Copyright: Autodata Ltd. 2000


Index

Safety precautions

Test procedures

Oscilloscope testing

Tools and equipment


Oscilloscope testing

Why use an oscilloscope?

Interpreting wave forms

Component testing


Why use an oscilloscope?

•	Digital multi-meters are entirely satisfactory for checking circuits in a static condition and for instances where any change in reading is a gradual one, but for dynamic checking (with the engine running) and the diagnosis of intermittent faults, the oscilloscope is a very powerful workshop tool.

•	Unlike older analogue oscilloscopes dedicated to HT ignition testing, a modern digital oscilloscope has a variable voltage scale, enabling low voltages (typically 0-5 V or 0-12 V) to be displayed and also an adjustable time scale, enabling any wave form to be displayed in the ideal manner.

•	Most oscilloscopes designed for automotive use can be hand-held and are therefore ideal for use in the workshop. They can also be used inside the car, while the vehicle is driven, to capture dynamic data.

•	Usually it is possible to store wave forms and associated data in an internal memory and then print it out or download it onto a PC, enabling the scope patterns to be studied in detail.

•	The oscilloscope display can show the amplitude, frequency, pulse width, shape and pattern of the signal received, by effectively drawing a graph of voltage (vertically) and time (horizontally).

•	It is easy to connect (normally just two leads) and the speed of sampling can be far in excess of even the best of digital multi-meters.

•	This fast response time enables diagnosis of intermittent problems and also enables the effect of disturbing parts of the system to be observed. When necessary, the response time can be slow enough to display signals such as the throttle position sensor.

•	Once the cause of a problem has been diagnosed and rectified, the repair can be verified by retesting with the oscilloscope.

•	The oscilloscope can also be used to check the overall condition of an engine management system equipped with a catalytic converter, by monitoring the activity of the oxygen sensor.

•	The complex electronic engine management systems fitted to catalyst equipped vehicles are designed to maintain the mixture level between quite close tolerances so that the oxygen sensor is able react to small changes in the exhaust oxygen level and feed this information back to the ECM in the form of a voltage signal. By watching the oxygen sensor signal with an oscilloscope any irregularity in the overall system performance can be detected. If the wave form displayed is satisfactory this is a reliable indication that the whole system is operating correctly.

•	Currently available oscilloscopes are easy to connect and use, enabling a trace to be displayed on the screen without any specialist knowledge or experience. The interpretation of this trace can be greatly assisted by reference to the typical wave forms illustrated in  this chapter.

Wave forms

•	Each oscilloscope wave form has one or more of the following parameters:

•	Amplitude - voltage (V)
The signal voltage at a certain moment in time

•	Frequency - cycles per second (Hz)
The time between points of the signal

•	Pulse width - duty cycle (%)
The period during which the signal is ON - expressed as a percentage (%) of the total

•	Shape - spike, curve, saw-tooth etc.
The overall 'picture' of the signal

  Wave form  parameters

•	Pattern - repeated shapes
The pattern of repetition of the overall shape of the signal

•	The oscilloscope will show all these parameters in one display and by comparing the scope traces from the vehicle under test with those illustrated, a judgement can be made about the condition of each circuit and its components.

•	The scope trace for a faulty circuit or component will usually appear very different to that for a satisfactory one, thus simplifying fault identification.

•	The five parameters listed above can be categorised as follows:

Direct current (DC) voltage signals - amplitude only

Analogue signal voltages from components such as:

Engine coolant temperature (ECT) sensor - 2

Intake air temperature (IAT) sensor - 3

Throttle position (TP) sensor - 4

Heated oxygen sensor (HO2S) - 5

Volume air flow (VAF) sensor - 6

Mass air flow (MAF) sensor - 7

Alternating current (AC) voltage signals - amplitude, frequency and shape

•	AC voltage signals are generated by components such as:

Knock sensor (KS) - 8

Engine speed (RPM) sensor - inductive type - 9

Frequency modulated signals - amplitude, frequency, shape and pulse width

•	Frequency modulated signals are generated by components such as:

Crankshaft position (CKP) sensor - inductive type - 10

Camshaft position (CMP) sensor - inductive type - 11

Vehicle speed (VSS) sensor - inductive type - 12

Hall effect speed and position sensors - 13

Optical speed and position sensors - 14

Mass air flow (MAF) and manifold absolute pressure (MAP) sensors - digital type - 15

Pulse width modulated signals - amplitude, frequency, shape, pulse width

•	Pulse width modulated signals from components such as:

Injectors - 16

Idle air control (IAC) devices - 17

Ignition coil primary circuits - 18

Evaporative emission (EVAP) canister purge  valve - 19

Exhaust gas recirculation (EGR) valves - 20

Serial data - amplitude, frequency, shape, pulse width and pattern

•	Serial data signals will be generated by the engine control module (ECM), if it has a self-diagnosis facility - 21.

•	By noting the pulse width, pattern and frequency, the short pulses can be counted in groups and interpreted as a trouble code, in this case 1223.

•	The amplitude and shape remain constant and the pattern will be repeated until the trouble code has been erased.

Trouble code wave form - 21


Interpreting wave forms

Typical wave forms - 22 & 23

•	Oscilloscope wave form patterns can vary greatly and are dependant on many factors. Therefore prior to making a diagnosis or changing components the following points should be considered when the wave form obtained does not appear to be correct when compared with the 'typical' wave form in the pin data table being used.

Voltage

•	Typical wave forms indicate the approximate position of the wave form in relationship to the 'zero grid', but this may vary 22 [1], dependant on the system being tested and may be positioned anywhere within the approximate 'zero range' 22 [2] & 23 [1].

•	The amplitude or overall height of the pattern (the voltage) 22 [3] & 23 [2], will depend on the circuit's operating voltage.

•	For direct current (DC) circuits this will depend on the voltage being switched, for example, idle speed control device voltage will be constant and will not vary with engine speed.

•	For alternating current (AC) circuits this will depend on the speed of the signal generator, for example, an inductive crankshaft position (CKP) sensor's output voltage will increase with engine speed.

•	Therefore if the oscilloscope pattern is too high (or the upper part is missing) increase the voltage scale to obtain the view required. If the pattern is too low decrease the voltage scale.

•	Some circuits operating solenoid components, for example idle speed control devices, may display voltage spikes 22 [4], when the circuit is switched off. This voltage is generated by the component and can normally be ignored.

•	Some circuits that have a square wave type of typical wave form may display the voltage decaying at the end of the switching period 22 [5]. This is a characteristic of some systems and can normally be ignored, as it does not by itself indicate a fault.

Frequency

•	The overall width of the pattern (the frequency), will depend on the circuit's operating speed.

•	The typical wave forms illustrated show the wave form viewed with the oscilloscope's time scale set to enable detailed observation.

•	In direct current (DC) circuits the time scale will be dependant on the speed at which the circuit is switched 22 [6], for example, the frequency of an idle speed control device will vary with engine load.

•	In alternating current (AC) circuits the time scale will depend on the speed of the signal generator 23 [3], for example, an inductive crankshaft position (CKP) sensor's frequency will increase with engine speed.

•	If the oscilloscope pattern is too compressed, decrease the time scale to obtain the view required. If the pattern is too expanded, increase the time scale.

•	If the pattern is reversed 23 [4], this indicates that the system being tested has its component connected in the opposite polarity to the typical wave form shown and can normally be ignored as it does not by itself indicate a fault.

Digital wave form - 22

Analogue wave form - 23


Component testing

•	Wave forms for a variety of components can be displayed. A few of the more common examples are described below.

•	Most modern oscilloscopes have just two test leads, used with a variety of interchangeable test probes. The red lead is the positive and is normally connected to the ECM pin. The black lead is the negative and is normally connected to a good earth.

•	If the leads are inadvertently connected with the wrong polarity, usually the only consequence is that the wave form will be displayed upside down.

Injectors

•	All electronically controlled intermittent injection systems operate by varying the opening time of the injectors to match the quantity of fuel supplied with the engine operating conditions.

•	The duration of the electrical impulses from the control unit is measured in milliseconds (ms) and typically ranges from 1 to 14. The oscilloscope on most engine testers can be used to display the injector pulse, enabling the duration to be measured.

•	A typical oscilloscope trace is shown in 24.

•	A series of smaller pulses, which hold the injector open after the initial negative pulse and a sharp positive voltage spike may be displayed as the injector closes.

•	It is therefore possible to check that the control unit is operating correctly by observing the changes in injector opening times during various engine operating conditions.

•	Pulse duration during cranking and at idle speed will be higher than at low engine speeds, but will increase as the engine speed rises, up to a maximum at full throttle.

•	This effect will be particularly evident if the throttle is 'blipped'.

Injector wave form - 24

Injector pulse

•	Using a thin probe, connect the oscilloscope test probe to the ECM injector terminal and the second test probe to earth.

•	Crank the engine and check the wave form.

•	Start engine and observe the wave form at idle speed.

•	Open the throttle rapidly to increase engine speed to around 3000 rpm.

•	The measured pulse duration should increase during acceleration and then stabilise at a reading equal to or slightly below the idle speed value.

•	Close the throttle rapidly and the trace should become a straight line with no pulse indicating that injection has been cut-off (for systems fitted with overrun injection cut-off).

•	When the engine is started from cold the quantity of fuel required is increased and therefore the pulse duration or dwell will be greater.

•	During warm-up the injection period should progressively decrease until the engine reaches normal operating temperature.

•	Systems without a cold start injector usually give additional injector pulses during the cold start, which may be seen as long and short pulses on the scope.

Typical injector duration periods

Condition	Duration
Idle speed	1,5-5 ms
2000-3000 rpm	1,1-3,5 ms
Full throttle	8,2-8,4 ms

Inductive sensors - 25

The general procedure is as follows:

•	Select sensor pin from pin data list with wave form reference.

•	Connect oscilloscope probe to ECM pin and second probe to earth.

•	Start engine and observe test conditions.

•	Compare scope trace with wave form reference.

•	Raise the engine speed and watch for the voltage (amplitude) display to increase.

Inductive sensor wave form - 25

Idle air control (IAC) valve

•	Idle air control (IAC) valves come in many different types, each with a different wave form.

•	In each case the duty cycle (or ON time) of the valve should increase when any additional engine load starts to reduce the idle speed.

•	If the duty cycle varies but the idle speed is not maintained under load, this indicates a faulty valve.

•	If the wave form shows a straight line around the zero mark, or the line is constant at the 5 V or 12 V level, this indicates a fault in the IAC valve circuit or ECM signal output.

•	The commonly used 4 pin stepper motor type is described below. Two and three pin IAC valves can be tested in a similar way, but will of course generate very different wave forms.

•	The stepper motor responds to an oscillating signal from the ECM, enabling small adjustments to be made to the engine idle speed, in reaction to variations of operating temperature and load.

•	This voltage signal can be checked by connecting the oscilloscope  test probe to each of the four stepper motor ECM pins in turn.

•	Ensure engine is at normal operating temperature.

•	Start the engine and allow idle speed to stabilise.

•	Increase the load on the engine by switching on the headlights, air conditioning or by turning the steering (power steering only).

•	The idle speed should momentarily drop, but then be stabilised by the action of the IAC valve.

•	Compare scope trace with wave form reference 26.

Idle air control (IAC) valve wave form - 26

Oxygen sensor (O2S)

NOTE: The following voltage figures refer to the almost universally  used Zirconium type O2S, without a 0,5 V control reference. A few recent models are fitted with a Titanium sensor which has an operating range of 0-5 volts and shows a high voltage signal with a weak mixture and a low voltage signal with a rich mixture.

•	Connect oscilloscope test probes between oxygen sensor ECM pin and earth.

•	Ensure engine is at normal operating temperature.

•	Compare scope trace with wave form reference 27.

•	If the trace shows no wave form but instead is a straight line, this usually indicates a weak mixture if the voltage is approximately 0-0,15, or a rich mixture if the voltage is approximately 0,6-1 - refer to Autodata fuel injection/engine management manuals or CD for possible causes of this condition.

•	If the wave form is satisfactory at idle, open the throttle briefly several times in succession.

•	The wave form should show the signal voltage 'cycling' between approximately 0-1 Volt.

•	The increasing voltage corresponds to the engine speed rising and the decreasing voltage corresponds to the engine speed falling.

Oxygen sensor (O2S) wave form  - 27

Knock sensor

•	Connect oscilloscope test probes between knock sensor ECM pin and earth.

•	Ensure engine is at normal operating temperature.

•	Briefly snap throttle open.

•	Wave form should display an AC signal showing a considerable increase in amplitude 28.

•	If this signal is not distinctly displayed, tap the cylinder block lightly in the region of the sensor.

•	If the signal is still not satisfactory this indicates a faulty sensor or associated circuit.

Knock sensor (KS) wave form - 28

Ignition amplifier

•	Connect oscilloscope test probes between ignition amplifier ECM pin and earth.

•	Ensure engine is at normal operating temperature.

•	Start the engine and allow to idle.

•	The signal should show a digital DC voltage pulse.

•	Compare scope trace with wave form reference 29.

•	If the signal is satisfactory the amplitude, frequency and shape of each pulse should be closely matched.

•	Increase the engine speed and check that the signal frequency increases in proportion to the engine rpm.

Ignition amplifier wave form  29

Ignition coil - primary

•	Connect oscilloscope test probes between ignition coil ECM pin and earth.

•	Ensure engine is at normal operating temperature.

•	Start the engine and allow to idle.

•	Compare scope trace with wave form reference 30.

•	Positive voltage spikes should be of even amplitude.

•	Major differences of amplitude could indicate either a high resistance in the secondary circuit or a faulty spark plug or HT lead (where applicable).

Ignition coil wave form - 30


Tools & equipment

General recommendations

Using multi-meters


General recommendations

The majority of electrical tests described in this manual require the use of a digital multi-meter.

Many suitable instruments are available, with a wide range of prices and specifications, as described below.

Certain other tools, such as a breakout box, digital oscilloscope (see separate section), vacuum pump, and memory saver will be necessary, or desirable to make best use of this manual.

Breakout box -1

•	Whenever possible a breakout box should be used for testing pinout signals.

•	A breakout box [1] is a means of connecting test equipment to the ECM pins, without the need to probe into the back of the harness multi-plug. It is connected by an adaptor lead [2], between the ECM [3] and the car's wiring harness [4].

•	The pin connections are laid out in rows in numerical order and have large connecting sockets, enabling test leads to be securely plugged into a circuit, and avoiding the possibility of connecting to the incorrect pin, due to miscounting.

•	The possibility of damaging components or wiring, due to shorting live pins to ground or to other components, is also much reduced.

•	The limiting factor is the range of adaptor leads. Several equipment manufacturers produce breakout boxes, but none are able to supply a complete range of leads to cover all the models included in this manual.

Breakout box -1

ECM harness multi-plug testing - 2

•	If a breakout box and suitable adaptor leads are not available, testing must be carried out at the wire side of the ECM harness multi-plug 2.

•	To access the terminals the protective cover must be removed from the plug. Many different types of plug are used for engine management wiring harnesses and two examples are shown in 3 and 4.

•	Use the appropriate ECM harness multi-plug - wire side diagram and identify the pin(s) to be tested.

•	Use only very fine test probes to access the connections and measure the signal between the pin and earth 2 [1], or between two ECM pins 2 [2], where a second pin number is given in brackets after the first number.

ECM harness multi-plug testing - 2

ECM harness multi-plug testing  3

ECM harness multi-plug testing  4

Multi-meters

•	A high impedance (10 K ohms/volt minimum) multi-meter that includes a  0-20 V voltage scale and a low (0-200) and high (0-20 K) range ohm scale  is recommended for measuring  the voltage   and resistance of the system components.

•	Analogue 5 and digital 6 electrical multi-test meters are available in a wide range of prices and specifications.  In view of the fact that the instrument will be used in the harsh environment of the workshop, the extra expense of a shock-proof digital type will be a good investment.

•	In addition to the usual volts, ohms and milliamp scales, features such as the facility to ready frequency (Hz) and duty cycle (%), will also be useful.

Analogue  5

Digital  6


Using multi-meters

Voltage checking

•	Connect negative test lead to a good earth, or direct to the negative battery terminal.

•	Observe conditions for test, e.g. ignition OFF, ignition ON etc.

•	If the meter does not automatically select the appropriate voltage range, ensure correct meter scale selected, e.g. 0-12 volts.

•	Connect positive test lead to ECM pin being tested.

Continuity checking

•	Ensure that battery supply voltage is not present at test terminals.

•	Select appropriate resistance scale.

•	Connect meter leads together and check zero reading (analogue meters only).

NOTE:	If unable to zero meter, this usually indicates that the internal battery requires replacement.

•	Connect ohmmeter leads to each end of component or wire to be tested.

•	Read off resistance.

NOTE:	If meter indicates zero 7 this indicates continuity between the two test points.  If the meter indicates infinity 8, this indicates an open circuit (broken wire, burnt out component etc.) between the two test points.

Continuity indicated  7

Open circuit indicated - 8

Diode checking

•	To check a diode connect the meter negative lead to the positive side of the diode and the positive lead to the negative side of the diode. The meter should indicate continuity 9.

•	When the meter connections are reversed the meter should indicate an open circuit 10.

•	If continuity is indicated in each direction, the diode is faulty.

Continuity indicated  9

Open circuit indicated - 10

Vacuum pump

A hand operated vacuum pump, with gauge, will be necessary for testing certain components, such as manifold absolute pressure (MAP) sensors.

Several types are available and are usually supplied with a connecting hose and a selection of adaptors 11.

Vacuum pump  11

Memory savers

With the increasing number of electronic devices incorporating a memory, whenever the battery is disconnected inconvenience is caused by the loss of such information as radio security codes and station settings, clock settings and ECM memories.

This situation can be avoided by the use of an alternative power source such as the memory savers marketed by several of the leading test equipment manufacturers.


Safety precautions

Air bags (Supplementary Restraint System - SRS)

Many of the models covered by this database are fitted with air bags as standard equipment. When working on a vehicle fitted with such a system, extreme caution must be taken to avoid accidental firing of the air bag, which could result in personal injury.

Unauthorised repairs to the system could render it inoperative, or cause it to inflate accidentally.

NOTE: All related wiring is encased in a yellow outer covering.

When the engine is started the AIR BAG warning lamp should go out after approximately 5-10 seconds, if not this indicates a fault in the system. The system should be checked and the fault corrected by a competent technician before any other work is undertaken.

•	NEVER attempt to test the system using a multi-meter.

•	NEVER tamper with or disconnect the air bag wiring harness.

•	NEVER make extra connections to any part of the system wiring harness or terminals.

•	ALWAYS ensure that the air bag wiring harness has not been trapped or damaged in any way when working on adjacent components or systems.

Electrical

CAUTION: To prevent the engine starting and to avoid damaging the catalytic converter(s), disconnect the fuel injector valve multi-plug(s) before cranking tests.

•	ALWAYS ensure that the battery is properly connected before attempting to start the engine.

•	DO NOT attempt to start the engine using a source in excess of 12 volts, such as a fast charger (16 volts) or by connecting two batteries in series (24 volts). ALWAYS disconnect the battery before charging it.

•	DO NOT disconnect the battery while the engine is running.

•	DO NOT connect the battery with reverse polarity.

•	DO NOT disconnect or touch the HT leads when the engine is being cranked or when it is running.

•	DO NOT connect or disconnect the electronic control module (ECM), or any other component of the fuel injection system while the ignition is switched ON.

•	DO NOT disconnect ECM Multi-plug within 30 seconds of switching ignition OFF.

•	DO NOT connect or disconnect multi-meters, voltmeters, ammeters or ohmmeters with the ignition switched ON.

•	DO NOT reverse the polarity of the fuel pump.

•	ALWAYS ensure that all electrical connections are in good condition and making good contact, PARTICULARLY the ECM connector.

•	ALWAYS disconnect the ignition coil, ECM, fuel pump relay/fuse before carrying out a compression test.

•	DO NOT flash a wire or circuit to ground to check that continuity exists.

•	Modern ignition systems operate at very high voltages and these high voltages can severely damage transistorised components such as a wrist-watch if electrical contact is made. Wearers of heart pacemaker devices, therefore, should not at any time carry out work involving ignition systems. In addition to the danger from electric shock, further hazards can arise through sudden uncontrolled body movement causing involuntary contact with moving parts of the engine, i.e. fan blades, pulleys and drive belts.

•	ALWAYS ensure that any replacement fuel or ignition system parts are correct for the application in question. Many units share common external features, but differ internally.

Mechanical

CAUTION: To minimise fire risk, fuel system must be depressurised before disconnecting any fuel lines or fuel system components.

•	ALWAYS disconnect the distributor before carrying out a fuel pump pressure or delivery check.

•	AVOID the risk of fire - ALWAYS disconnect the ignition coil supply and ground the coil HT lead, so that NO HT spark can be emitted, before checking the fuel injector valves, or any other component of the fuel injection system likely to result in the presence of fuel in or around the engine bay.

•	AVOID the risk of fire - NEVER work on the fuel injection system when SMOKING or close to a NAKED FLAME.

•	ALWAYS keep a fire extinguisher close at hand when working on the fuel injection system.

•	ALWAYS ensure that test equipment, leads, tools and especially items of clothing, are clear of moving parts and are not liable to fall into the engine bay, due to vibration, when the engine is running.


Test procedures

General notes

•	All pin data in this database refers to a closed circuit with all components connected.

•	Typical values are an approximate guide only and should not be regarded as specific.

•	Typical values should be measured with the engine at normal operating temperature, unless otherwise stated.

•	All test voltages are measured by connecting the positive meter probe to the engine control module (ECM) pin and the negative probe to earth or a second ECM pin.

•	Only ECM pins connected to a circuit are listed in pin data tables. Pins not listed may have a wire connected and even a measurable voltage output, but are not used.

•	If available, connect a breakout box between harness multi-plug and ECM.

•	Refer to Safety Precautions section of this manual before proceeding with any tests.

Test procedure - without breakout box

•	Find the model related chapter from vehicle manufacturer, model name, year of manufacture, engine code and fuel system type.

•	If required locate ECM position within vehicle by referring to ECM locations section.

•	Turn to appropriate pin data table.

•	Ensure ignition switched OFF.

•	Connect fine test probes to multi-meter.

•	Pull out ECM for easy access to multi-plug(s).

•	Access wire side of harness multi-plug(s) by removing cover or dust boot.

•	Ensure ECM multi-plug(s) are securely connected.

•	Identify location of pin(s) to be tested from multi-plug diagram at beginning of appropriate pin data table.

•	Connect test probe between ECM terminal and a good earth or second ECM terminal (shown in brackets), from the wire side of the harness plug.

•	CAUTION: Avoid shorting a live connector terminal to earth with the test probe as this could damage the ECM, wiring and sensor.

•	Note conditions (e.g. Ignition ON, engine cranking, 3000 rpm etc.).

•	Measure signals at selected pins and compare results with data provided.

•	If the test involves a wave form pattern, refer to the Oscilloscope testing section of this manual.

•	If any test reading is incorrect, reference should be made to wiring diagrams and component test procedures as published in our Fuel Injection, Electronic Ignition and Engine Management manuals which are also available on CD.

•	Verify any repairs by rechecking pin data.

Test procedure - with breakout box

•	Find the model related chapter from vehicle manufacturer, model name, year of manufacture, engine code and fuel system type.

•	If required locate ECM position within vehicle by referring to ECM locations section.

•	Turn to appropriate pin data table.

•	Ensure ignition switched OFF.

•	If required, pull out ECM for easy access to multi-plug(s).

•	Disconnect ECM harness multi-plug(s).

•	Connect T-harness between ECM, breakout box and vehicle harness.

•	Fit appropriate overlay card to breakout box (refer to equipment manufacturer's instructions).

•	Connect suitable test probes to multi-meter.

•	Ensure ECM and breakout box multi-plugs are securely connected.

•	Connect test probe between breakout box terminal and a good earth or second breakout box terminal.

•	CAUTION: Avoid shorting a live breakout box terminal to earth with the test probe as this could damage the ECM, wiring and sensor.

•	Note conditions (e.g. Ignition ON, engine cranking, 3000 rpm etc.).

•	Measure signals at selected pins and compare results with data provided.

•	If the test involves a wave form pattern, refer to the Oscilloscope testing section of this manual.

•	If any test reading is incorrect, reference should be made to wiring diagrams and component test procedures as published in our Fuel Injection, Electronic Ignition and Engine Management manuals which are also available on CD.

•	Verify any repairs by rechecking pin data.