Chapter 10 Bennet

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Chapter 10

Chassis Electrical Circuits


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Objectives (1 of 2)

Describe how a light bulb functions.

Explain the operating principles of halogen andhigh-intensity discharge (HID) lamps.

Describe the function of the reflector and lens in aheadlamp assembly.

Aim truck headlights.

Troubleshoot lighting circuit malfunctions.

Describe the operation of typical truck auxiliaryequipment.

Explain how a trailer electrical plug and connectorare connected.


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Objectives (2 of 2)

Outline the operating principles of truck instrument clustercomponents.

Diagnose and repair some typical truck instrument clusterfailures.

Explain the function and operation of warning and shutdownsystems.

Identify the types of circuit protection used in truck electricalsystems including fuses and cycling and non-cycling circuitbreakers.

Describe the procedure and material required to solder a pair ofcopper wires.

Outline the procedure required to quickly check out a truckelectrical system.


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Lighting Systems (1 of 2)


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Lighting Systems (2 of 2)

Light bulb operation

Fluorescent lights

Halogen lamps

Halogen infrared (HIR) headlamps

Xenon headlamps

HID optics

Managed headlamp voltage


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Light-emitting Diodes (LEDS)


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Caution

When replacing

halogen bulbs, take

care not to contact the

bulb with fingers

because this can cause

a rapid failure. When

installing a bulb, handle

it by the base only.


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Shop Talk

Some manufacturers recommend coating the

prongs and base of the new sealed beam

with terminal grease for corrosion protection.

Use an electrical terminal protectiveapproved by the manufacturer.


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Night Vision Systems

Although a typical truck operates only 30 percent of itsoperational life at night, more than half of truck accidents occurwhen it is dark.

This has led to the emergence of infrared thermal imagingsystems such as the Bendix Xvision system.

XVision consists of a front-mounted infrared camera thatsenses heat from the environment and processes the signalelectronically to produce a virtual display as an output on thevehicle dash.

The driver display consists of a black-and-white image. Warmer

objects such as people, animals, and headlights appear inshades of white, while cooler objects such as guardrails, trees,and abutments appear in shades of gray and black.


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Shop Talk

Lights should be turned off when cranking the

engine to avoid transient voltage spikes.

As the engine is cranked, 650 to 1,200 amps

are drawn through the system.

At the precise moment when the starter is

disengaged, electricity may surge into any

closed electrical circuits. This random surge can shorten the life of the

lights, causing them to burn out prematurely.


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Headlight Dimmer Switch

The headlight dimmer switch, or courtesy

switch, can either be mounted on the floor or

it can be a part of the turn signal assembly.


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Stalk Switch

Trucks today may use either electromechanicalswitches or smart switches.

The procedures you use for diagnosing andrepairing switches differ vastly so make sure you

know what you are working on before attemptingany repairs.

The stalk switch or turn light switch may be eithersmart or electromechanical.

A smart stalk switch is designed to send nothingmore than signals to a module.

The module then converts the signals into outputs.


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Turn Signal Switch Diagnosis

Before replacing the turn signal switch, ensure thatthe trouble is in the switch and not elsewhere in thecircuit.

Check that the circuit breaker and fuse are

functional, and inspect the signal light bulbs forbroken filaments.

Also, check the flasher relay, and replace it ifnecessary.

If the turn signal switch must be replaced, makesure the key is off and the battery negative cable isdisconnected.


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Auxiliary Electrical Equipment

Trailer circuit connector

Instrument panels and gauges

Dash design

Dash components


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Thermistor Resistance Check

To check thermistor-type

temperature sensors, the

approximate temperature of

the component being

checked must be known todetermine the resistance

that should be read at the

cluster harness connector.

Consult a chart such as this:

See Table 10-3 on

page 263 of the

textbook.


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Caution

Static electricity can cause permanent

damage to the cluster.

Before working on the cluster, be sure to

remove all static electricity from your body bytouching grounded metal.

Do not touch pin connectors during removal

and installation of gauges.


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Fuel Gauge

The fuel level sensor used isa float/rheostat type.

When a sending unit sensoris shorted to ground, thegauge should read empty.

Locate the fuel sensorgauge input in the clusterharness.

If resistance for the fueltank level matches OEM

spec and the gaugereading does notcorrespond, replace thefuel gauge.

See Table 10-4 on

page 264 of the

textbook.

Sh T lk


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Shop Talk

Before checking the fuel gauge, be sure the

cab interior is warmed up (during cold

weather), and that the vehicle has been

sitting still long enough to allow the fuel tosettle.


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Circuit-protection Devices

Fuses

Circuit breakers

Cycling circuit breakers

Non-cycling circuit breakers


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Relays

Terminal Assignments

Terminal Designation Old Number New Number

Coil circuit

coil circuit (Control Pos) 86 1 coil circuit (Control Neg) 85 2

Switched circuit

common (supply current or B+ ) 30 3

normally closed (NC) 87a 4

normally open (NO) 87 5

Standard SAE


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Standard SAE

Relay Terminal Assignments


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R id Ch ki f


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Rapid Checking of

a Truck Electrical Circuit General voltmeter test

Battery voltage

Cranking voltage

Charging voltage

Voltage-drop test

These tests are used to determine the generalcondition of a vehicle electrical system.

The idea is to produce a report card on the battery,the cranking circuit, and the charging circuit.

In fact, the test is so fast and easy to perform, itshould become part of routine service procedure.


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Cranking Voltage

Ensure the engine is not capable of starting by no-fueling the engine.

Connect the voltmeter leads across the crankingmotor terminals.

Crank the engine for 15 continuous seconds. Thevoltmeter reading should read above 9.6V for thefull 15 seconds.

If the reading falls below 9.6V, a problem with oneof the following is indicated:

Defective or corroded battery cables or terminals Defective or discharged batteries

Defective cranking motor, solenoid, or relay


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Charging Voltage

Start the engine and run at 75 percent of

rated speed with no load.

Now turn on all the electrical accessories on

the vehicle. Use the voltmeter to test battery voltage. It

should read between 13.5V and 14.5V.


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Summary (2 of 6)

Truck chassis lighting systems use

incandescent, gaseous-discharge, or LED

operating principles.

Halogen headlamps use a tungsten filamentwithin a gas envelope.


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Summary (3 of 6)

Xenon or HID headlamps are becoming

popular due to longer service life and a

brighter, whiter light characteristic.

LED light units are increasingly being usedon truck chassis marker and taillight

assemblies because they last longer and

illuminate faster.

Faster illumination makes them ideal as brake

lights.


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Summary (4 of 6)

The wiring output junction block for the trailer

is usually located inside the tractor cab, often

directly behind the driver seat.

Most current trucks use computer-managedinstrument clusters that are networked with

the chassis data bus.

For this reason, the OEM service literatureshould be consulted before attempting

troubleshooting.

S


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Summary (5 of 6)

Relays are a means of using a low-currentcontrol circuit to switch a high-current circuit.Standard SAE relays use standard numericcoding to indicate the terminal assignments.

Fuses and cycling and non-cycling circuitbreakers are all circuit-protection devices.

Fuses fail when overloaded. Circuit breakers

trip when overloaded, opening the circuit; theyreset either automatically or when the circuit isswitched open.

S


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Summary (6 of 6)

SAE Type 1 circuit breakers are cycling andautomatically reset.

SAE Type 2 circuit breakers are non-cycling and

the circuit must be switched open to allow them to

cool after tripping.

Voltage-drop testing is a key to diagnosing truck

chassis electrical systems.

Note the procedures for performing a fast electricalsystem assessment on a truck in this chapter, and

study voltage-drop testing with a DMM in the next.

Chapter 10 Bennet

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