Automotive Wave forms

Oscilloscope's

An oscilloscope can be a priceless price of test equipment used for observation of electrical/electronic wave patterns.

Oscilloscope's

To properly use an oscilloscope you must understand the scale selections.

Must automotive oscilloscope will automatically adjust to the proper scale.

It’s the technician job to interpret the reading and determine if the wave form is normal or defective.

Not all devices operate in the same manner please check manufactures specifications before replacing any sensor.

Oscilloscope's

Oscilloscope are used to measure voltage over a period of time.

An event that happens over a period of time is called frequency and in measured in Hertz per second.

1 Second Scale

TIME

VOLTAGE

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10 1 Second scale

12.2 filter DC voltage

6 – 9 volts unfiltered DC voltage

4 Volts Peak to Peak AC Voltage

Grid lines

Oscilloscope's

Filtered voltage: A smooth supply of voltage.

Unfiltered voltage: Normally a ripple type wave pattern.

Oscilloscope's

An AC wave form change direction from positive to negative.

Peak voltage: is the voltage from the highest positive voltage to the lowest negative voltage reading.

+5

0

-5

Peak to Peak voltage is determine by multiplying:

Grid division x voltage setting = Peak to peak voltage

Oscilloscope's

Oscilloscope can be used to measure an event over a period of time. Frequency: Number of complete cycles per

second measured in hertz (Hz). Hertz (Hz): Unit of measurement for frequency. 1

hertz equal 1 cycle per second.

Oscilloscope's

To check frequency you may need some math.

Number of grid lines covered for 1 cycle divided by the time scale.

Wheel speed sensor

The Anti-lock Braking System (ABS) relies upon information coming in from speed sensors.

The wave form produced is used by the ABS control module to determine if wheel lock-up is occurring.

Two ABS wave forms. Both are good the difference is the meter set-up

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SCALE

5 mS Time Scale

NORAM ABS PATTERN

10 mS Time Scale

Normal ABS Pattern

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10 mS2 Volts

Throttle Position Sensor

A throttle position is normally produces an analog signal.

The throttle position is normally used to tell the PCM the position of the throttle

Analog signal: an electrical signal that varies in strength.

200 mS Time Division

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Normal Throttle Position Wave Form

Most throttle position can be easily seen on the 200 mS scale

Throttle Held open Throttle Snapped

200 mS Time Division

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Normal Throttle Position Wave Form

Most throttle position can be easily seen on the 200 mS scale

Defective TPS

Air Flow Meters

The voltage output from the internal track of an Air Flow Meter (AFM) should be linear to flap movement,

This can be measured on an oscilloscope

MASS AIR FLOW METER

The waveform should show approximately 1.0 volt when the engine is at idle,

Voltage will rise as engine when engine speed is increased.

A peak voltage of around 4.5 is normal due to natural inertia.

Voltage will drop and then level off

Spike due to acceleration

Normal fall-off caused by inertia

Normal float back to match air flow

Throttle Released

Around 1 Volt at idle

Hot Wire Air flow Meters

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0At idle around 1 volt normal

Voltage spike due to quick throttle opening

Cold air cooling of wire

Temperature stabilizes

Throttle Released

250 mS time division

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A defective Mass Air Flow will have low voltage or operate erratic at all throttle angles.

A mass air flow angle should change in relation to throttle angle.

250 mS time division

Wave Forms

Another common type wave form is the square wave.

Most electronic components are controlled by a computer PCM, BCM, ECM, ECU or a host of other names.

When the computer turns on a component it usually completes a ground circuit sometime controlling a relay.

Square Wave

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Normal Square wave.

NOTE: Any wave that is on more than off or off ore than on is called a pulsed wave.

Normal voltage spikes

Pulsed Wave

Pulsed waves are used to control several electrical components such as: Fuel pump Canister solenoids EGR solenoids

Wave length: The time required for on complete cycle of a waveform.

Pulsed Wave

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The difference in wave length is caused by circuit demands

NORMAL PULSE WAVE

Wave length

Pulsed Wave

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Bad pulse wave

Voltage did not drop properly

Voltage did increase properly

Coolant Temperature Sensor(CTS) Most coolant sensors are

Negative Temperature Coefficient (NTC) devices.

The sensor resistance starts out high around 200 Ohms and drops as engine temperature increases.

Bad CTS wave form

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Good CTS waveform

Bad CTS wave forms

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Bad CTS sensor or electrical connection disconnected waveform

Bad CTS erratic voltage pattern

Crankshaft Sensor

This signal should be test when the engine fail to start due to lack ignition.

The crankshaft sensor produces an AC voltage that is low while cranking and increases after the engine starts.

AC voltage output is critical, low output can prevent an engine from starting. Even if the signal pattern

look good.

Crankshaft Sensor

Normal due to missing tooth on reluctor or flywheel.

The missing tooth is used by the PCM to determine crankshaft position

Crankshaft Sensor

Voltage at start-up Voltage at higher cranking RPM

NOTE: Operating parameters vary from manufactures

Crankshaft Sensor

Engine Running: Note the higher voltage

Crankshaft Sensor

A defective crankshaft sensor will show little or no output on the oscilloscope

Proper output voltage is critical.

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Defective Crankshaft sensor

Voltage to LOW

Knock Sensor

A knock sensor is used with some management systems,

The Knock sensor is a small piezo-electrical device. That produces electricity when vibrated.

Knock sensor are used to reduce ignition timing when a spark knock occurs. Usually no more than 10˚ Knock Sensor

Knock Sensor

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Point where knock occurred

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50 mS scale

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NOTE: Knock sensor output should normally be zero

Knock Sensor

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Bad knock sensor pattern

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Oscilloscope's

Its important to remember that all electrical wiring will have some type electrical pattern that can be view on an oscilloscope.

Open Circuit

NOTE: Voltage is “0” on an open circuit

Good ground circuit

Oscilloscope's Quiz

1. Technician A says pattern A is DC voltage. Technician B says Pattern B is DC voltage. Who is correct.

A. Technician A only

B. Technician B only

C. Both A and B

D. Neither A nor B

Pattern A

Pattern B

Oscilloscope's Quiz

2. How many seconds pass from point A to Pont B?A. 1

B. 2

C. 3

4. 4

1 Second Scale

A B

Oscilloscope's Quiz

3. Technician A says this pattern is an AC wave form. Technician B says this is DC unfiltered. Who is correct?

A. Technician A Only B. Technician B only C. Both A and B D. Neither A nor B

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Oscilloscope's Quiz

4. What is peak Negative voltage in this pattern? A. - 1 volt B. - 2 volts C. - 4 volts D. -.25 volts

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Oscilloscope's Quiz

5. What is peak to peak voltage in this pattern? A. 1 volt B. 2 volts C. 4 volts D. .25 volts

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Oscilloscope's Quiz

6. The number of complete cycles per second measured is: A. Amperage B. Frequency C. Ohms D. Voltage

Oscilloscope's Quiz

7. Technician A says voltage is measured from point A - B. Technician B says Time is measure from point C-D. Who is correct?A. Technician A onlyB. Technician B onlyC. Both A and BD. Neither A nor B

A

B C D

Oscilloscope's Quiz

8. What time scale is this oscilloscope set for? A. 2 mS

B. 5 mS

C. 10 mS

D. 100 mS

Oscilloscope's Quiz

9. Technician A says the wave form is of a defective TPS (Throttle position sensor) Technician B says the pattern is of a good CTS sensor. Who is correct? A. Technician A only B. Technician B only C. Both A and B D. Neither A nor B

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50 Second Scale

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Oscilloscope's Quiz

10. Which diagram is of a good ground circuit? A. 1 B. 2 C. 3 D. 4

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