect of low circuit resistancerather than the cause of the problem, figure 1-14.

Understanding the inverse relationship of amperage to re-sistance at a steady voltage is an important diagnostic aid. Anycircuit damage, whether an open or short, poor or corroded

connection, frayed wire, broken insulation, or a defective com-ponent, will change the designed resistance of the circuit.When the battery and charging system are in good condition,a change in resistance will increase or decrease amperage in thecircuit. Excessive amperage will cause blown fuses, while reduced amperage can cause slow motor operation, dim bulbs,sluggish solenoid or relay response, and less than peak perfor-mance from other circuit devices.

DiodesDiodes serve as one-way check valves in an electrical system.They allow current to flow in one direction, but prevent cur-rent flow in the other. A diode is used to direct current flowand protects solid state devices from voltage spikes. Each diodehas two halves, an anode and a cathode. The diode allows cur-rent flow through the cathode to the anode. Diodes are used incircuits to re-direct current flow. A good example of diodesbeing used is in an alternator, where they modify current fromalternating current to direct. A standard silicon diode causes avoltage drop of approximately 0.6V.

Clamping Diodes Clamping diodes are diodes placed in a circuit in parallel witha magnetic coil. When the magnetic field produced by the coilcollapses because power is removed, it produces a voltage spikewith polarity opposite that of normal current flow. The diodeis wired in parallel with the coil so when the field collapses, thespike is blocked from flowing in the circuit. The diode pre-vents the spike from reaching a computer or other solid statecomponent.

For example, when a relay is de-energized, the resultingvoltage spike can exceed 40 volts. A starter relay can produce avoltage spike of nearly 200 volts. Clamping diodes protect thevehicle computers from these spikes.

Alternator: See Generator.Diode: An electronic component designed to allow current flow in one direction only. Used in control circuits and in rectifier assembliesin the generator.

0

00

0

10 10

1010

AMPERAGE

VOLTAGE

FIXED RESISTANCE

INCREASE

INCREASE

DECREASE

DECREASE

Fig. 1-12. Voltage and amperage increase or decrease directly inproportion to each other as long as resistance remains constant.

RESISTANCE

LOW

VOLTAGE

CONSTANT

HIGH

AMPERAGE

Fig. 1-14. High amperage overloading a fuse is often the effectof low resistance allowing too much current to flow.HIGH

RESISTANCE

AMPERAGE

LOW

VOLTAGE

CONSTANT

Fig. 1-13. Amperage and resistance are inversely proportionalwhen voltage is constant, so if either one increases the othermust decrease.



Series CircuitsIn a series circuit, the current has only one path to follow. Infigure 1-15, using conventional current flow theory, you seethat the current must flow from the battery through the resis-tor, and back to the battery. The circuit must be continuous, orhave continuity. If one wire is disconnected from the battery,the circuit is broken and there is no current. If electrical loadsare wired in series, they must all be switched on and workingor the circuit is broken and none of them work. A simplifiedexample of a series circuit is shown in figure 1-16. Currentflows from the battery through the horn switch, through thehorn, and then back to the battery.

Series Circuits and Ohm’s LawOhm’s law can easily be applied to a series circuit. If any two ofthe values are known, the third can be calculated using Ohm’sLaw. Some characteristics of a series circuit are:

• Current is the same everywhere in the circuit. Since thereis only one path for current, the same amount of currentmust be available at all points of the circuit

• Voltage drops may vary from load to load if the individualresistances vary, but the sum of all voltage drops in the se-ries is equal to source voltage

• The total resistance is the sum of all individual resistancesin the series

In figure 1-15, the circuit consists of a 3 ohm resistor connect-ed to a 12 volt battery. The amperage is found by using Ohm’s Law:

E � R � I12 � 3 � 4 amperes

When you know the current and the individual resistances ofa series circuit, you can calculate the voltage drop across eachload. The sum of these drops equals the source voltage. For the2 ohm resistor in figure 1-17:

I � R � E2 � 2 � 4 volts

For the 4 ohm resistor in figure 1-17:

I � R � E2 � 4 � 8 volts

The sum of the volts is 4 volts � 8 volts � 12 volts, which is thesource voltage.

Parallel CircuitsWhen current can follow more than one path to complete acircuit, that circuit is called a parallel circuit. The points wherecurrent paths split and rejoin are called junction points. Theseparate paths that split and meet at junction points are calledbranch circuits or shunt circuits. A parallel circuit is shown infigure 1-18. In an automobile, the headlamps are wired in par-allel with each other, figure 1-19.

Parallel Circuits and Ohm’s LawThe features of a parallel circuit are:

• The voltage applied to, or measured across, each branchof the circuit is the same

• The total current in a parallel circuit is the sum of the cur-rent in each branch

• The total resistance of a parallel circuit is always less thanthe lowest individual resistance. The reason is that whenyou add resistors in parallel, you are actually adding moreconductors, or paths in which current can flow, which re-duces the total resistance

Fig. 1-17. A series circuit with more than 1 resistor.

Fig. 1-18. A parallel circuit.

Parallel Circuit: An arrangement that provides separate power supplies and ground paths to several loads.Series Circuit: An arrangement in which current must flow through one load before another. Each load shares the power supply withthe other loads in the circuit.Shunt: A parallel electrical connection or branch circuit, in parallel with another branch circuit or connection.

Fig. 1-15. A simple series circuit.

Fig. 1-16. This horn circuit diagram illustrates a simple seriescircuit.

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There are two ways to calculate the total resistance, or equiva-lent resistance, in a parallel circuit. One formula for any num-ber of resistors is:

Rt � 1 � (1 � R1 � 1 � R2 � 1 � R3)

Note: Rt � Total circuit resistance.

For the circuit illustrated in figure 1-18:

R � 1 � (1 � 6 � 1 � 3) � 2 ohms

Another way to calculate total resistance is the product-over-the-sum method:

R1 � (R1 � R2) � (R1�R2)

This formula can be used for only two resistances at a time. Ifmore than two are wired in parallel, you must calculate theirvalues in pairs until you determine one total resistance for thecircuit. For the circuit in figure 1-18:

R1 � (6 � 3) � (6 � 3) � 2 ohms

To apply Ohm’s Law to a parallel circuit, sometimes you musttreat branches as independent circuits and sometimes youmust deal with the entire circuit, depending upon which val-ues are unknown. To find current, you must treat each branchseparately because of the different current in each branch.Voltage is applied equally across all branches, so the sourcevoltage is divided by the branch resistance to determine thecurrent through that branch. Adding the current in all thebranches gives the total current in the circuit. In the circuitshown, figure 1-18, current through the 6 ohm resistor is:

E � R � I12 volts � 6 ohms � 2 amps

Through the 3 ohm resistor, it is:

E � R � I12 volts � 3 ohms � 4 amps

Total circuit current is 2 amps � 4 amps � 6 amps.

If the resistance of a branch is unknown, dividing the sourcevoltage by the branch current gives the branch resistance. Infigure 1-18, for the first branch:

R1 � 12 volts ÷ 2 amps � 6 ohms

For the second branch:

R2 � 12 volts ÷ 4 amps � 3 ohms

Total resistance of the circuit can be calculated using the product-over-the-sum method:

Rt � (6 ohms � 3 ohms) (6 ohms � 3 ohms) � 2 ohms

Or, if all you need is the equivalent circuit resistance, divide thesource voltage by the total circuit amperage as follows:

Rt � 12 volts � 6 amps � 2 ohms

To determine source voltage, multiply the total circuit currentby the total circuit resistance. Or, since the voltage is the sameacross all branches, multiply one branch current by the samebranch resistance. In figure 1-18:

I � R � E6 amps � 2 ohms � 12 volts

Or, (branch I) � (branch R) � E:

Branch 1: 2 � 6 � 12 volts

Branch 2: 4 � 3 � 12 volts

Series-Parallel CircuitsAs the name suggests, series-parallel circuits combine the twotypes of circuits already discussed. Some of the loads are wiredin series, but there are also some loads wired in parallel, figure1-20. The entire headlamp circuit of an automobile is a series-parallel circuit, figure 1-21. The headlamps are in parallel witheach other, but the switches are in series with the battery andwith each lamp. Both lamps are controlled by the switches, butone lamp still lights if the other is burned out. Most of the cir-cuits in an automobile electrical system are series-parallel.

Series-Parallel Circuits and Ohm’s LawValues in a series-parallel circuit are figured by reducing the par-allel branches to equivalent values for single loads in series. Thenthe equivalent values and any actual series loads are combined.

To calculate total resistance, first find the resistance of allloads wired in parallel. If the circuit is complex, it may be

Series-Parallel Circuit: An arrangement that combines two or more loads in parallel with one or more loads in series.

Fig. 1-20. A series-parallel circuit.

Fig. 1-19. The headlamps are wired in parallel with each other inall headlamp circuits.



handy to group the parallel branches into pairs and treat eachpair separately. Then add the values of all loads wired in seriesto the equivalent resistance of all the loads wired in parallel. Inthe circuit shown in figure 1-20:

Rt � (6 � 3) � (6 � 3) � 2 � 4 ohms

In the illustration, the equivalent resistance of the loads in par-allel is:

(6 � 3 ) � (6 � 3) � 2 ohms

The total of the branch currents is 1 � 2 � 3 amps, so the volt-age drop is:

I � R � E3 � 2 � 6

The voltage drop across the load in series is 2 [�] 3 � 6 volts.Add these voltage drops to find the source voltage:

6 � 6 � 12 volts

To determine the source voltage in a series-parallel circuit, youmust first find the equivalent resistance of the loads in paral-lel, and the total current through this equivalent resistance.Figure out the voltage drop across this equivalent resistanceand add it to the voltage drops across all loads wired in series.

To determine total current, find the currents in all paral-lel branches and add them together. This total is equal to thecurrent at any point in the series circuit. In figure 1-20:

I � (E � R1) � (E � R2) � (6 � 6) � (6 �3) � 1 � 2 � 3amps

Notice that there are only 6 volts across each of the branch cir-cuits because another 6 volts have already been “dropped”across the 2 ohm series resistor.

With this summary of electrical theory, you can performmore accurate diagnoses, resulting in more efficient repairs,and a higher percentage of satisfied customers.

MORE DIAGNOSTICSCheck the BasicsSome mechanical and electrical systems are not monitored bythe electronic powertrain control system. Failures here cancause driveability or emission problems without setting codes.Other problems may not be detected by a scan tool or labscope. Some problems may be the root cause of a code or asensor that is out of range, even though it is on a system that isnot monitored by the electronic powertrain control system.

The following tests, described over the next several pages,may be performed to detect these types of problems. The testsare not necessarily in the order they should be performed. Thisis a reminder list. The list does not have a specific order or spe-cific procedures.

No-Start DiagnosisTo run, an engine requires four things: air, fuel, compressionand ignition, all at the right time. Perform the following teststo find what the problem is:

• Observe the engine’s cranking speed; if it is too slow, checkthe battery and starting system

• Check fuel pressure and volume• Verify the electrical signal to the injector with a 12V test

light, figure 1-22, depending on the OEM’s recommen-dation

• Use a properly gapped spark tester to check for spark• Check compression by performing a cranking vacuum or

compression test• Check the ignition timing• Verify camshaft drive integrity and valve timing

BatteryPerform a preliminary visual inspection and check the elec-trolyte level. The battery should measure 12.6V or higher, if itis fully charged. The minimum state of charge needed to per-form a load or capacity test is 12.4V.

If the state of charge is too low, perform a “3 minute (sulfa-tion) test” while charging. To do this, connect the charger and setit on high. In three minutes, check the charging voltage. If at theend of three minutes, the charging voltage is above 15.8V, the bat-tery may be considered sulfated. It should be replaced because itmay never accept a full charge. If the battery passes, continuecharging at a normal rate until it is fully charged. A capacity testshould be performed with a load of half the cold cranking amperes applied for 15 seconds. By the end of this time, the bat-tery should not have dropped below 9.6V. If it does, replace it.

Sometimes, a battery’s state of charge is low because of akey OFF drain. To test for a key OFF drain, disconnect the negative battery cable connection. Connect a (known good)

Electrolyte: The chemical solution in a battery that conducts electricity and reacts with the plate materials.Integrity: Soundness, intactness of a component, or a person’s adherence to a code of values.Lab Scope: An oscilloscope used to observe electronic sensor and actuator waveforms, usually not capable of reading high secondaryignition voltage.

Fig. 1-21. A complete headlamp circuit, with all bulbs andswitches, is a series-parallel circuit.

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12-volt test light in series with the battery post and the batterycable terminal connector. If the light illuminates, there is alarge drain. If the test light does not illuminate, remove it andproceed to the next test.

Temporarily connect a jumper (shunt) in series between thenegative battery post and the disconnected cable’s battery ter-minal connector. Connect an ammeter across the battery postand the battery terminal connector, figure 1-23. The shunt willprotect your meter from a current increase while the vehicle’s capacitors are charging up. Wait 3–4 minutes, then disconnectthe jumper before measuring. Use the highest meter range first,usually 10 or 20 amps. Then scale down to milliamps to check

for a small (parasitic) drain. If you measure zero, check the am-meter’s circuit protection. There are electronic control devicesthat need power even with the key OFF. You will need to look uptheir normal parasitic drain, to know if there is an abnormaldrain on the system.

StartingDisable both ignition and fuel, or just the fuel system. This notonly prevents startup, but also prevents crankcase oil dilutioncaused by gasoline washing past the rings while performingcranking tests. Limit cranking tests to 15 seconds to protect thestarter from overheating. The starter should crank the engineat normal speed and not draw more current than specified.Battery voltage during a “15-second starter draw test” shouldnot drop below 9.6V and the amperes should stay within OEMspecifications. Keep in mind that some electronic engine con-trol systems require at least 10.5V during normal startup. Volt-age lower than 10.5V may cause a no-start.

When the starter cranks too slowly and draws high current,the problem may be caused by:

• A short in the starter • Excessive mechanical load

When the starter cranks too slowly and draws low current, theproblem may be caused by:

• Poor battery capacity • Excessive resistance in the circuit• Excessive resistance in the starter

When the starter cranks too fast and draws low current, theproblem is probably a low compression problem—often acamshaft drive defect.

If the starter engages the flywheel but does not release, ormakes unusual noise during cranking, the problem may becaused by:

• Improper pinion to flywheel clearance• Bad starter drive• Shorted starter solenoid or relay• Starter not aligned properly

When the starter spins but does not engage the flywheel, thecause may be:

• Defective starter drive• Starter mounting bolts loose

When the solenoid clicks but the starter does not spin, theproblem may be caused by:

• A defective solenoid switch• Excessive resistance in the starter control circuit

ChargingBegin by checking the alternator belt condition and tension.Check battery voltage with the ignition key in the OFF position.Test the charging system voltage at the battery, with the enginerunning at idle speed and accessories turned on. If there is noOEM specification available, it should maintain a minimum ofat least 0.5V above the battery’s key OFF voltage with the ac-cessory loads on.

If the system voltage is low, first be sure engine idle speedis correct, then look for high resistance in a wire or connection.

Fig. 1-22. Using a 12V test light to verify electrical signal to aninjector.

VHz~~

10A

RPM+

COM

A °C°F

RPM

V

° %

OFFVHz

TEMPORARYSHUNT

+ -

BATTERY

Fig. 1-23. Using a jumper (shunt) to protect an ammeter duringa battery drain test.



To do this, perform a voltage drop test on the charging circuit,figure 1-24. Check both the positive and ground side of the cir-cuit. It is important to turn on enough accessories to cause aload of at least 20 amperes on the alternator. This will ensurethat the flaw in the circuit is revealed. If this does not uncoverthe problem, verify that the field circuit voltage or amperage isat specification.

Check for overcharging. Measure the voltage at the batterywith the engine running at 2000 rpm and the accessoriesturned off. It should not be above the OEM’s specified charg-ing system voltage limit.

Other tests include performing:

• An oscilloscope alternator diode ripple test• Alternating Current (AC) volt leakage test. AC voltage

above 0.5V is the rule of thumb for a failed alternator• Current leakage test using an ammeter in the charging

circuit

InsightThink of the powertrain systems affected by a weak battery ordefective charging system: starting, ignition, fuel delivery, fuelcontrol, emission controls, and transmission controls. Low orhigh system voltage will affect tailpipe emissions. When a engine control module (ECM) goes into a “limited operationstrategy” because of low system voltage, it may disable theEGR, causing higher emission of Nitrogen Oxides (NOx).

CoolingCheck the coolant condition and the level with a coolant tester(hydrometer). Most manufacturers recommend a 50/50 mix of

antifreeze/water in all but the coldest climates. A 70/30 mix isthe maximum ratio allowed for all but a few vehicle applica-tions. Look for corrosion or contamination in the system. Usethe radiator cap pressure specification when pressurizing thesystem to perform a leak check.

Proper engine temperature is critical for clean emissions andoptimal engine operation. Use a scan tool to accurately determineand confirm thermostat operation. A lower temperature, stuck-open thermostat, or no thermostat may cause a long warmuptime, or in cool weather no warmup. This in turn may cause:

• Extended high idle speed • High Carbon Monoxide (CO) tailpipe emission

Left uncorrected, other symptoms may be:

• Fouled spark plugs• High hydrocarbon (HC) tailpipe emission• High fuel consumption• Abnormal fuel trim readings• Carbon build up• Oxygen (O2) sensor carbon contamination• Catalyst damage

Engine cooling fan systems are often controlled by the ECM.The ECM uses the coolant temperature signal to know whento activate a relay to control the cooling fan. A system that isbypassed to make the fan run constantly may cause the samehigh CO symptoms as a thermostat that is stuck open.

Overheating problems are caused by:

• Low coolant level• Poor or no coolant circulation• Inoperable auxiliary fan• Lack of airflow

An engine overheating during an emissions test may cause thetest to be aborted. However, an engine running hotter than nor-mal, but not overheating, may cause a NOx emission test failure.

Engine Cylinder Power Contribution TestA cylinder power contribution test tells you which cylinder orcylinders’ combustion is not as efficient as the others. It doesnot tell you which system is at fault, figure 1-25.

A low power contribution by one or more cylinders maybe caused by:

• A vacuum leak• Compression loss• Poor valve lift• Weak spark• Fuel injector defect• Primary ignition wiring fault

+-

VHz~~

10A

RPM+

COM

A °C°F

RPM

V

° %

OFFVHz

Fig. 1-24. Voltage drop testing the positive side of the chargingcircuit.

Carbon Monoxide: An odorless, colorless, tasteless poisonous gas. A pollutant produced by the internal combustion engine.Fouled: Contaminated, like a spark plug contaminated (fouled) with carbon.Fuel Trim (FT): Fuel delivery adjustments based on closed-loop feedback. Values above the central value (0%) indicate increasedinjector pulse width. Values below the central value indicate decreased injector pulse width. Short Term Fuel Trim is based on rapidlyswitching oxygen sensor values. Long Term Fuel Trim is a learned value used to compensate for continual deviation of the Short TermFuel Trim from its central value. (Term means time. Short Term Fuel Trim makes an immediate correction for O2 sensor bias. LongTerm Fuel Trim makes a correction for Short Term Fuel Trim bias).Ripple Test: A test that checks for unwanted A/C. voltage leaking from an alternator rectifier bridge.

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• ECM failure• Fuel injector electrical circuit defect

Do not forget that on a multiport system, a fuel injector with abad intake O-ring seal can cause a vacuum leak that has moreeffect on its own cylinder. So use your favorite vacuum leak detection method and include injector O-ring seals in yoursearch. Exhaust gas leaking into the intake from an Exhaust GasRecirculation (EGR) valve has an unequal effect on cylinderpower loss. The cylinders closest to the EGR are affected most.

This effect is more severe at low rpm than at high rpm.

Diagnosing Different ConfigurationsSince there are so many different valvetrain, fuel system, and ig-nition system configurations, seriously consider system config-uration when analyzing test results. To understand this better,look at the following example:

ConfigurationInline 4-cylinder engineDistributor ignitionThrottle-body fuel injection

Power Contribution Test ResultsCYLINDER NUMBER RPM DROP

1 1102 303 1154 105

Compression Test ResultsAll cylinders within specification

Ignition Scope CheckAll cylinders appear O.K.

The above engines’ symptoms are: runs rough, has pooridle quality, and HC emission is high. The cylinder power contribution test result shows number 2 does not contribute itsshare of power because the drop in engine speed is only 30 rpm.The fuel system configuration dictates that it cannot be a fuel

distribution problem, since fuel is delivered by a TBI system. Itwould be a good idea, at this point, to perform a running com-pression test. It may show low-running compression on cylin-der number 2. A worn camshaft lobe should be suspect, but anypart of the valvetrain that is causing this cylinder’s valve to notopen enough would reduce its power contribution.

However, if this engine in the example had a multiportfuel-injection system, a fuel injector problem would have to beconsidered a possible cause. The O-ring seal on number 2 in-jector should be checked for a vacuum leak. A fuel-injectorvolume or pressure drop test should be performed, to see ifnumber 2 injector delivers a proper amount of fuel.

As this example illustrates, you must consider system con-figuration in your analysis, or you will not be aware of all thepossible causes of a symptom.

InsightMost problems that affect engine power contribution cause arise in HC emission. Only those problems that cause a richcondition cause a proportional rise in CO emission. The moreeffect the problem has on power contribution, the higher theHC emission will be.

Engine Mechanical Condition TestsA cranking compression test will reveal a cylinder with a seal-ing problem. Testing dry and then wet with a few squirts of oilwill indicate whether you have ring or valve problems. Thefastest compression test is an automated relative compressiontest performed on an engine analyzer.

Use a leak down test to locate the cause of a compressionleak. A leak of 20 psi or greater during a leak down test is seri-ous. Listen to find where the air is escaping. You will hear itcoming from the:

• Tailpipe when the exhaust valve is leaking• Intake if an intake valve is leaking• Crankcase if the rings are bad or the piston is damaged• Cooling system filler if the block is cracked, the head is

cracked, or the head is warped and the head gasket is leaking

But none of these tests will disclose an engine breathing prob-lem, such as a worn camshaft lobe or a valvetrain problem thatprevents the proper amount of air from entering the cylinder.However, a running compression test will uncover this problemand you should perform it when other tests are inconclusive.

Carbon deposits on intake valves can be a difficult prob-lem to diagnose. Intake valve deposits can cause an engine torun lean while cruising and accelerating, and rich during deceleration.

During lean conditions, NOx emission is high. Duringrich conditions, CO emission is high. Intake valve deposits canalso cause driveability problems such as a rough idle, stumble,hesitation, and loss of power under load.

Often, an engine that displays a rough idle problem runssmooth after a fuel-injector cleaning service is performed. Intakevalve deposits that were also removed by the fuel-injector service

Fig. 1-25. Oscilloscope power balance test control panel.

Configuration: The organization of related components in a specific order.Hesitation: A sudden loss of power or forward motion.



could have been the reason performance improved. All that wasreally needed was a carbon clean solution administered throughthe intake manifold, by way of a manifold vacuum port.

A borescope inspection is one way to know for sure ifthere are excessive deposits on the intake valves. By inserting itthrough the intake, you can see the back sides of the valves.

Try a borescope inspection to actually see some problemssuch as intake valve deposits, or to check for a cracked head orblock before condemning the head gasket, figure 1-26. Aborescope can eliminate some tear down inspections and im-prove on others, figure 1-27. This can be a real time-saver foryou and a great value for the customer.

InsightA final thought to remember that will aid your diagnosis: Mostmechanical engine problems that cause engine performancesymptoms do so by affecting combustion efficiency, which in-creases HC emission.

Air Intake System ProblemsRemember to check for vacuum leaks. Keep in mind that a vac-uum leak does not cause a power imbalance but increases en-gine speed. If the system uses an airflow sensor, any leak, evenin an intake air duct, is air that was not measured, figure 1-28. Whether it’s an air duct leak or vacuum leak, if too large,the system cannot compensate, resulting in a lean combustionproblem. Lean combustion will cause HC and NOx emissionsto increase. When the lean combustion problem is severe

enough to cause a misfire, NOx emission will fall and HCemission will increase dramatically.

On systems that use a Manifold Absolute Pressure(MAP) sensor, a vacuum leak will cause the engine speed to in-crease. The faster speed of the engine produces more total massor volume of emissions. However, the emissions remain pro-portionally the same. HC emission will increase if the vacuumleak causes a power imbalance.

Dirty air filters, unless extremely restricted, are usuallycompensated for by today’s modern fuel injection systems.However, air filters must still be changed when needed becausethey do protect an expensive airflow sensor and engine.

InsightCarbureted vehicles built in the ’80s experiencing an air intakeproblem could cause a power imbalance and an rpm decreaseat idle.

Fig. 1-27. A borescope can help spot defects even after a teardown.

Borescope: A device used to look inside areas of the engine that usually cannot be seen without disassembly.Misfire: Incomplete combustion resulting in increased emissions and the possibility of catalyst damage.Manifold Absolute Pressure (MAP): The pressure in the intake manifold referenced to a perfect vacuum. Since manifold vacuum is thedifference between manifold absolute pressure and atmospheric pressure, all the vacuum readings in the Composite VehiclePreparation/Reference Booklet are taken at sea level (where standard atmospheric pressure equals 101 kPa or 0 in. Hg).

Fig. 1-26. Use a flexible fiber optic borescope to see where younormally cannot see.

THROTTLE BODY ASSEMBLY

AIRDUCT

AIRFLOWSENSOR

Fig. 1-28. Air leaks at the air duct connections or breaks in theair duct would cause a lean condition.

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Idle System ProblemsCurb idle is usually controlled by the ECM on most late-modelvehicles. However, most have what is called a minimum air rate,minimum throttle angle, or minimum idle speed adjustment.Check the tune-up procedure section of your shop manual forthe proper procedure. If the minimum air rate is incorrect, thevehicle may suffer from:

• Off-idle hesitation• Idle load compensation problems• Low or high idle speed• Rough idle quality• Stall on deceleration

If the Throttle Position (TP) sensor is adjustable, adjustmentusually accompanies a minimum idle adjustment.

Inspect behind the throttle plate for carbon build-up.Look in the throttle bore and the bypass port, figure 1-29. Thisbuild-up of carbon will affect the minimum air rate. Check theOEM recommendations before cleaning. Some throttle boreshave a special coating that may be removed by cleaner, expos-ing it to corrosion and carbon build-up. Carbon build-up inthe throttle bore or bypass port may cause:

• Low idle speed• Rough idle quality• Off-idle hesitation

Fuel QualityCheck for fuel quality problems such as water contaminationor alcohol content. Too much alcohol not only decreases en-gine power, it also damages fuel delivery system components.Test kits are available to check for fuel contamination.

Stale or old gasoline that has been stored for a long timemay cause hard starting. Old gas can also increase HC due tomisfires, but seldom prevents starting.

Low octane gasoline can cause spark knock or engine pingand increase NOx emission.

Refineries control seasonal gasoline volatility. Higher fuelvolatility on an unseasonably warm day can increase NOxproduction. It can also vaporize in the fuel delivery system,causing a leaner fuel mixture. The leaner mixture will cause anincrease of HC emission. When the vaporization problem is se-vere enough, fuel starvation from vapor lock occurs. Lowergasoline volatility in cold weather can cause hard starting anddriveability problems during cold engine operation.

When in doubt about the fuel, it is best to test it or replaceit with fresh fuel of the proper octane.

Fuel System TestsVisual inspection for fuel leaks is a first step when there is alean combustion problem, or a fuel odor complaint. In tightplaces where there is poor visibility, the gas analyzer can helpyou search for a leak. Watch the HC reading on the analyzerand use the sample probe to sniff out the leak.

Do not neglect to perform fuel pressure and volume tests,figure 1-30. Even if access is difficult, they are absolutely nec-essary for diagnosing both rich and lean mixture problems.Use pressure and volume tests to help diagnose defects of the:

• Fuel pump• Rest or static pressure check valve• Fuel pressure regulator • Fuel injector

Fuel injectors can fail in many different ways. Most defectsaffect a pressure drop or volume flow test. Refer to OEM spec-ifications. Some shops try injector cleaning first, replacinginjectors only if cleaning does not solve the problem. Be sureto check the OEM’s recommendations because cleaning dam-ages some types of injectors. In this case, the only safe alterna-tive is replacement.

Check Valve: A valve that permits flow in only one direction.Compensation: To correct for too much or too little of something.Fuel Starvation: The lack of fuel available for efficient combustion.Fuel Volatility: The lower the temperature at which a fuel vaporizes, the higher the volatility.Vapor Lock: When fuel vaporizes in a line or device and blocks the flow of liquid fuel, usually causes engine stall.

THROTTLEBORE

BYPASSPORT

EGRPASSAGE

Fig. 1-29. Check the throttle bore, throttle plate, and bypass portfor carbon build-up. Fig. 1-30. Fuel system pressure test with pressure gauge.



An increase of CO emission, caused by the fuel deliverysystem, is usually one of the following high-pressure problems:

• Defective pressure regulator• Pinched return hose• Crushed return line• Fuel injector nozzle leak

If the system is allowed to continue with these rich conditions,it may foul the spark plugs, causing a misfire, resulting in in-creased HC emission.

If a high CO emission, rich condition is so severe it causesall available combustion O2 to be used up, HC emission willalso increase.

An increase of HC and NOx emissions caused by the fuel-injection system is usually due to one of the following low pres-sure or volume problems:

• Weak fuel pump• Restricted fuel filter• Restricted fuel line• Pinched fuel delivery hose• External fuel leak• Dirty fuel injector or poor spray pattern• Punctured fuel pressure regulator diaphragm

If the lean condition is so severe it causes a misfire, NOx emis-sion will decrease and HC emission will increase dramatically.

Ignition System ProblemsFirst, let’s clear up some OBD II ignition terms: EI is an Elec-tronic Ignition system that is a direct ignition system using eithertwo spark plugs per coil, or a direct ignition system with one coilper spark plug. DI is an ignition system that uses a distributor.

Unless you have an engine that is a no-start or an obviousignition wire problem to repair, start by checking the ignitiontiming first. Regardless of whether it is an EI type, or not, if ithas a specification and a procedure available, be sure to checkit. Few crank sensors are adjustable and will change initial timing if not adjusted properly. You should verify the timingadvance capability of most models.

Acceptable type spark testers, like a High Energy IgnitionTester (HEI), are great for a no-start diagnosis. However, it isdifficult to say for certain that the spark is adequate, just bywatching it jump the gap of a spark tester. Oscilloscope checksof secondary voltage are best for showing a spark plug, sparkplug wire, distributor cap, or rotor problem.

EI systems give some problems with scope hook-up, butmost manufacturers now have methods for connecting, evenon EI systems that have no plug wires.

Remember, two things will not change: High voltage onthe scope means high resistance in the circuit. Low voltage on the scope means low resistance, or short circuit problems,figure 1-31.

Any ignition problem that affects combustion increasesHC emission.

InsightWhen any type of misfire occurs, it may cause the O2 sensor tosend a low voltage signal to the ECM. The ECM interprets thisto mean the system is lean, when in reality it is not. Unless the ECM recognizes this as a fault, the ECM will adjust the

Diaphragm: A thin flexible wall, separating two cavities, used to turn a change of vacuum or pressure into mechanical movement, suchas the diaphragm in a vacuum advance.Electronic Ignition (EI): An ignition system that has coils dedicated to specific spark plugs (one or two spark plugs) and does not use adistributor; often referred to as distributorless ignition.On Board Diagnostics (OBD): A diagnostic program contained in the PCM that monitors computer inputs and outputs for failures.OBD II is an industry-standard, second generation OBD system that monitors emissions control systems for degradation as well asfailures.Nozzle: The opening through which a substance flows.Short Circuit: A condition in which a path is provided around the circuit load to another circuit or ground.

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15

10

5

0

30

20

10

0

15

10

5

0

30

20

10

0

Fig. 1-31. Examples of common ignition secondary problemsdisplayed on an oscilloscope.



injector on time and fuel trim to increase fuel delivery. How-ever, no amount of fuel will lower the amount of oxygen pass-ing the O2 sensor from the misfire. During this time, the sys-tem will be rich and CO emission will be higher.

If the ECM recognizes the signal is false, it will set a codeand remain in open-loop mode. This mode does not necessar-ily mean the system will be rich because some systems actuallydefault lean.

Exhaust System ProblemsSometimes, a loss of power can be caused by a restricted orclogged exhaust system. A first step to check for a restricted ex-haust is a vacuum test. To perform this test, warm the engine andattach a vacuum gauge to the manifold vacuum. Run the engineto at least 1500 rpm. Vacuum should be steady at 17 to 21 inchesof mercury (in-hg), depending on engine condition. If the exhaustis restricted severely enough, the vacuum may never reach thisvalue. The vacuum will drop as the exhaust backpressure builds.

Be aware, some other engine performance problems cancause the same vacuum test results. These include a loss of fuelpressure or volume, weak spark, or low charging voltage.

The next step is exhaust backpressure testing. Use a poundsper square inch (psi) pressure gauge in the O2 sensor threadedmounting hole, figure 1-32, in the EGR backpressure transducerexhaust port hose, an OEM gas analyzer exhaust gas test port (ifyou are lucky), or an aftermarket exhaust gas backpressure testkit. If you cannot find an OEM specification, use a specificationof 3psi maximum backpressure at 1500 rpm. Another option isto disassemble and visually inspect to find the restriction.

Exhaust system air leaks can cause a safety problem for pas-sengers. Poisonous emissions from the leak could reach the pas-senger compartment. Exhaust leaks can cause annoying poppingsounds in the exhaust and cause your gas analyzer to give some

pretty strange readings. The problem with an exhaust leak is thatit lets air into the exhaust, as well as letting exhaust gas out.

The extra air can cause sudden combustion of hot exhaustgas. The extra air also dilutes the exhaust stream, causing unre-liable gas analyzer readings.

To check for the location of an exhaust leak, have a helpercover the tailpipe outlet to cause backpressure. Listen for ahissing sound, at the site of a leak, along the length of the ex-haust system.

Transmission and Final Drive ProblemsState emission inspections performed on dynamometers havecreated another reason for periodic drivetrain inspection andrepair.

A drivetrain problem can be the cause of an aborted or failedemission test. It is important to realize during your drivetrain in-spection that any modification of tire circumference, or final gearratio, will change the ratio of engine speed to vehicle speed. Asimple change of tire size now has an effect on emissions.

One example would be certain OEM’s ECM strategies forEGR system operation. Some systems monitor a ratio of en-gine speed to vehicle speed for decisions about opening and

Drivetrain: A reference that describes the parts from the engine to the drive axle(s).Monitor: To watch, observe, or check something.On time: The time when an actuator is energized, as when a fuel injector is signaled to allow fuel to flow.

SOLENOID OFF

SOLENOID ON

ARMATURE

ARMATURE

EXHAUSTTO SUMP

SEAT

FROM SOLENOIDREGULATOR VALVE

FROM SOLENOIDREGULATOR VALVE

TO SHIFT VALVE(PRESSURE LOWERTHAN INPUT)

BALL VALVE (SEATED)

FULL PRESSURE TO SHIFT VALVE

SPRING

Fig. 1-33. Two position solenoids are on/off switches that openand close passages to regulate fluid flow in the transmission.

GAUGE

ADAPTER

O SENSOR2

EXHAUSTMANIFOLD

Fig. 1-32. Using a pressure gauge to test exhaust systembackpressure.



modulating the EGR valve. A modification here could cause astate emission test failure.

A drivetrain problem can create an unsafe condition thatwould cause an emissions test to be aborted. Diagnosing drive-train safety problems requires much the same approach as en-gine problems. Visual inspection of CV boots, U-joints, or axlevibration dampers requires grasping, pushing, and pulling tocheck for worn joints. Listen while test driving to see if you hearthat telltale clicking noise on turns, indicating a bad CV joint.

Some defects can affect emissions by adding or changingengine load conditions. Check levels and condition of fluids.Use pressure tests to help diagnose automatic transmissionproblems. Perform electrical checks of transmission fluid tem-perature (TFT), transmission turbine speed (TSS), and thetransmission range (TR) switch, as well as engine and vehiclespeed (VSS) sensors. Electrically check the torque converterclutch lock-up and shift control solenoids, figure 1-33.

EXHAUST GAS ANALYZERSTo properly diagnose fuel system concerns an exhaust gas an-alyzer should be used. The analyzers are available in manystyles and designs. Current models are designed to sample andanalyze either four or five gasses present in the exhaust fromthe vehicle. The newest models are designed for five gas detec-tion and normally provide digital and/or printed results ofeach test. Either piece of equipment is generally suitable for di-agnosing basic fuel system abnormalities and driveabilityproblems.

Five-Gas AnalyzersFive-gas analyzers measure the parts per million (ppm) of hy-drocarbons (HC), the percentage of carbon monoxide (CO),the percentage of oxygen (O2), the percentage of carbon diox-ide (CO2) and the percentage of oxides of nitrogen (NOx).Most properly tuned computer-controlled vehicles will pro-duce about 50 ppm of HC, less than 0.5 percent CO, 1.0 to 2.0percent O2 and 13.8 to 15.0 percent CO2.

Four-Gas AnalyzersFour-gas analyzers measure HC, CO, CO2 and O2. They donot provide data as to the levels of NOx in the exhaust.

Diagnosing Exhaust GassesFor an accurate analysis of fuel combustion on catalytic con-verter-equipped vehicles, prevent the air injection system fromsupplying oxygen into the exhaust stream. This decreases theamount of O2 at the tailpipe and the efficiency of the converter.The air injection system may be disabled by several means. Onsome vehicles, disconnecting the air injection pump or plug-ging the pulse air injection system is effective. For others, theprobe of the analyzer can be connected to a port installed up-stream of the catalytic converter or to the exhaust opening forthe EGR valve. Next, make sure that the engine is at operatingtemperature, in closed loop, and the HO2S is transmitting a

variable signal. Sample the exhaust gases both at idle and at2,500 RPM. If a dynamometer is used, test under simulatedhighway load conditions as described by the manufacturer.

Refer to Chapter 6 for additional information.

Abnormal HC and CO ReadingsHigh HC levels indicate unburned fuel in the exhaust causedby incomplete combustion. The source of high HC emissionscan often be traced to the ignition system, but mechanical orfuel system problems also can increase HC emissions. Highlevels of HC emissions result from:

• Advanced ignition timing• Ignition misfire from defective spark plug wires or fouled

spark plugs• An excessively rich or lean air-fuel mixture• Leaking vacuum hoses, vacuum controls, or seals• Low engine compression• Defective valves, valve guides, valve springs, lifters,

camshaft, or incorrect valve lash• Defective rings, pistons, or cylinder walls• Clogged fuel injectors causing a lean misfire

The amount of CO in the exhaust stream is directly propor-tional to the amount of O2 contributing to the combustionprocess. Richer air-fuel mixtures, with lower oxygen content,produce higher CO levels; leaner air-fuel mixtures, with higheroxygen content, produce lower CO levels. High CO emissionsmay result from one or more of the following abnormal con-ditions:

• Clogged or dirty intake air passages• Plugged air filter element• Throttle body coking• Rich fuel mixture• Incorrect idle speed• Excessive fuel pressure • Leaking fuel injectors

Both HC and CO levels reading high at the same time may becaused by the following conditions:

• Defective positive crankcase ventilation system• Defective catalytic converter• Defective manifold heat control valve• Defective air pump• Defective thermostatic air cleaner

Abnormal CO2 and O2 ReadingsSince the catalytic converter reduces HC and CO, these emis-sions are unreliable for determining the air fuel ratio. Howev-er, CO2 and O2 readings can be useful, provided that the airinjection system has been disabled.

When air and fuel entering the engine burns with the leastamount of wasted energy, at the stoichiometric air-fuel ratio,the engine emits the highest amount of CO2. Look for readings

Carbon Monoxide: An odorless, colorless, tasteless poisonous gas. A pollutant produced by the internal combustion engine.CV Boot: The flexible cover used to prevent road dirt contamination of a CV joint.Hydrocarbons: Chemical compounds in various combinations of hydrogen and carbon. A major pollutant from an internal combustionengine. Gasoline, itself, is a mixture of hydrocarbons.

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between 13.8 and 15 percent. As the air-fuel ratio of the mix-ture leans or enriches, the CO2 level drops. To determinewhether a low CO2 level indicates a lean or rich condition, ex-amine the O2 reading. Levels of O2 below approximately 1.0percent indicate a rich running engine; above 2.0 percent indi-cates a lean running engine.

To perform adequately and operate efficiently, an enginemust be in sound mechanical condition. Therefore it is im-portant to determine the overall mechanical condition of theengine before attempting to isolate or repair the cause of a drive-ability or performance problem.

Perform a compression or cylinder leakage test to deter-mine the internal sealing capabilities of the engine. When testresults are marginal and indicate valve seating problems, per-formance can often be restored by adjusting lash or servicing

hydraulic valve lifters. If test results are below specifications,internal engine repairs are required to restore performance.

Variable Valve TimingThe variable valve timing system advances or retards camshafttiming to increase engine output, improve fuel efficiency and de-crease emissions. A hydraulic actuator on the cam drive uses oilpressure to rotate the cam’s position slightly, increasing valve du-ration. Cam timing is determined by the engine control module(ECM) using the crankshaft position (CKP) sensor andcamshaft position sensor (CMP 1 and CMP 2) signals.

Each intake camshaft has a separate camshaft positionsensor, hydraulic actuator, and control solenoid. If little or nooil pressure is received by a hydraulic actuator, it is designed tomechanically default to the fully retarded position.



1. Voltage drop in a circuit always equals:a. The resistance of each componentb. The current flow through the

ground circuitc. The source voltaged. None of the above

2. A diode is designed to:a. Allow current flow in both directionsb. Prevent current flow in both

directionsc. Add extra resistance to control

circuitsd. Allow current flow in one direction

only

3. When diagnosing a starting problem,disabling the ignition and fuel or justthe fuel system prevents start up.What else is prevented whileperforming cranking tests?a. Crankcase oil dilutionb. Excessive fuel backwashc. False scan tool readingsd. Parasitic drains

4. In a series circuit with three 4 ohmbulbs and 12 volts applied, the totalcircuit voltage drop will be:a. 12 voltsb. 4 voltsc. 8 voltsd. 1 volt

5. The unit of measure for current flow ina circuit is:a. Ampsb. Voltsc. Ohmsd. Watts

6. The unit of measurement forresistance in a circuit is:a. Voltsb. Ohmsc. Wattsd. Amps

7. A circuit that has one path for currentflow is called a:a. Complex circuitb. Series circuitc. Parallel circuitd. Bias circuit

8. The voltage drop across an electricalcomponent depends on the voltageapplied and the _______ of thecomponent.a. Sizeb. Resistancec. Electrond. Weight

9. When performing a no-start diagnosis,if fully charged, a battery shouldmeasure:a. 12.3Vb. 12.4Vc. 12.5Vd. 12.6V

10. A long warm-up time may be causedby:a. Low ambient temperatureb. Stuck-open thermostatc. No thermostatd. All of the above

11. An engine breathing problem, such asa worn camshaft lobe or valvetrainproblem that prevents the properamount of air entering the cylinder,may be diagnosed by running whichtest?a. Compression testb. Running compression testc. Engine mechanical testd. Standing rhinostatic test

12. Technician A says that watching aspark jump a gap of a spark tester isadequate to determine whether there isa spark problem. Technician B says it’simportant to run an oscilloscope checkof secondary voltage to determinewhether the problem exists in a sparkplug wire. Who is right?a. A onlyb. B onlyc. Both A and Bd. Neither A nor B

13. True of false? When you believe a lossof power is being caused by a restrictedor clogged exhaust system, the firststep to check for a restricted exhaustsystem is to perform an exhaustbackpressure test.a. Trueb. False

14. Technician A says a five-gas exhaustgas analyzer is used to measure HC,CO, O2, CO2, and NOx. Technician Bsays a five-gas analyzer is used tomeasure HC, CO, O2, CO2, and N2.Who is right?a. A onlyb. B onlyc. Both A and Bd. Neither A nor B

15. While diagnosing a starting problem, itis determined that the solenoid clicksbut does not spin.Technician A saysthat a defective solenoid may becausing the problem.Technician Bsays it might be due to excessiveresistance in the starter control circuit.Who is right?a. A onlyb. B onlyc. Both A and Bd. Neither A nor B

16. In a series circuit containing three 4 ohm bulbs with 12 volts applied,resistance total is:a. 3 ohmsb. 12 ohmsc. 4 ohmsd. 1 ohm

17. In a series circuit with three 4 ohmbulbs and 12 volts applied, currentflow is:a. 1 ampb. 12 ampsc. 4 ampsd. 8 amps

CHAPTER QUESTIONS

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CHAPTER OBJECTIVES• The technician will complete the ASE task list on Computer-

ized Powertrain Controls Diagnosis Including OBD II.• The technician will be able to answer 13 questions dealing with

the Computerized Powertrain Controls Diagnosis IncludingOBD II section of the L1 ASE Test.

This chapter focuses on the operation and diagnosis of com-puterized powertrain control systems. The industry is placingmore emphasis on the technician’s ability to diagnose thesecomplex control system defects and failures. If you need addi-tional information on engine control systems refer to the A8study guide.

As with any diagnostic routine once you have verified thecustomer’s concern, begin your diagnosis by checking formissing, modified, inoperative, or tampered computerizedpowertrain control components. If any are found, repair or re-place and retest the system before continuing.

Most OEMs provide diagnostic information for the com-puterized powertrain control systems either in the ServiceManual or in a separate Diagnostic Manual. Locate the correctdiagnostic information for the vehicle being serviced. Keep inmind that it is very important to take into account the follow-ing when looking up information:

• Model year• Manufacturer/Make• Model• Production date• VIN• Engine size• Emissions certification type

Most service and diagnostic procedures begin with a short description of system operation to familiarize you with thedesigned operating strategies for the system. Make it a habitto always read this information before jumping into the diag-nostic routine.

COMPOSITE VEHICLE TYPE 3 INFORMATIONGeneral DescriptionThis generic four cycle, V6 engine has four overhead chain-driven camshafts, 24 valves, distributorless ignition, and a mass

airflow-type closed-loop sequential multiport fuel injectionsystem. The Engine Control Module (ECM) receives inputfrom sensors, calculates ignition and fuel requirements, andcontrols engine actuators to provide the desired driveability,fuel economy, and emissions control, figure 2-1. The ECM alsocontrols the vehicle’s charging system. The powertrain controlsystem has OBD II-compatible sensors and diagnosticcapabilities. The ECM receives power from the battery andignition switch and provides a regulated 5 volt supply for mostof the engine sensors. The engine is equipped with a singleexhaust system and a three-way catalytic converter, without anysecondary air injection. Engine control features include variablevalve timing, electronic throttle actuator control (TAC), a datacommunications bus, a vehicle anti-theft immobilizer system,and onboard refueling vapor recovery (ORVR) EVAP com-ponents. The control system software and OBD II diagnosticprocedures stored in the ECM can be updated using factorysupplied calibration files and PC-based interface software, alongwith a reprogramming device or scan tool that connects the PCto the vehicle’s data link connector (DLC).

Fuel System• Sequential Multiport Fuel Injection (SFI)• Returnless Fuel Supply with electric fuel pump mounted

inside the fuel tank• Fuel pressure is regulated to a constant 50 psi (345 kPa) by

a mechanical regulator in the tank. Minimum acceptablefuel pressure is 45 psi (310 kPa). The fuel system shouldmaintain a minimum of 45 psi (310 kPa) for two minutesafter the engine is turned off.

Ignition System• Electronic (Distributorless) Ignition (EI) with six ignition

coils (coil-over-plug)• Firing order: 1-2-3-4-5-6• Cylinders 1, 3, and 5 are on Bank 1; cylinders 2, 4, and 6

are on Bank 2• Ignition timing is not adjustable• Timing is determined by the ECM using the Crankshaft

Position (CKP) sensor signal• The ignition control module is integrated into the ECM

CHAPTER TWO

COMPUTERIZED POWERTRAIN CONTROLS DIAGNOSIS INCLUDING OBD II

Sequential Multiport Fuel Injection (SFI): A fuel injection system that uses one electronic fuel injector for each cylinder. The injectorsare pulsed in the sequence of each cylinder’s intake stroke.


Chapter Two: Computerized Powertrain Controls Diagnosis Including OBD II 625

Fig. 2-1. The ASE composite vehicle Type 3 wiring diagram shows ECM sensors, actuators, and related circuits. (Part 1 of 3)

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626 Chapter Two: Computerized Powertrain Controls Diagnosis Including OBD II





Idle Speed• Non-adjustable closed throttle stop (minimum air

rate)• Normal no-load idle range is 850 to 900 rpm with an idle

air control value of 15% to 25%

Automatic Transmission• Four-speed automatic overdrive transaxle, with shifting

controlled by a transmission control module (TCM). TheTCM communicates with the ECM and other modulesthrough the data bus.

Overdrive: A condition in which the drive gear rotates slower than the driven gear. Output speed of the driven gear is increased, whileoutput torque is reduced. A gear ratio of 0.70:1 is an overdrive gear ratio.Transaxle: The combination of a transmission and differential gears, used in front wheel drive and rear engine vehicles.

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• The TCM provides its own regulated 5 volt supply, per-forms all OBD II transaxle diagnostic routines, and storestransaxle diagnostic trouble codes (DTCs). The controlsystem software and OBD II diagnostic procedures storedin the TCM can be updated in the same way as the ECM.

• Failures that result in a pending or confirmed DTC relat-ed to any of the following components will cause the TCMto default to fail-safe mode: range switch, shift solenoids,turbine shaft speed sensor, and the vehicle speed sensor.The TCM will also default to fail-safe mode if it is unableto communicate with the ECM.

• When in fail-safe mode, maximum line pressure will becommanded, the transmission will default to 2nd gear andthe torque converter clutch will be disabled.

Variable Valve Timing• Intake camshaft timing is continuously variable using a

hydraulic actuator attached to the end of each intakecamshaft. Engine oil flow to each hydraulic actuator iscontrolled by a camshaft position actuator control sole-noid. Exhaust camshaft timing is fixed.

• A single timing chain drives both exhaust camshafts andboth intake camshaft hydraulic actuators. While valveoverlap is variable, valve lift and duration are fixed.

• Cam timing is determined by the ECM using the crank-shaft position (CKP) sensor and camshaft position sensor(CMP 1 and CMP 2) signals. At idle, the intake camshaftsare fully retarded and valve overlap is zero degrees. Athigher speeds and loads, the intake camshafts can be ad-vanced up to 40 crankshaft degrees.

• Each intake camshaft has a separate camshaft positionsensor, hydraulic actuator, and control solenoid. If little orno oil pressure is received by a hydraulic actuator (typi-cally at engine startup, at idle speed, or during a fault con-dition), it is designed to mechanically default to the fullyretarded position (zero valve overlap), and is held in thatposition by a spring-loaded locking pin.

Electronic Throttle Control• The vehicle does not have a mechanical throttle cable, a

cruise control throttle actuator, or an idle air control(IAC) valve. Throttle opening at all engine speeds andloads is controlled directly by a throttle actuator control(TAC) motor mounted on the throttle body housing.

• Dual accelerator pedal position (APP) sensors provideinput from the vehicle operator, while the actual throttleangle is determined using dual throttle position (TP)sensors.

• If one APP sensor or one TP sensor fails, the ECM willturn on the malfunction indicator lamp (MIL) and limitthe maximum throttle opening to 35%. If any two (ormore) of the four sensors fail, the ECM will turn on theMIL and disable the electronic throttle control.

• In case of failure of the electronic throttle control system,the system will default to limp-in operation. In limp-inmode, the spring-loaded throttle plate will return to adefault position of 15% throttle opening, and the TACvalue on the scan tool will indicate 15%. This default

position will provide a fast idle speed of 1400 to 1500rpm, with no load and all accessories off.

• Normal no-load idle range is 850 to 900 rpm at 5% to10% throttle opening.

• No idle relearn procedure is required after componentreplacement or a dead battery.

Data Communications Bus• The serial data bus is a high-speed, non-fault tolerant,

two wire twisted pair communications network. Itallows peer-to-peer communications between variouselectronic modules, including the engine controlmodule (ECM), transmission control module (TCM),instrument cluster (including the MIL), immobilizercontrol module, and a scan tool connected to the datalink connector (DLC).

• The Data-High circuit switches between 2.5 (rest state)and 3.5 volts (active state), and the Data-Low circuitswitches between 2.5 (rest state) and 1.5 volts (activestate). The data bus has two 120 ohm terminating resis-tors: one inside the instrument cluster, and another oneinside the ECM.

• Any of the following conditions will cause the data com-munications bus to fail and result in the storage of net-work DTCs: either data line shorted to power, to ground,or to the other data line.

• The data bus will remain operational when one of the twomodules containing a terminating resistor is not connect-ed to the network. The data bus will fail when both termi-nating resistors are not connected to the network.

• Data communication failures do not prevent the ECMfrom providing ignition and fuel control.

Immobilizer Anti-Theft System• When the ignition switch is turned on, the immobilizer

control module sends a challenge signal through the an-tenna around the ignition switch to the transponder chipin the ignition key. The transponder key responds with anencrypted key code. The immobilizer control modulethen decodes the key code and compares it to the list ofregistered keys.

• When the engine is started, the ECM sends a request tothe immobilizer control module over the data bus to ver-ify the key validity. If the key is valid, the immobilizercontrol module responds with a “valid key” message tothe ECM to continue normal engine operation.

• If an attempt is made to start the vehicle with an invalid ig-nition key, the immobilizer control module sends a messageover the data bus to the instrument cluster to flash the anti-theft indicator lamp. If the ECM does not receive a “validkey” message from the immobilizer control module within2 seconds of engine startup, the ECM will disable the fuelinjectors to kill the engine. Cycling the key off and crankingthe engine again will result in engine restart and stall.

• The immobilizer control module and ECM each have theirown unique internal ID numbers used to encrypt theirmessages, and are programmed at the factory to recognizeeach other. If either module is replaced, the scan tool must



be used to program the replacement module, using theVIN, the date, and a factory-assigned PIN number.

• Up to eight keys can be registered in the immobilizer con-trol module. Each key has its own unique internal keycode. If only one valid key is available, or if all keys havebeen lost, the scan tool can be used to delete lost keys andregister new keys. This procedure also requires the VIN,the date, and a factory-assigned PIN number.

• The ECM, TCM, and the immobilizer control module donot prevent operation of the starter motor for anti-theftpurposes.

On-Board Refueling Vapor Recovery (ORVR) EVAP System

• The on-board refueling vapor recovery EVAP systemcauses fuel tank vapors to be directed to the EVAP char-coal canister during refueling, so that HC vapors do notescape into the atmosphere

• The following components have been added to the tradi-tional EVAP system for QRVR capability: a one inch I.D. fillpipe, a one-way check valve at the bottom of the fill pipe, afuel vapor control valve inside the fuel tank, and a 1⁄2 inch I.D.vent hose from the vapor control valve to the canister.

• The fuel vapor control valve has a float that rises to seal thevent hose when the fuel tank is full. It also prevents liquidfuel from reaching the canister and blocks fuel from leak-ing in the event of a vehicle roll-over.

SENSORSCrankshaft Position (CKP) SensorA magnetic-type sensor that generates 35 pulses for eachcrankshaft revolution. It is located on the front engine cover,with a 35-tooth iron wheel mounted on the crankshaft just

behind the balancer pulley. Each tooth is ten crankshaft de-grees apart, with one space for a “missing tooth” located 60 de-grees before top dead center of cylinder number 1, figure 2-2.

Camshaft Position (CMP 1 and CMP 2) SensorsA pair of three-wire solid state (Hall-effect or optical-type)sensors that generate a signal once per intake camshaft revolu-tion. The leading edge of the bank 1 CMP signal occurs on thecylinder 1 compression stroke, and the leading edge of the bank2 CMP signal occurs on the cylinder 4 compression stroke,figure 2-2.When the intake camshafts are fully retarded (zerovalve overlap), the signals switch at top dead center of cylinders1 and 4. When the intake camshafts are fully advanced (maxi-mum valve overlap), the signals switch at 40 crankshaft degreesbefore top dead center. These signals allow the ECM to deter-mine fuel injector and ignition coil sequence, as well as theactual intake valve timing. Loss of one CMP signal will set aDTC, and valve timing will be held at the fully retarded posi-tion (zero valve overlap). If neither CMP signal is detected dur-ing cranking, the ECM will store a DTC and disable the fuelinjectors, resulting in a no-start condition. Located at the rearof each valve cover, with an interrupter mounted on the intakecamshafts to generate the signal.

Mass Airflow (MAF) SensorSenses airflow into the intake manifold. The sensor readingvaries from 0.2 volt (0 gm/sec) at key-on, engine-off, to 4.8 volts(175 gm/sec) at maximum airflow, figure 2-3. At sea level, no-load idle (850 rpm), the sensor reading is 0.7 volt (2.0 gm/sec).Located on the air cleaner housing.

Manifold Absolute Pressure (MAP) SensorSenses intake manifold absolute pressure. The MAP sensorsignal is used by the ECM for OBD II diagnostics only. Thesensor reading varies from 4.5 volts at 0 in. Hg vacuum I 101 kPa

Fig. 2-2. CKP and CMP sensor waveforms.

Hall-Effect Sensor: A signal-generating switch that develops a transverse voltage across a current-carrying semiconductor whensubjected to a magnetic field.Magnetic Type Sensor: Magnetic pulse generator, a signal-generating device that creates a voltage pulse as magnetic flux changesaround a pickup coil.Optical Sensor: Uses a light-emitting diode and shutter blade to trigger the switching of a photo-sensitive transistor, sends a squarewave signal used for engine rpm and/or piston position.

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pressure (key on. engine off, at sea level) to 0.5 volts at 24 in.Hg vacuum /20.1 kPa pressure, figure 2-4. At sea level, no-loadidle with 18 in. Hg vacuum (40.4 kPa absolute pressure); thesensor reading is 1.5 volts. Located on the intake manifold.

Throttle Position (TP 1 and TP 2) SensorsA pair of redundant non-adjustable potentiometers thatsense throttle position The TP 1 sensor signal varies from 4.5 volts at closed throttle to 0.5 volts at maximum throttleopening (decreasing voltage with increasing throttle posi-tion), figure 2-5. The TP 2 sensor signal varies from 0.5 voltsat closed throttle to 4.5 volts at maximum throttle opening(increasing voltage with increasing throttle position). Failure

of one TP sensor will set a DTC and the ECM will limit themaximum throttle opening to 35%. Failure of both TP sen-sors will set a DTC and cause the throttle actuator control tobe disabled, and the spring-loaded throttle plate will returnto the default 15% position (fast idle). Located on the throt-tle body.

Engine Coolant (ECT) SensorA negative temperature coefficient (NTC) thermistor thatsenses engine coolant temperature. The sensor values rangefrom -40°F to 248°F (-40°C to 120°C). At 212°F (100°C), thesensor reading is 0.46 volt, figure 2-6. Located in the engineblock water jacket.

Fig. 2-3. MAF signal voltage increases as airflow increases.

Fig. 2-4. MAP sensor signal voltage increases as intake manifold vacuum decreases and manifold absolute pressure increases.

Potentiometer: A variable resistor with three terminals. Signal voltage comes from a terminal attached to a movable contact thatpasses over the resistor.

Fig. 2-5. TPS signal voltage increases as the throttle is opened.



Fig. 2-7. APP sensors signal voltage increases as the accelerator pedal is depressed.

Accelerator Pedal Position (APP 1 and APP 2) SensorsA pair of redundant non-adjustable potentiometers that senseaccelerator pedal position. The APP 1 sensor signal varies from0.5 volts at the released pedal position to 3.5 volts at maximumpedal depression (increasing voltage with increasing pedal po-sition), figure 2-7. The APP 2 sensor signal varies from 1.5 voltsat the released pedal position to 4.5 volts at maximum pedaldepression (increasing voltage with increasing pedal position,offset from the APP 1 sensor signal by 1.0 volt). The ECM in-terprets an accelerator pedal position of 80% or greater as a re-quest for wide open throttle. Failure of one APP sensor will seta DTC and the ECM will limit the maximum throttle openingto 35%. Failure of both APP sensors will set a DTC and causethe throttle actuator control to be disabled, and the spring-loaded throttle plate will return to the default 15% position(fast idle). Located on the accelerator pedal assembly.

EGR Valve Position SensorA three-wire non-adjustable potentiometer that senses theposition of the EGR valve pintle. The sensor reading variesfrom 0.50 volts when the valve is fully closed to 4.50 volts

when the valve is fully opened, figure 2-8. Located on top ofthe EGR valve.

Knock SensorA two-wire piezoelectric sensor that generates an AC voltagespike when engine vibrations within a specified frequencyrange are present, indicating spark knock. The signal is usedby the ECM to retard ignition timing when spark knock is de-tected. The sensor signal circuit normally measures 2.5 voltsDC with the sensor connected. Located in the engine block.

Intake Air Temperature (IAT) SensorA negative temperature coefficient (NTC) sensor that sensesair temperature. The sensor values range from -40°F to 248°F(-40°C to 120°C). At 86°F (30°C), the sensor reading is 2.6volts, figure 2-6. Located in the air cleaner housing.

Vehicle Speed Sensor (VSS)A magnetic-type sensor mat senses rotation of the final driveand generates a signal that increases in frequency as vehiclespeed increases. The TCM uses the VSS signal to controlupshifts, downshifts, and the torque converter clutch. The

Downshift: To shift into a lower gear ratio.Frequency: A measurement in Hertz (cycles per second) of how often something occurs in a specific amount of time.Upshift: To shift into a higher gear ratio.

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Fig. 2-6. ECT, IAT, and TFT sensors signal voltage decreases as temperature increases.



TCM communicates the VSS signal over the data communica-tions bus to the ECM to control high-speed fuel cutoff, and tothe Instrument Cluster for speedometer operation The signalis displayed on the scan tool in miles per hour and kilometersper hour. Located on the transaxle housing.

Heated Oxygen Sensors (HO2S 1⁄1, HO2S 2⁄1and HO2S 1⁄2)Electrically heated zirconia sensors that measure oxygen con-tent in the exhaust stream. Sensor 1⁄1 is located on the Bank1 exhaust manifold (cylinders 1, 3, and 5). Sensor 2⁄1 is located onthe Bank 2 exhaust manifold (cylinders 2, 4, and 6). Both up-

stream sensor signals are used for closed loop fuel control andOBD II monitoring. Sensor 1⁄2 is mounted in the exhaust pipeafter the catalytic converter (downstream). See figure 2-9 toview the relative locations of upstream and downstreamHO2S sensors. The HO2S sensor signal is used for OBD IImonitoring of catalytic converter operation. The sensor out-puts vary from 0.0 to 1.0 volt. When a sensor reading is lessthan 0.45 volt, oxygen content around the sensor is high;when a sensor reading is more than 0.45 volt, oxygen contentaround the sensor is low. No bias voltage is applied to thesensor signal circuit by the ECM. With the key on and engineoff, the sensor readings are zero volts. Battery voltage is con-tinuously supplied to the oxygen sensor heaters whenever theignition switch is on.

Fig. 2-8. EGR valve position sensor signal voltage increases as sensor is opened.

Fig. 2-9. ASE Composite Type 3 vehicle. (Part 1 of 2). Fig. 2-9. ASE Composite Type 3 vehicle. (Part 2 of 2).



Power Steering Pressure (PSP) SwitchA switch that closes when high pressure is detected in thepower steering system. The signal is used by the ECM to adjustidle airflow to compensate for the added engine load from thepower steering pump. Located on the P/S high pressure hose.

Brake Pedal Position (BPP) SwitchA switch that closes when the brake pedal is depressed (brakesapplied). The signal is used by the ECM to release the torqueconverter clutch. Located on the brake pedal.

A/C On/Off Request SwitchA switch that is closed by the vehicle operator to request A/Ccompressor operation. Located in the climate control unit onthe instrument panel.

A/C Pressure SensorA three-wire solid-state sensor for A/C system high-side pressure, figure 2-10. The sensor reading varies from 0.25 voltat 25 psi to 4.50 volts at 450 psi. The signal is used by the ECMto control the A/C compressor clutch and radiator fan, and to adjust idle air flow to compensate for the added engine loadfrom the A/C compressor. The ECM will also interrupt com-pressor operation if the pressure is below 40 psi or above 420 psi. Located on the A/C high side vapor line.

Fuel Level SensorA potentiometer that is used to determine the fuel level.The reading varies from 0.5 volt/0% with an empty tank to4.5 volts/100% with a full tank. When the fuel tank is 1⁄4 full,the sensor reading is 1.5 volts. When the fuel tank is 3⁄4 full,

the sensor reading is 3.5 volts. Used by the ECM when test-ing the evaporative emission (EVAP) system. Located in thefuel tank.

Fuel Tank (EVAP) Pressure SensorSenses vapor pressure or vacuum in the evaporative emission(EVAP) system compared to atmospheric pressure, figure 2-11.The sensor reading varies from 0.5 volt at 1/2 psi (14 in. H2O)vacuum to 4.5 volts at 1/2 psi (14 in. H20) pressure. With nopressure or vacuum in the fuel tank (gas cap removed), thesensor output is 2.5 volts. Used by the ECM for OBD II evap-orative emission system diagnostics only. Located on top of thefuel tank.

Transmission Fluid Temperature (TFT) SensorA negative temperature coefficient (NTC) thermistor thatsenses transmission fluid temperature. The sensor values rangefrom -40°F to 248°F (-40°C to 120°C). At 212°F (100°C), thesensor reading is 0.46 volts. This signal is used by the TCM todelay shifting when the fluid is cold, and control torque con-verter clutch operation when the fluid is hot. Located in thetransaxle oil pan.

Transmission Turbine Shaft Speed (TSS) SensorA magnetic-type sensor that senses rotation of the torque con-verter turbine shaft (input/mainshaft) and generates a signalthat increases in frequency as transmission input speedincreases. Used by the ECM to control torque converter clutchoperation and sense transmission slippage. Located on thetransaxle housing.

A/C Compressor Clutch: An electromagnetic device that engages the otherwise freewheeling A/C pulley.Atmospheric Pressure: The pressure caused by the weight of the earth’s atmosphere. At sea level, this pressure is 14.7 psi (101 kPa).

Fig. 2-10. A/C pressure sensor signal voltage increases as high-side pressure increases.

Fig. 2-11. Fuel Tank (EVAP) pressure sensor signal voltage increases as pressure increases.

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Transmission Range (TR) SwitchA six-position switch that indicates the position of thetransaxle manual select lever: Park/Neutral, Reverse, ManualLow (1), Second (2), Drive (3), or Overdrive (OD). Used by thePCM to control transmission line pressure, upshifting, anddownshifting. Located on the transaxle housing.

ACTUATORSAll coils, injectors, solenoids, and relays receive a constant bat-tery positive voltage feed from the ignition switch and are con-trolled by the ECM providing a ground connection.

Fuel Pump RelayWhen energized, this relay supplies battery voltage (B+) to thefuel pump. The relay coil resistance spec is 48 ± 6 ohms.

Fan Control (FC) RelayWhen energized, this relay provides battery voltage (B+) to theradiator/condenser fan motor. The ECM will turn the fan onwhen engine coolant temperature reaches 210°F (99°C) andoff when coolant temperature drops to 195°F (90°). The fanalso runs whenever the A/C compressor clutch is engaged. Therelay coil resistance spec is 48 ± 6 ohms.

A/C Clutch RelayWhen energized, this relay provides battery voltage (B+) to theA/C compressor clutch coil. The relay coil resistance spec is48 ± ohms.

Throttle Actuator Control (TAC) MotorA bidirectional pulse-width modulated DC motor that controlsthe position of the throttle plate. A scan tool data value of 0%indicates an ECM command to fully close the throttle plate, anda value of 100% indicates an ECM command to fully open thethrottle plate (wide open throttle). Any throttle control actua-tor motor circuit fault will set a DTC and cause the throttle ac-tuator control to be disabled, and the spring-loaded throttleplate will return to the default 15% position (fast idle). Whendisabled, the TAC value on the scan tool will indicate 15%.

Malfunction Indicator Lamp (MIL)The MIL is part of the instrument cluster and receives com-mands from the ECM and TCM over the data communica-tions bus. If the instrument cluster fails to communicate withthe ECM and TCM, the MIL is continuously lit by default.Under normal conditions, when the ignition switch is turnedon the lamp remains lit for 15 seconds for a bulb check. After-ward, the MIL will light only for emissions related concerns.Whenever an engine misfire severe enough to damage the cat-alytic converter is detected, the MIL will flash on and off.

Camshaft Position Actuator Control SolenoidsA pair of duty cycle–controlled solenoid valves that increase ordecrease timing advance of the intake camshafts by controllingengine oil flow to the camshaft position actuators. When theduty cycle is greater than 50%, the oil flow from the solenoidcauses the actuator to advance the camshaft position. Whenthe duty cycle is less than 50%, the oil flow from the solenoid

causes the actuator to retard the camshaft position. When theECM determines that the desired camshaft position has beenachieved, the duty cycle is commanded to 50% to hold the ac-tuator so that the adjusted camshaft position is maintained.The solenoid winding resistance spec is 12 ± 2 ohms.

Exhaust Gas Recirculation (EGR) ValveA duty cycle–controlled solenoid that, when energized, lifts thespring-loaded EGR valve pintle to open the valve. A value of0% indicates an ECM command to fully close the EGR valve,and a value of 100% indicates an ECM command to fully openthe EGR valve The solenoid is enabled when the engine coolanttemperature reaches 150°F (66°C) and the throttle is not closedor wide open. The solenoid winding resistance spec is 12 ± 2ohms.

Fuel InjectorsElectro-mechanical devices used to deliver fuel to the intakemanifold at each cylinder. Each injector is individually ener-gized once per camshaft revolution timed to its cylinder’s in-take stroke. The injector winding spec is 12 ± 2 ohms.

Ignition CoilsThese six coils, mounted above the spark plugs, generate a highvoltage to create a spark at each cylinder individually. Timingand dwell are controlled by the ECM directly, without the useof a separate ignition module. The coil primary resistance specis 1 ± .5 ohms. The coil secondary resistance spec is 10K ± 2K.

Generator FieldThe ECM supplies this variable-duty cycle signal to ground thefield winding of the generator (alternator), without the use of aseparate voltage regulator. Increasing the duty cycle results in ahigher field current and greater generator (alternator) output.

Evaporator Emission (EVAP) Canister PurgeA duty cycle-controlled solenoid that regulates the flow of va-pors stored in the canister to the intake manifold. The solenoidis enabled when the engine coolant temperature reaches 150°F(66°C) and the throttle is not closed. A duty cycle of 0% blocksvapor flow, and a duty cycle of 100% allows maximum vaporflow. The duty cycle is determined by the ECM, based on en-gine speed and load. The solenoid is also used for OBD II test-ing of the evaporative emission (EVAP) system. The solenoidwinding resistance spec is 48 ± 6 ohms. There is also a serviceport with a Schrader valve and cap installed on the hose be-tween the purge solenoid and the canister.

Evaporative Emission (EVAP) Canister Vent SolenoidWhen energized, the fresh air supply hose to the canister isblocked. The solenoid is energized only for OBD II testing ofthe evaporative emission (EVAP) system. The solenoid wind-ing resistance spec is 48 ± 6 ohms.

Torque Converter Clutch (TCC) Solenoid ValveA duty cycle–controlled solenoid valve that applies the torque converter clutch by redirecting hydraulic pressure in the

Duty Cycle: Describes the time of a complete cycle of action, including both the on (energized) and off (deenergized) time of a solenoid.



transaxle. With a duty cycle of 0%, the TCC is released. Whentorque converter clutch application is desired, the pulse widthis increased until the clutch is fully applied. The solenoid willthen maintain a 100% duty cycle until clutch disengagement iscommanded. Then the pulse width is decreased back to 0%. Ifthe brake pedal position switch closes, the duty cycle is cut to0% immediately. The solenoid is enabled when the enginecoolant temperature reaches 150°F (66°C), the brake switch isopen, the transmission is in 3rd or 4th gear, and the vehicle is atcruise (steady throttle) above 40 mph. In addition, wheneverthe transmission fluid temperature is 248°F (120°C) or more,the ECM will command TCC lockup. The solenoid windingresistance is 48 ± 6 ohms.

Transmission Pressure Control (PC) SolenoidThis pulse width modulated solenoid controls fluid in thetransmission valve body that is routed to the pressure regula-tor valve. By varying the duty cycle of the solenoid, the ECMcan vary the line pressure of the transmission to control shiftfeel and slippage. When the duty cycle is minimum (10%), theline pressure will be maximized. When the duty cycle is maxi-mum (90%) the line pressure will be minimized. The solenoidwinding resistance spec is 6 ± 1 ohms.

Transmission Shift Solenoids (SS1 and SS2)These solenoids control fluid in the transmission valve body thatis routed to the 1-2, 2-3, and 3-4 shift valves. By energizing orde-energizing the solenoids, the ECM can enable a gear change,figure 2-12. The solenoid winding resistance is 12 ± 4 ohms.

SFI SYSTEM OPERATION AND COMPONENT FUNCTIONSStarting ModeWhen the ignition switch is turned on, the ECM energizes thefuel pump relay for 2 seconds, allowing the fuel pump to buildup pressure in the fuel system. Unless the engine is crankedwithin this two-second period, the fuel pump relay is de-ener-gized to turn off the pump. The fuel pump relay will remainenergized as long as the engine speed (CKP) signal to the ECMis 100 rpm or more.

Clear Flood ModeWhen the throttle is wide open (throttle opening of 80% orgreater) and the engine speed is below 400 rpm, the ECM turnsoff the fuel injectors.

Run Mode: Open and Closed Loop

Open LoopWhen the engine is first started and running above 400 rpm,the system operates in open loop. In open loop, the ECM doesnot use the oxygen sensor signal. Instead, it calculates the fuelinjector pulse width from the throttle position sensor, thecoolant and intake air temperature sensors, the MAF sensor,and the CKP sensor. The system will stay in open loop until allof these conditions are met:

• Both upstream heated oxygen sensors are sending varyingsignals to the ECM

• The engine coolant temperature is above 150°F (66°C)• Ten seconds has elapsed since startup• Throttle position is less than 80%

Closed LoopWhen the oxygen sensor, engine coolant temperature sensor, andtime conditions are met, and the throttle opening is less than80%, the system goes into closed loop. Closed loop means thatthe ECM adjusts the fuel injector pulse widths for Bank 1 andBank 2 based on the varying voltage signals from the upstreamoxygen sensors. An oxygen sensor signal below 0.45 volt causesthe ECM to increase injector pulse width. When the oxygensensor signal rises above 0.45 volt in response to the richer mix-ture, the ECM reduces injector pulse width. This feedback trimsthe fuel control program that is based on the other sensor signals.

Acceleration Enrichment ModeDuring acceleration, the ECM uses the increase in mass airflowand the rate of change in throttle position to calculate in-creased fuel injector pulse width. During wide open throttleoperation, the control system goes into open loop mode.

Deceleration Enleanment ModeDuring deceleration, the ECM uses the decrease in mass air-flow, the vehicle speed value, and the rate of change in throttleposition to calculate decreased fuel injector pulse width.

Fuel Cut-Off ModeThe ECM will turn off the fuel injectors, for safety reasons,when the vehicle speed reaches 110 mph, or if the engine speedexceeds 6000 rpm.

OBD II SYSTEM OPERATIONComprehensive Component MonitorThe OBD II diagnostic system continuously monitors allengine and transmission sensors and actuators for shorts,opens, and out-of-range values, as well as values that do notlogically fit with other powertrain data (rationality).

Fig. 2-12. This chart shows transmission solenoid applicationsfor the complete vehicle.

Comprehensive: Inclusive or complete.Freeze Frame: Operating conditions that are stored in the memory of the PCM at the instant a diagnostic trouble code is set. (The currentstored PCM data of what was sensed and what commands were being given at the instant in time the most current trouble was set).

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On the first trip during which the comprehensive com-ponent monitor detects a failure that will result in emissionsexceeding a predetermined level, the ECM will store a DTC,illuminate the MIL, and store a freeze frame.

System MonitorsThe OBD II diagnostic system also actively tests some systemsfor proper operation while the vehicle is being driven; fuelcontrol and engine misfire are checked continuously. Oxygensensor response, oxygen sensor heater operation, catalyst effi-ciency, EGR operation, EVAP integrity, variable valve timing,and thermostat operation are tested once or more per trip.When any of the System Monitors detects a failure that will re-sult in emissions exceeding a predetermined level on two con-secutive trips, the ECM will store a diagnostic trouble code(DTC) and illuminate the malfunction indicator lamp (MIL).Freeze frame data captured during the first of the two consec-utive failures are also stored.

FUEL CONTROL—This monitor will set a DTC if the systemfails to enter Closed Loop mode within 5 minutes of startup,or the Long Term Fuel Trim is excessively high or low anytimeafter the engine is warmed up, indicating the loss of fuel con-trol. This is always the case when the Long Term Fuel Trimreaches its limit (�30% or -30%).

ENGINE MISFIRE —This monitor uses the CKP sensor sig-nal to continuously detect engine misfires both severe andnon-severe. If the misfire is severe enough to cause catalyticconverter damage, the MIL will blink as long as the severemisfire is detected.

CATALYTIC CONVERTER — This monitor compares the sig-nals of the two upstream heated oxygen sensors to the signalfrom the downstream heated oxygen to determine the abilityof the catalyst to store free oxygen. If the converter’s oxygenstorage capacity is sufficiently degraded, a DTC is set.

EGR SYSTEM — This monitor uses the MAP sensor signal todetect changes in intake manifold pressure as the EGR valve iscommanded to open and close. If the pressure changes too lit-tle or too much, a DTC is set.

EVAP SYSTEM — This monitor first turns on the EVAP ventsolenoid to block the fresh air supply to the EVAP canister.Next, the EVAP purge solenoid is turned on to draw a slightvacuum on the entire EVAP system, including the fuel tank.Then the EVAP purge solenoid is turned off to seal the system.The monitor uses the fuel tank (EVAP) pressure sensor signalto determine if the EVAP system has any leaks. If the vacuumdecays too rapidly, a DTC is set. In order to run this monitor,the engine must be cold (below 86°F/30°C) and the fuel levelmust be between 1⁄4 and 3⁄4 full.

VARIABLE VALVE TIMING — This monitor compares thedesired valve timing with the actual timing indicated by theCMP sensors. If the timing is in error, or takes too long to reachthe desired value, a DTC is set.

ENGINE THERMOSTAT — This monitor confirms that the en-gine warms up fully within a reasonable amount of time. If thecoolant temperature remains too low for too long, a DTC is set.

OXYGEN SENSORS — This monitor checks the maximum andminimum output voltage, as well as switching and response timesfor all oxygen sensors. If an oxygen sensor signal remains too lowor too high or switches too slowly or not at all, a DTC is set.

OXYGEN SENSOR HEATERS — This monitor checks thetime from cold start until the oxygen sensors begin to operate.If the time is too long, a DTC is set. Battery voltage is contin-uously supplied to the oxygen sensor heaters whenever theignition switch is on.

Monitor Readiness StatusThe monitor readiness status indicates whether or not a par-ticular OBD II diagnostic monitor has been run since the lasttime that DTCs were cleared from ECM and TCM memory. Ifthe monitor has not yet run, the status will display on the ScanTool as “Not Complete.” If the monitor has been run, the sta-tus will display on the scan tool as “Complete.” This does notmean that no faults were found, only that the diagnostic mon-itor has been run. Whenever DTCs are cleared from memoryor the battery is disconnected, all monitor readiness status in-dicators are reset to “Not Complete.” Monitor readiness statusindicators are not needed for the Comprehensive Component,Fuel Control, and Engine Misfire monitors because they runcontinuously. The readiness status of the following systemmonitors can be read on the scan tool:

Oxygen SensorsOxygen Sensor HeatersCatalytic ConverterEGR SystemEVAP SystemVariable Valve TimingEngine Thermostat

Warm-Up CycleWarm-up cycles are used by the ECM for automatic clearing ofDTCs and Freeze Frame data. To complete one warm up cycle,the engine coolant temperature must rise at least 40°F (22°C)and reach a minimum of 160°F (71°C).

TripA trip is a key-on cycle in which all enable criteria for a particu-lar diagnostic monitor are met and the diagnostic monitor is run.The trip is completed when the ignition switch is turned off.

Drive CycleMost OBD II diagnostic monitors will run at some time duringnormal operation of the vehicle. However, to satisfy all of thedifferent trip enable criteria and run all of the OBD II diagnos-tic monitors, the vehicle must be driven under a variety of con-ditions. The following drive cycle will meet the enable criteriato allow all monitors to run on the composite vehicle.

Decay: To decline or decrease gradually in activity, strength, or performance.Degraded: Worn down, performing at less than usual standards.



1. Ensure that the fuel tank is between 1⁄4 and 3⁄4 full2. Start cold (below 86°F/30°C) and warm up until engine

temperature is at least 160°F (71°C) — one minute minimum

3. Accelerate to 40–55 mph at 25% throttle and maintainspeed for five minutes

4. Decelerate without using the brake (coast down) to 20 mph or less, then stop the vehicle. Allow the engine toidle for 10 seconds, turn the key off, and wait one minute

5. Restart and accelerate to 40–50 mph at 25% throttle andmaintain speed for two minutes

6. Decelerate without using the brake (coast down) to 20mph or less, then stop the vehicle. Allow the engine toidle for 10 seconds, turn the key off, and wait one minute

Freeze Frame DataA Freeze Frame is a miniature “snapshot” (one frame of data)that is automatically stored in the ECM/TCM memory when anemissions-related DTC is first stored. If a DTC for fuel controlor engine misfire is stored at a later time, the newest data arestored and the earlier data are lost. All parameter ID (PID) val-ues listed under “Scan Tool Data” are stored in freeze framedata. The ECM/TCM stores only one single freeze frame record.

Storing and Clearing DTCs & Freeze Frame Data, Turning the MIL On & OffONE TRIP MONITORS: A failure on the first trip of a “onetrip” emissions diagnostic monitor causes the ECM to immedi-ately store a DTC and freeze frame, and turn on the MIL. Allcomprehensive component monitor faults require only one trip.

TWO TRIP MONITORS: A failure on the first trip of a “twotrip” emissions diagnostic monitor causes the ECM to store atemporary DTC. If the failure does not recur on the next trip, thetemporary DTC is cleared from memory. If the failure does recuron the next trip, the ECM will store a DTC and freeze frame, andturn on the MIL. All the system monitors are two trip monitors.Engine misfire that is severe enough to damage the catalytic con-verter is a two trip monitor, with the additional condition thatthe MIL will blink while the severe misfire is occurring.

AUTOMATIC CLEARING: If the vehicle completes three con-secutive “good trips” (three consecutive trips in which themonitor that set the DTC is run and passes), the MIL will beturned off, but the DTC and freeze frame will remain stored inECM memory. If the vehicle completes 40 warm-up cycleswithout the same fault recurring, the DTC and freeze frame areautomatically cleared from the ECM memory.

MANUAL CLEARING: Any stored DTCs and Freeze Framedata can be erased using the scan tool, and the MIL (if lit) willbe turned off. Although it is not the recommended method,DTCs and Freeze Frame data will also be cleared if the ECMpower supply of the battery is disconnected.

Scan Tool DataFigure 2-13 shows the different types of information that canbe displayed on the OBD II scan tool.

OBD II SYSTEM DIAGNOSTICSOBD II General DescriptionOn-board Diagnostics Second Generation (OBD II) is agovernment-mandated system designed to monitor fuel sys-tem performance, engine misfire, and emission systems oper-ation during normal vehicle operation.

The system includes industry-wide standardization in-tended to improve the diagnostic process by allowing all tech-nicians (dealership and aftermarket) equal access to on-boardcomputer information using a Generic Scan Tool (GST). Im-portant features common to all OBD II vehicles include:

• A common Data Link Connector (DLC)• Access to on-board vehicle information using a GST• Standardized Diagnostic Trouble Codes (DTCs)• MIL operation• Standardized terminology for fuel, ignition, and emission

systems components• Expanded emissions related on-board testing (readiness

tests and system monitors)• New emission related diagnostic procedures• Performance feedback from selected actuators (bi-direc-

tional actuator control)

Data Link Connector (DLC)OBD II standards establish guidelines for the DLC. It is a 16-pin connector, figure 2-14, used to access on-board computerinformation through a GST. The DLC must be located in astandard position, in plain view under the driver’s side dash,and be easily accessed by the technician. Between 1994 and1996, locations varied slightly because manufacturers were al-lowed a grace period to make production changes.

Generic Scan Tool (GST)The GST connects to the 16 pin DLC connector and relays spe-cific OBD II information used in enhanced diagnosis. Thetechnician can also use the GST to activate selected actuatorswhen performing a system diagnosis. Although manufacturedby numerous companies, the GST has the following featuresthat are required by OBD II regulation:

• Record and display the OBD II alphanumeric, five digitDTCs

• Display the status of on-board computer “readiness tests”• Record and display freeze frame data• Display sensor and actuator information when requested

by technician• Clear DTCs and freeze frame data from vehicle computer

memory

Diagnostic Trouble Codes (DTCs)DTCs identify faults in ECM system sensors and circuits or indicate individual system conditions. An OBD II DTC is a five-character, alphanumeric fault identifier, figure 2-15. Since a let-ter is included in every DTC, the only way to retrieve codes iswith a scan tool. The first character of an OBD II DTC is a let-ter. Composite vehicle questions in the L1 test will refer to

Snapshot: A technician-recorded scan tool record or “movie” of PCM data during an event, so that the data can be played back.

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powertrain codes, designated by a capital “P.” Powertrain codestell the technician there is a problem in the fuel, air metering, ig-nition, or an emission control system. Refer to figure 2-15 for anexplanation of other letter codes. The second character is anumber that indicates if the code is common to all OBD IIvehicles (0) or specific to one vehicle manufacturer (1). Re-member, only emissions related, P0 codes will activate the MIL.The third character is a number used by all manufacturers toidentify which system has a fault. This designation will be thesame for P0 (OBD II) or P1 (manufacturer’s) codes. Followingis the established numbering system:

• 1 - Air/Fuel metering system input faults• 2 - Air/Fuel metering output faults

• 3 - Ignition system or misfire faults• 4 - Auxiliary emission controls• 5 - Vehicle speed control and idle control system• 6 - Computer output circuit faults• 7 - Transmission• 8 - Transmission

The fourth and fifth characters indicate the actual problem as-sociated with the code, (e.g., signal voltage low, system alwayslean, etc.) The intent of OBD II code designation is to help thetechnician identify the system at fault, then pinpoint the ac-tual problem or specific circuit causing the fault. Once a prob-lem is identified by code, the technician must use appropriateservice manuals to complete the diagnosis and repair.

MIL OperationThe most significant difference to remember when using theMIL to begin diagnosis on an OBD II vehicle is that there areno soft codes. If the MIL is on, a DTC and freeze frame data arerecorded in computer memory and there is definitely a prob-lem. The OBD I practice of clearing codes and driving the vehicle to see if codes reset must not be used on OBD II vehi-cles. All system monitor codes and many comprehensive com-ponent monitor codes require specific driving conditions before they will test a system or set a DTC. A quick trip aroundthe block to confirm repairs often will not set a DTC, so the

Fig. 2-13. The above data can be accessed by the technician using the OBD II scan tool and the Data Link Connector (DLC).

Fig. 2-14. The Data Link Connector (DLC) has the same shapeand pin designations for all OBD II vehicles.



technician has no way of knowing if the problem still exists. It is best to clear DTCs only when instructed to do so by themanufacturer’s diagnostic procedure, because freeze frame dataand readiness test status are also erased when DTCs are cleared.Instead, use the stored freeze frame data to see what drivingconditions were present when the code was set. Look for un-usual readings from other sensors that may give a clue to thecause of the problem. Try to develop a “total picture” of vehicleoperating conditions at the time the DTC was recorded. Thesame information is useful to help simulate driving conditionson a test drive as you verify the symptoms.

Remember, the MIL will be activated only for failures thatcause excessive emissions. Problems in related systems or com-ponents may be recorded in ECM memory as OBD II (P0) ormanufacturer-designated (P1) DTCs. All powertrain codesshould be reviewed and investigated as part of the diagnosticprocess for driveabilty complaints.

Comprehensive Component MonitorsComprehensive component monitors are most like the OBD Imonitoring system that watches engine and transmission sensorinputs and actuator outputs for shorts, opens, and out-of-rangevalues. OBD II computer (ECM) programs are enhanced to include identification of sensor values that don’t logically fit withother powertrain data. For instance, if the Throttle PositionSensor (TPS) is reporting wide-open throttle (4.5 volts on thecomposite vehicle), but other sensors are reporting idle speedvalues, the ECM will set a DTC for the TPS.

Remember, comprehensive component monitors are onetrip monitors. The ECM will activate the MIL and store DTCand freeze frame data the first time an emissions-related faultis detected. If a misfire or fuel control problem is detected afterthe original DTC was recorded, freeze frame date for the mis-fire or fuel control code will replace the original data.

Readiness Status and System MonitorsYou will recall that the monitor readiness status tells the techni-cian if a particular diagnostic monitor (test) has been completedsince the last time DTCs were cleared from memory. There aretwo important concepts to understand when viewing monitorreadiness status: First, the vehicle must be driven under specificconditions for some monitors to run, and second, the emissionssystem being monitored must be operational. If battery power isdisconnected and the vehicle isn’t driven through an entire drive

cycle, the readiness status will be “NO.” If there is an electricalproblem or component failure in a monitored system, the mon-itor will not run. A DTC may be recorded that points to the elec-trical or component failure, but the system cannot be tested bythe monitor, so the readiness status will be “NO.” A readiness sta-tus of “NO” for any of the five monitored systems, catalyst, EGR,EVAP, Oxygen sensors, and Oxygen sensor heaters, does notmean a failed monitor, only that the monitor has not been com-pleted. At the same time, a “YES” status does not mean the sys-tem passed the monitor, only that the test was completed. In bothcases, you must check for codes to investigate further.

Fuel Control MonitorThe fuel control monitor is designed to constantly check theability of the ECM to control the air/fuel ratio. On the com-posite vehicle, the ECM program that fine tunes the air/fuelratio is called Fuel Trim. It is divided into a short term programand a long term program. Both trim programs are presentedas diagnostic data when a freeze frame is recorded. Separateshort term and long term data are displayed for cylinder bank1 and cylinder bank 2.

The oxygen sensor (HO2S) drives the fuel trim programanytime the vehicle is in closed loop. The starting point for fueltrim is 0% correction, figure 2-16. When the ECM sees a lean(low voltage) signal from an upstream HO2S, the fuel trimprogram adds fuel to compensate for the detected leaness. Theshort term fuel trim display on the scan tool will move to thepositive (+) side of 0% to indicate more fuel is being added.When the ECM sees a rich (high voltage) signal from theHO2S, the fuel trim program subtracts fuel to lean the mix-ture. The scan tool will display a percentage on the negative(-) side of 0%. If short term fuel trim is necessary in one di-rection (rich or lean correction) for a period of time, the ECMwill command a correction of long term fuel trim. When A/Fcontrol is out of acceptable range for too long a time, a DTCwill set. On the composite vehicle, if long term fuel trim reaches+30% (lean correction) or -30% (rich correction) on two

Fig. 2-16. On the composite vehicle, fuel trim corrections aredisplayed on the scan tool as percentage of correction.

Fig. 2-15. OBD II DTCs use a standard format to help all technicians interpret problems more easily.

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consecutive trips, the ECM will activate the MIL and record aDTC and freeze frame data.

Long term fuel trim represents correction to fuel deliveryover time. If the oxygen sensor voltage is fluctuating, but ismainly below 450 mV, indicating a lean A/F ratio, long termfuel trim will increase and the ECM will command longer in-jector pulse width. If oxygen sensor voltage is fluctuating, butremains mostly above 450 mV, indicating a rich mixture, longterm fuel trim will decrease and the ECM will command short-er injection pulse width to adjust fuel delivery.

Short term fuel trim is useful when confirming fuel con-trol. Observe short term fuel trim on the scan tool while addingpropane through the intake system. The additional fuel willcause a rich mixture. If the fuel system is in closed loop, shortterm fuel trim will move in a negative direction as the fuel trimprogram shortens fuel injector pulse width in response to ahigher HO2S voltage signal. Driving the system lean by pullinga vacuum line will cause short term fuel trim to increase injec-tor pulse width. The scan tool display will move in a positive direction.

During diagnosis, be sure to look at both short and longterm fuel trim. A problem that has existed for some time willcause long term fuel trim to record high or low. Once the prob-lem is repaired, long term fuel trim will not change for a while,but short term fuel trim will begin immediately to move in theopposite direction. A restricted fuel filter, for instance, will causea lean mixture. Long term fuel trim will eventually show a pos-itive percentage (more fuel) as the system compensates for thelean mixture. Once the fuel filter is replaced, the A/F ratio is sud-denly too rich. Comparing short and long term fuel trim im-mediately after the filter is replaced will reveal opposite readings:a negative percentage reading in short term fuel trim because theECM is attempting to return the A/F ratio to normal by sub-tracting fuel, and a positive percentage reading in long term fueltrim because the long term program still “remembers” the leancorrection and is waiting to see what happens.

Misfire MonitorEngine misfire monitoring uses the CKP signal as the primarysensor. When a misfire occurs, whether due to engine

compression, ignition, or fuel, crankshaft speed is affected. TheECM is programmed to notice the intermittent change in CKPpulses, figure 2-17.

Camshaft position is used to identify which cylinder mis-fired. Because outside factors such as electrical interference andrough roads can mimic a misfire, most ECM programs keeptrack of how many times a cylinder misfires in a given numberof engine rotations. The ECM activates the MIL when misfirereaches a predetermined percentage of rpm.

Remember, misfire monitoring, like fuel trim monitoring,is a two trip monitor. The MIL will glow steadily once a mis-fire is detected. If misfiring becomes severe enough to damagethe catalytic converter, the MIL will blink continuously untilthe misfire becomes less severe.

Catalytic Converter MonitorAs mentioned earlier, the catalytic converter monitor checksconverter efficiency by comparing upstream HO2S signals withthe downstream HO2S signal. In normal operation the upstream HO2S signals will switch frequently between 200 mVand 900 mV and the downstream HO2S signal will show verylittle fluctuation and a voltage that tends to stay above the 450 mV threshold, figure 2-18. As catalyst performance beginsto degrade, less oxygen is used in the converter and so less endsup in the exhaust, causing voltage fluctuations and a lower volt-age bias, figure 2-19. When the downstream HO2S voltage sig-nal begins to fluctuate within about 70% of the upstream HO2Ssignal on two consecutive trips, the ECM will record freezeframe data, set a DTC, and actuate the MIL.

EVAP MonitorA vehicle will fail the EVAP monitor if the ECM, using in-formation from the fuel tank pressure sensor, sees vacuumdecrease too quickly after the EVAP vent and EVAP purgesolenoids have been closed. Keep in mind that simple prob-lems like a loose, damaged, or missing gas cap will cause thiscode to set.

Be careful when making quick repairs. For example, afterreplacing a damaged gas cap on a vehicle brought in for a litMIL, you may be tempted to clear the DTC and return the car

Fig. 2-17. The ECM is programmed to notice the sudden change in CKP sensor pulses.

Intermittent: Occurring infrequently, not often, or rarely.Threshold: The upper limit of or beginning of something.



Some technicians use a sensor simulator to simulate a coldstart so the monitor will run. A scan tool can be used to checkEVAP system integrity, even with a full tank of gas. Refer tofigure 2-9 to trace the following test procedure on the com-posite vehicle. First, idle the engine. Then, using a scan tool,close the EVAP vent solenoid and open the EVAP purge sole-noid. Intake manifold vacuum will draw a vacuum in theEVAP system. Now close the EVAP purge solenoid to trap vac-uum in the system. Observe the fuel tank pressure sensor read-ing on the scan tool. The composite vehicle will show 0.5 voltat 1⁄2 psi vacuum. If the system is leaking, voltage will climb to-ward 2.5 volts as pressure increases. As always, test and repairprocedures must be followed exactly. Some test procedures,the IM 240 for example, specify testing EVAP system integritywith pressure instead of vacuum.

Diagnostic StrategyThe most valuable aspect of diagnosis with a scan tool is theability to compare data from many sensors and actuators.However, scan tool data should not be used alone. Vehiclesymptoms, driving conditions, and an understanding of oper-ational principles are also important diagnostic tools.

Today’s vehicles require today’s technicians to be aware ofthe ways traditional technology blends with newer, more com-plex system-based technologies. Vehicles manufactured beforethe 1970s controlled fuel and ignition timing through vacuumand mechanical weights and springs. Exhaust emissions werenot seriously considered until the early 1970s. Modern vehiclesuse computerized controls to control fuel and ignition timingprecisely. The tradeoff, however, for this technological ad-vancement is that today’s drivetrain problems can result inrepeated and multiple component failures that require a sys-tem-based approach. For example, a late-model vehicle has thefollowing symptoms: hard starting when cold, an engine miss,and a failed emission test. The initial diagnosis finds a fouledspark plug. Replacing the spark plug and retesting emissions re-sults in a passing report and a smoothly running engine. Whilethis approach addresses the immediate symptom, it does notdeal with the underlying cause of the fouled spark plug.

The result?The customer returns the next day with the same symp-

toms. Only then does the technician examine further to deter-mine that a faulty injector is leaking when the engine is turnedoff. This leak floods a cylinder. The flooded cylinder causes itsassociated spark plug to fail. This would also drain the fuel rail,causing extended cranking on a cold start.

This example clearly illustrates that a systems-basedapproach to the diagnostic process is vital to help eliminatemultiple and repeated component failures that result in dissat-isfied customers.

When approaching any diagnostic problem, take thetime to define vehicle symptoms. How is the vehicle running?Does it have rich symptoms like poor gas mileage or a failedemissions test? Does it surge or idle rough? Is it hard to start?Next, do a thorough inspection for obvious problems such as

Fig. 2-18. When the catalyst is working efficiently, most oxygenis used for oxidation and reduction, so post converter voltagefluctuations are minimal.

Fig. 2-19. As catalyst performance becomes less efficient, lessoxygen is used and voltage fluctuations from the post converterbegin to increase.

to the customer after a short road test. However, the EVAPmonitor won’t run if the engine is warm (above 86°F) or if thefuel level is not between 1⁄4 and 3⁄4 full. If the EVAP system hasother problems and the EVAP monitor doesn’t run during theroad test, the MIL will come on after you return the vehicle tothe customer.

Scan Tool Data: Information from the computer that is displayed on the scan tool, including data stream, DTCs, freeze frame, andsystem monitor readiness status.

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vacuum leaks and damaged electrical connections. Don’t for-get to consider the basics such as low fuel pressure, incorrectignition timing, low or uneven engine compression, and fuelquality. If possible, a review of recent vehicle service mayyield valuable diagnostic clues. For recent vehicle service in-formation, check dealership resources and communicatewith the vehicle owner.

Connect the scan tool and retrieve stored DTCs and freezeframe data. Record your findings, then check the service man-ual to learn the specific conditions that cause the DTC. Takethe time to thoroughly understand what caused the DTC.

Check the readiness status of system monitors. If the readi-ness status is “NO” for all monitors, review recent service his-tory; the battery may have been changed or the vehicle mayhave been in another shop where DTCs were erased. The vehi-cle must be driven through the complete drive cycle to ensureall monitors run. If readiness status is “NO” for only one or twosensors, check sensors, actuators, and related circuitry for prob-lems that would prevent the monitor from running. Again, thevehicle may have to complete an entire drive cycle to providethe time and conditions to run the remaining monitors.

When there is more than one DTC in memory, diagnoseand correct component-related DTCs before diagnosing sys-tem failure DTCs. A sensor or actuator problem may preventa monitor from running or cause a system to fail the monitor.Once a component failure is repaired, drive the vehiclethrough the specified drive cycle to be sure the system is fullyrepaired. For example, when discovering a code P0125, “exces-sive time to enter closed loop,” and a code P0155, “HO2S/1,Bank 2 Heater Malfunction”, the best procedure is to diagnoseand repair the HO2S/1 heater malfunction first, even thoughits DTC is a higher number.

Next, clear codes and drive the vehicle as directed in theservice manual. In this example, it is probable that the failedoxygen sensor heater caused the system to be slow enteringclosed loop. Misfire and fuel control DTCs are considered pri-ority codes and should always be diagnosed first.

When using the drive cycle to confirm repairs, reviewfreeze frame data for the driving conditions present at the timethe DTC was recorded. It is especially important when con-firming misfire and fuel control repairs to closely match theengine rpm, calculated load, and engine temperature valuesrecorded in the freeze frame.

How close is close? Before the PCM will deactivate theMIL for misfire and fuel control codes, engine speed must bewithin 375 rpm of the engine speed when the code was set, andthe calculated load value must be within ±10% of the load pre-sent when the code was set. Be aware that some manufacturersmay direct you to drive a portion of the drive cycle to confirma particular repair. Drive cycles vary between manufacturersand must always be followed exactly. Freeze frame and scantool data must be analyzed with care. Use service manuals tolearn the normal parameters for each sensor and actuator.

Review freeze frame data to identify all sensors and actuatorsthat are out of range. Many times a sensor will be out of rangeand not set a DTC, especially when the out-of-range sensor isresponding to an unusual condition. Try to determine if thesuspect sensor is reporting an unusual vehicle condition orsending a signal that doesn’t match the actual symptoms orother sensor data. When you have gathered all necessary in-formation—vehicle symptoms, driving conditions, DTCs, andsensor/actuator data—use your knowledge and experience topick out the most probable cause of the symptom. Always referto appropriate service manuals for proper test procedureswhen testing sensors, actuators, and related circuits.

InsightThe following section will present some examples of ECMinputs and explain how unusual readings might affect vehi-cle operation. Tips for testing various components are alsoincluded.

Battery VoltageThe ECM uses battery voltage as an input for the computer-controlled charging system. A low voltage signal may cause theECM to increase both idle speed and alternator field current togenerate higher alternator output. When idle speed is abovespecification and fuel system control based on HO2S and fueltrim data appears normal, check battery voltage, generator,and idle air control (IAC) data. If battery voltage is low andgenerator field and IAC percentages are higher than normal,test the battery and charging system for defects. Also, check ac-cessory load sensors for false signals. A power steering pressureswitch that sticks closed, for example, will cause the ECM toraise idle speed.

Brake Pedal Position (BPP) SwitchThe BPP switch is used on the composite vehicle as an input tocontrol the torque converter clutch. On some systems, how-ever, the BPP switch is also part of the ABS (anti lock brake)system. Many of these vehicles use information from the ABSwheel speed sensors as an input for the misfire monitor. Whentraveling over rough roads, tire slip and driveline torque affectthe smooth rotation of the crankshaft, simulating engine mis-fire. At the same time, wheel speed sensors send erratic signalsto the ECM. When the ECM sees the erratic signals, it suspendsthe misfire monitor. If the BPP switch fails to close or open asexpected, the ECM disables ABS braking and ignores wheelspeed data. The misfire monitor is again suspended becausethe ECM has incomplete information.

Idle Air Control (IAC) ValveThe IAC valve regulates idle speed by controlling the amountof air that bypasses the throttle plate. Lower than-normal IACpercentage means the ECM is trying to reduce idle speed; higherpercentage means the ECM is trying to increase idle speed. Forexample, a vacuum leak will cause idle speed to increase. TheECM will command a lower percentage opening from the IAC

Priority Codes: Codes that are more important than, and take precedence over, others.



to compensate. An EGR that doesn’t fully close at idle will re-duce idle speed and quality. The ECM will command a largerIAC opening, in an attempt to maintain specified idle speed.

A quick way to test IAC performance is to view IAC per-centage on the scan tool while increasing engine load at idle. Ifthe IAC percentage increases when the A/C is turned on (or thesteering wheel is turned) and the idle speed remains steady, thesystem is working normally. If the IAC doesn’t respond or idlespeed decreases with increased load, physically check the IACvalve for damage or passages clogged with carbon.

Intake Air Temperature (IAT) SensorThe IAT sensor measures the temperature of air in the intakesystem. IAT data are used as the air density input for air/fuelratio calculations. In the composite vehicle, the IAT sensor hasthe same temperature/voltage signal relationship as the EngineCoolant Temperature (ECT) and Transmission Fluid Temper-ature (TFT) sensors. To confirm IAT sensor accuracy, measureair temperature near the sensor and compare with the tem-perature reading on the scan tool. After turning off the engineand waiting for 10 minutes with the hood down, the measuredtemperature should be within 5°F of the IAT temperature onthe scan tool. Compare IAT with ECT temperature readingsafter turning off the engine and waiting for 15–20 minutes.The two readings should be almost identical. When the vehi-cle is cold, before being started in the morning, IAT signal volt-age should be the same as voltage signals from the ECT andTFT sensors.

Manifold Absolute Pressure (MAP) SensorThe MAP sensor is used on the composite vehicle to monitorEGR operation. It senses changes in manifold pressure as theEGR valve opens and closes. Typical MAP sensor problems likea cracked vacuum hose or a poor electrical connection willlead to EGR trouble codes. It is important to remember thatmanifold pressure can be described two ways, as pressure orvacuum. When the EGR valve opens, the intake manifold fillsmore quickly. Intake manifold vacuum drops toward 0 in. Hg,but manifold absolute pressure rises toward 100 kPa. Pay at-tention to your scan tool displays and read all MAP sensorquestions carefully.

Mass Airflow (MAF) SensorThe MAF sensor measures the volume of air flowing into theintake manifold. The voltage values of the composite vehiclesensor range from 0.2 volt with no flow (0 gm/sec) to 4.8 voltsat maximum air flow (175 gm/sec). The sensor is located inthe air intake system before the throttle plate, usually near theair cleaner. When diagnosing driveability problems, observeMAF and RPM on the scan tool as the engine is accelerated.MAF signal voltage (or gm/sec value) will increase at aboutthe same rate as engine RPM. Don’t forget, the ECM is capa-ble of computing a default MAF value based on engine speedand throttle position signals. The only sure way to check theMAF signal is to verify the signal at the sensor, not on the scantool. If the MAF signal increases more slowly than engine

RPM and there is a low power complaint, suspect a restrictedair filter. If there are lean symptoms, suspect air leaks betweenthe MAF sensor and the throttle plate usually caused bycracked air ducts. When the complaint is hesitation on accel-eration, check that cracks in the air ducting aren’t opening asthe engine torques on the motor mounts.

When faced with a no-start problem, unplug the MAFsensor. If the vehicle starts, check the electrical circuit for ashorted 5-volt reference wire. Some systems will shut downignition and fuel injection if the 5-volt reference is lost. Un-plugging the sensor restores the signal. Also, check for a leansystem. Some vehicles will default to a rich mixture when theMAF signal is lost. The vehicle will start because added fuelcompensates for the lean problem.

No Start DiagnosisTo run, an engine requires four things: air, fuel, compressionand ignition, all at the right time. Perform the following teststo find what the problem is:

• Observe the engine’s cranking speed; if it is too slow checkthe battery and starting system.

• Check fuel pressure and volume• Verify the electrical signal to the injector with a 12V test

light, depending on the OEM’s recommendation• Use a properly gapped spark tester to check for spark• Check compression by performing a cranking vacuum or

compression test• Check the ignition timing• Verify camshaft drive integrity and valve timing

Hard Start DiagnosisA variety of sensor or physical conditions may result in a hardstart condition without setting a diagnostic trouble code(DTC). In order to determine if any of these conditions exist,perform the following actions:

• Inspect for an engine coolant temperature (ECT) sensorthat has shifted in value.

• Inspect the mass air flow (MAF) sensor for proper instal-lation.

• Inspect the camshaft position (CMP) sensor for propermounting and/or a bad connection. An extended crankoccurs if the engine control module (ECM) does not re-ceive a CMP signal.

• Verify proper operation of the manifold absolute pressure(MAP) sensor.

• Inspect the exhaust gas recirculation (EGR) system forproper sealing/connections and operation.

Engine Misfire Diagnosis• Inspect the engine control module (ECM) grounds for

being clean, tight, and in the proper locations.• Inspect the heated oxygen sensors (HO2S). The HO2S

should respond quickly to different throttle positions. Ifthey do not, inspect the HO2S for silicon or other conta-minants from fuel or the use of improper RTV sealant.

Default Value: A value used in place of another value known to be unreliable.

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The sensors may have a white, powdery coating and resultin a high but false signal voltage rich exhaust indication.The ECM will then reduce the amount of fuel delivered tothe engine, causing a severe driveability problem.

• Inspect the air intake ducts for being collapsed, damaged,loose, improperly installed, or leaking, especially betweenthe mass air flow (MAF) sensor and the throttle body.

• Test the exhaust gas recirculation (EGR) system for properoperation.

• Inspect for proper operation of the manifold absolutepressure (MAP) sensor.


• Inspect the MAF sensor and intake air system for properoperation.

Engine Hesitation DiagnosisMomentary lack of response as the accelerator is pushed down.Can occur at any vehicle speed. Usually more pronouncedwhen first trying to make the vehicle move, as from a stop. Maycause the engine to stall if severe enough.

• Inspect the engine control module (ECM) grounds forbeing clean, tight, and in the proper locations.

• Inspect the heated oxygen sensors (HO2S). The HO2Sshould respond quickly to different throttle positions. Ifthey do not, inspect the HO2S for silicon or other conta-minants from fuel or the use of improper RTV sealant.The sensors may have a white, powdery coating and resultin a high but false signal voltage rich exhaust indication.The PCM will then reduce the amount of fuel delivered tothe engine, causing a severe driveability problem.

• Inspect the air intake ducts for being collapsed, damaged,loose, improperly installed, or leaking, especially betweenthe mass air flow (MAF) sensor and the throttle body.




• Inspect the MAF sensor and intake air system for properoperation.

Poor Fuel Economy DiagnosisFuel economy, as measured by an actual road test, is noticeablylower than expected. Also, fuel economy is noticeably lowerthan the economy was on this vehicle at one time, as previouslyshown by an actual road test.


• Discuss driving habits with the owner. • Is the A/C on or the defroster mode on full time?• Are the tires at the correct pressure?• Are the wheels and tires the correct size?• Are there excessively heavy loads being carried? • Is the acceleration rate too much, too often?• Remove the air filter element and inspect for dirt or for

restrictions.

• Inspect the air intake system and crankcase for air leaks.• Inspect the crankcase ventilation valve for proper

operation.• Inspect for an inaccurate speedometer.

Engine Surges DiagnosisEngine power variation under steady throttle or cruise. Feelslike the vehicle speeds up and slows down with no change inthe accelerator pedal position.


• Inspect the heated oxygen sensors (HO2S). The HO2Sshould respond quickly to different throttle positions. If it does not, inspect the HO2S for silicon or other contam-inants from fuel or the use of improper RTV sealant. Thesensors may have a white, powdery coating and result in ahigh but false signal voltage rich exhaust indication. TheECM will then reduce the amount of fuel delivered to theengine, causing a severe driveability problem.

• Inspect the mass air flow (MAF) sensor for any contami-nation on the sensing element.

• Inspect the air intake ducts for being collapsed, damaged,loose, improperly installed, or leaking, especially betweenthe MAF sensor and the throttle body.




Rough, Unstable, or Incorrect and Stalling DiagnosisEngine runs unevenly at idle. If severe, the engine or vehiclemay shake. Engine idle speed may vary in RPM. Either condi-tion may be severe enough to stall the engine.


• Remove and inspect the air filter element for dirt or for re-strictions.

• Inspect the air intake ducts for being collapsed, damagedareas, looseness, improper installation, or leaking, espe-cially between the MAF sensor and the throttle body.

• Inspect the Transaxle Range Switch input with the vehiclein drive and the gear selector in drive or overdrive.

Circuit Testing Using Serial DataUsing serial data to test ECM circuits can be of great value dur-ing driveability diagnosis; however, there are some items toremember. The data that are being read on the scan tool couldactually be a default value that the ECM substitutes to com-pensate for possible circuit failures. Also, serial data transmit-ted by the ECM to the scan tool is an interpretation of what theECM thinks it is seeing. The true readings may be different. Youcan confirm actual signal values by testing the circuit live witha DVOM, breakout box, or lab scope, depending on what youneed to test. False data stream values may be caused by aninternal ECM fault or an ECM ground circuit problem. The



following are examples of using serial data to test and diagno-sis driveability and intermittent problems:

• Thermistors: disconnect or short across thermistor circuitto check the maximum range of the sensor. For example,disconnect the ECT to create an open circuit. Temperaturereading should drop to -40°F (-40°C). Install a jumperwire across the connector to create a short circuit.Temperature should go to a maximum reading, about266°F (130°C)

• Create the opposite circuit problem to see if a DTC sets.For example, a P0117 code in memory tells you an ECTsensor circuit voltage went low, indicating a short. To cre-ate an open circuit, disconnect the ECT sensor and see ifthe ECM sets a P0118 (circuit high). If it does, then thecircuit and ECM are operational and the problem is prob-ably in the sensor

• Intermittent problem testing: Wiggle, tap, heat up, or cooldown a component or circuit to see if the serial data forthat circuit changes

• Testing the effect of one circuit on another by manipulatingthe input signal. Manipulate the signal by disconnectingcircuits or substituting values. Here are some examples:

• IAT, ECT, TP sensor, MAP, MAF, and HO2S signals’effect on injector pulse width.

• ECT, ACT, TP sensor signals’ effect on Idle speed control.• IAT, ECT, TP, MAP, and MAF signals’ effect on igni-

tion timing control.• ECT, TP sensor, and EVP signals’ effect on EGR control• ECT and TP sensor signals’ effect on canister Purge• VSS, TP sensor, ECT, and MAP signals’ effect on

torque convert clutch operation.

Circuit Testing OperationsWhile scan tools are an important part of any diagnosis, onceyou locate a problem you must use either a DVOM or lab scopeto accurately check a circuit. The following section covers cir-cuit testing procedures and guidelines for using the proper testequipment.

VoltageWhen using a DVOM to check voltage in and out of sensors,always check the voltage using the signal ground return at thesensor, rather than using an engine or battery ground, fig-ure 2-20. Sensors are grounded directly through the ECM,rather than being connected directly to a chassis ground. Thisway sensors avoid noise interference. Sensors need a “clean”ground for reliable operation.

An open signal ground return will cause the ECM to see ahigh voltage on the sensor signal line. An example would be aTP sensor that always sends a wide open throttle (high voltage)signal to the ECM.

ResistanceOhm’s law says that even very low resistance in an automotivecomputer circuit will cause sensors and actuators to work im-properly because of low voltage. For example, an on-boardECM ignition feed circuit drawing 365 milliamps with aresistance in the ignition feed wire of 2.5 ohms will cause avoltage supply drop of 1.5 volts. This voltage drop will cause

severe driveability problems. One example of this may be a carthat idles too high because the ECM monitors the battery volt-age. If the supply voltage is low the ECM may raise the idlespeed so the charging system could charge what the ECMthinks is a low battery.

To check resistance, make sure that the circuit to be testedis not under power. Place the leads across the circuit or com-ponent to be tested, figure 2-21. To read ohms, place the

Fig. 2-20. Checking voltage to a throttle position sensor.

Fig. 2-21. When checking resistance, the part must not be underpower or you will probably destroy your meter.

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meter on auto-ranging or start at the higher scales and workdown.

Voltage DropChecking voltage drop is one of the most important tests thata technician can perform on a circuit. A voltage drop test mea-sures the difference in electrical pressure between two pointsin a live circuit. Voltage drops can cause major driveabilitysymptoms in on-board computer systems. A voltage drop ona ECM power ground can cause sensor voltage references to behigher than normal, throwing off the overall sensor calibrationof the entire engine control system, figure 2-22.

Another example of a driveability symptom might be a carwith an idle speed that continuously hunts. To start diagnosis,you connect the scan tool to check trouble codes and the idlesmooths out. This is usually caused by a poor ground.

To check voltage drop, the circuit must be powered up andhave current flowing. The circuit also must have the maximumamount of current flowing under normal conditions by whichthe circuit was designed.

Although there is no exact amount voltage drop that isconsidered acceptable, you should remember that low currentcircuits that draw milliamps will be affected by very small volt-age drops. A good rule of thumb would be a drop of 0.2 volt orless. However, even this is too much for some circuits. A powerground circuit should have a voltage drop of no more than 0.1 volt. A computer ground circuit should have a voltage dropof no more than 0.05 volt.

AmperageToo much amperage flow through a ECM actuator driver cir-cuit can partially damage that circuit and cause severe drive-ability problems. Most ECM actuator components carry milliamps through their circuits. Using a ohmmeter and calcu-lating amperage draw from resistance and voltage readings isnot always accurate because the device under test does not carrythe actual load it was designed to carry. Most actuator devicescarry about 180 ma (12.6 volts at 70 ohms) to 500 ma (12.6volts at 25 ohms), but there are always exceptions to the rule.Fuel injectors may carry much more amperage through theircircuit (as much as 8 amps depending on the type of injector).

To check amperage draw, the circuit must be powered upand have current flowing. Set your meter for amperage drawand connect it in series between the solenoid negative termi-nal and ground, or the actual driver circuit if you can energizeit, figure 2-23. Start by checking amps first, then move downto milliamp scale. Leave the circuit energized for 1 to 2 min-utes to check draw.

Remember, this test is for solenoids such as CanisterPurge, EGR, and Air Management only. Do not check fuel in-jectors in this manner. Holding an injector on for any lengthof time destroys it.

AC RippleOn-board automotive computers do not like to see AC ripplespass through the internal components. This effect can causelogic problems as well as many other types of driveability prob-lems. For example, a bad alternator with a dropped diode canseverely affect an automotive computer system.

To check for AC ripple voltage, switch your DVOM toAC and connect the black lead to a good ground and the redlead to the “BAT,” or power, terminal on the back of the al-ternator (not the battery), figure 2-24. A good alternatorshould measure less than 0.5 volts AC with the engine run-ning and the headlights on. A higher reading indicates dam-aged alternator diodes.

FrequencyFrequency is the number of times a signal repeats itself in onesecond. Frequency is measured in hertz.

A signal that repeats itself 10 times a second is operatingat a frequency of 10 hertz. Many automotive computer systemsread the frequency of a signal instead of the voltage. Ford MAPsensors and AC Delco Mass Airflow Sensors are examples ofsensors that produce this type of signal.Fig. 2-22. Checking voltage drop at the ECM ground connection.

Fig. 2-23. Checking amperage draw through a solenoid drivercircuit.



For example, a Ford EEC-IV MAP sensor has a 5 volt ref-erence voltage applied to it. At a duty cycle of 50 percent (halfof the time on and half of the time off), the DVOM will aver-age the reading so you would see 2.5 volts. However, the num-ber of times the signal switches on and off in one second willchange depending on manifold vacuum. To accurately diag-nose these signals, you must have a meter that can read fre-quency, figure 2-25.

Duty CycleDuty cycle is the percentage of time a digital signal is high verseslow. When measuring duty cycle, one complete cycle is

considered 100 percent. For a 5 volt signal at a 50 percent dutycycle, the voltage would read 2.5 volts.

For automotive applications, when dealing with digitalwaves, and especially with ECM outputs, we are concernedwith the amount of time the signal is low, rather then high.This is because the low time is when the driving transistor ison, completing the circuit to ground.

You can measure duty cycle with a DVOM that has a dutycycle setting. Attach the red lead to the signal wire and theblack lead to a good engine ground, figure 2-26.

IMMOBILIZER ANTI-THEFT SYSTEM DIAGNOSISThe following are possible causes for Immobilizer Anti-TheftSystem failures:

• The ignition key is not registered with the immobilizer unit• Poor communication between the immobilizer antenna

and ignition key caused by low battery voltage or interfer-ence from a metal key chain

• Immobilizer unit failure• ECM failure• Ignition key failure• Incorrect ignition key used• Poor communication between the ECM and immobilizer

unit caused by low battery voltage or noise interference• Open or short in wiring harness• Blown fuse

Fig. 2-24. Checking for voltage ripple from an AC generator.

Fig. 2-25. Checking the frequency of a MAP sensor.

Fig. 2-26. Checking the duty cycle of a canister purge solenoid.

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1. True or false? The type 3 compositevehicle has a four cycle, V6 enginewith four overhead chain-drivencamshafts, 24 valves, distributorlessignition, and a speed density typeclosed loop sequential multiport fuelinjection system.a. Trueb. False

2. Which of the following statements isNOT true? The ECM on the compositevehicle:a. Controls the vehicle’s charging

system.b. Receives power from the battery

and ignition switch and provides aregulated 5 volt supply for most ofthe engine sensors.

c. Controls the shifting of thecomposite vehicle’s four speedautomatic overdrive transmission.

d. Receives input from sensors,calculates ignition and fuelrequirements, and controls engineactuators to provide the desireddriveability, fuel economy, andemissions control.

3. Which of the following sensor signals isNOT used during open loop engineoperation?a. MAF sensorb. O2 sensorc. CKP sensord. TPS

4. True or false? OBD II is a government-mandated system designed to monitorfuel system performance, enginemisfire, and emission systems duringnormal vehicle operation. It includesindustry-wide standardizationintended to improve the diagnosticprocess by allowing all techniciansequal access to on-board computerinformation using a GST.a. Trueb. False

5. On U.S designed vehicles built after1996, where would you find the DLC?a. In plain view under the passenger’s

side dashb. In plain view under the exact

center of the dashc. In plain view under the driver’s side

of the dashd. Location varies, depending on

manufacturer and/or model.

6. Technician A is diagnosing an OBD IIvehicle and is about to clear the DTCsand take the vehicle for a short drive tosee if the DTCs reset. Technician Bsays that a quick trip around the blockmay not set a DTC so it may not bepossible to confirm whether a problemhas actually been corrected.Who is right?a. A onlyb. B onlyc. Both A and Bd. Neither A nor B

7. True or false? Since a scan tool has theability to compare data from manysensors and actuators, its data, usedalone, provide sufficient diagnosticinformation to diagnose all problems.a. Trueb. False

8. Technician A says when viewingmonitor readiness, the vehicle must bedriven under certain specificconditions for some monitors to run.Technician B says the emissionssystem being monitored must beoperational.Who is right?a. A onlyb. B onlyc. Both A and Bd. Neither A nor B

9. The engine misfire monitor uses thesignal of which of the following primarysensors?a. ECTb. IATc. CKPd. MAP

10. Which of the following DTCs areconsidered priority codes and shouldbe diagnosed first?a. Auxiliary emission controlsb. Misfire and fuel controlc. Transmissiond. Vehicle speed control

11. True or false? The MAP sensor is usedon the composite vehicle to monitorEGR operation.a. Trueb. False

12. When using a DVOM to check voltagein and out of a sensor: Technician Asays to always use an engine or batteryground. Technician B says to alwaysuse the ground return at the sensor.Who is right?a. A onlyb. B onlyc. Both A and Bd. Neither A nor B

13. A computer ground circuit shouldhave a voltage drop of no more than:a. 0.1 voltb. 1.0 voltc. 0.5 voltd. 0.05 volt

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ASE L1

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