Copyright: Autodata Ltd. 1998


Index

Engine management component descriptions

Safety precautions

General test procedures


Engine management component descriptions

Engine control module (ECM)

Fuel supply system

Air flow measurement

Idle speed control

Sensors

Engine speed & position sensors

Valves


Engine control module (ECM)

•	Electrical signals from the fuel and ignition system sensors are received and processed by the electronic circuits in the ECM.

 Functions

•	Outputs from the ECM control the following:

•	Injectors - by controlling the length of time each injector valve remains open.

•	Fuel pump relay.

•	Idle speed control system.

•	Evaporative emissions system.

•	Exhaust gas recirculation.

•	Ignition timing and knock sensing.

•	Self-diagnosis of faults, indicated by an LED on the ECM Fig. 1 or a MIL or check engine warning light on the instrument panel Fig. 2.

•	Emergency `limp-home' programme, which substitutes an average value for any failed sensor enabling the vehicle to be driven to a workshop for correction.

Fig 1.

Fig 2.


Fuel supply system

Fuel pressure regulator  Fig 3.

•	Maintains an even supply pressure to the injectors.

•	Single point systems - incorporated in the throttle body.

•	Multi-point systems - fitted downstream of the injectors.

Fig. 3

1. Diaphragm

2. Spring

3. Vacuum connection

4. Pressurised fuel inlet

5. Fuel return

 Operation

•	The regulator body is divided into two chambers by a diaphragm.

•	The upper (spring) chamber is connected, by vacuum hose, to the inlet manifold downstream of the throttle valve.

•	The lower chamber receives pressurised fuel.

•	When the fuel supply pressure exceeds the pre-set regulator pressure the diaphragm valve opens and allows the excess fuel to flow back to the tank.

•	The vacuum exerted on the upper chamber modifies the spring pressure and ensures that the pressure difference between the fuel chamber and the inlet manifold is constant.

 Injector

•	Single point systems - the injector sprays fuel into the throttle body.

•	Multi-point systems - the injectors spray fuel into the inlet manifold.

 Operation

•	Electronically controlled injectors are opened by an integral solenoid - single point Fig. 4, multi-point Fig. 5, which responds to signals from the ECM.

•	The fuel is injected in a fine spray, in order to ensure thorough mixing with the intake air.

•	External resistors may be provided for each injector.

•	Injectors for mechanical systems (KE-Jetronic and derivatives) have a spring loaded valve operated by fuel pressure Fig. 6.

Fig.4

1. Solenoid

2. Fuel inlet

3. Fuel return

4. Needle valve

Fig.5

1. Solenoid

2. Fuel inlet

3. Needle valve

Fig. 6

1. Fuel inlet

2. Filter gauze

3. Needle valve - open

4. Needle valve - closed


Air flow measurement

Volume air flow (VAF) sensor  Fig. 7

•	The VAF sensor is fitted between the air filter and throttle body. It measures the amount of air drawn into the engine by the deflection of a flap inside the sensor.

Fig. 7

1. Sensor flap

2. Damper flap

3. Potentiometer

4. Intake air temperature sensor

Operation

•	The flap is connected to a potentiometer which sends a signal voltage to the ECM.

•	This is the primary signal which, in conjunction with those from the other system sensors, is used to determine the fuelling requirements of the engine.

•	In many applications the intake air temperature sensor is incorporated in this type of air flow sensor

Mass air flow (MAF) sensor - hot-wire type  Fig. 8

•	The MAF sensor has no moving parts and provides very little restriction to the air flow. It is fitted between the air filter and throttle body.

Operation

•	The MAF sensor consists of a venturi with a resistance wire exposed to the incoming air.

•	An air temperature sensor or compensating wire is also exposed to the air flow.

•	The ECM senses changes in temperature caused by variations in air mass passing through the sensor body and increases or decreases the current flowing through the measuring wire.

•	An internal electronic circuit maintains the approximate temperature of the measuring wire at a constant 120°C, above the intake air temperature.

•	As the mass of air passing the wire increases, so the current required to maintain the temperature also increases.

•	This increase in current is sensed by the ECM.

•	This is the primary signal, used in conjunction with signals from the other system sensors, for determining the correct amount of fuel to be supplied to the engine.

•	A `burn-off' facility operates when the system is switched off, to remove dirt from the filament.

Fig. 8

1. Protective mesh

2. Resistance wire

3. Intake air

4. Compensating wire

Mass air flow (MAF) sensor - hot-film type

•	Similar to the hot-wire type, but with the measuring element incorporated in a ceramic plate. This arrangement obviates the need for a `burn-off' facility.

Manifold absolute pressure (MAP) sensor  Fig. 9

•	Connected by vacuum hose to the engine inlet manifold. Usually located in the engine bay, or built into the ECM casing.

Operation

•	The sensor consists of a diaphragm and piezo-electric circuit which varies the resistance in proportion to the pressure in the manifold.

•	The sensor has a 5 volt supply and sends a signal voltage to the ECM which is proportional to the manifold pressure.

•	The ECM uses the voltage signals from the MAP sensor to adjust the signals to the injectors.

Fig.9

A. Full load

B. Idling

1. Vacuum connector

2. Piezo-electric crystal

3. Electrical connections


Idle speed control

•	Idle speed control devices are all fitted into a throttle by-pass system, allowing extra air to enter the engine. This extra air complements the mixture enrichment during a cold start and the subsequent warm-up period.

Idle air control (IAC) valve  Fig. 10

•	The auxiliary air valve allows air to by-pass the throttle valve when the engine is cold, to increase idle speed during the warm-up period.

Fig.10

1. Bi-metal strip

2. Heating element

3. Valve plate

4. By-pass air

Operation

•	The valve is mounted on the engine in a position where it is in direct contact with engine temperature.

•	The valve incorporates an electrically heated bi-metal strip which determines the cut-off point as the engine reaches normal operating temperature.

Idle air control (IAC) valve - solenoid type  Fig. 11

•	The solenoid idle air control actuator allows air to by-pass the throttle valve to govern the idle speed, under all conditions.

Fig.11

1. Solenoid

2. Air valve

3. Air outlet

4. Air inlet

Operation

•	It is controlled directly by the ECM, which interprets signals from sensors such as RPM and coolant temperature to maintain the idle speed at a pre-determined level.

•	The solenoid acts against a spring, either with a rotary or linear action.

Idle air control (IAC) valve - rotary type  Fig. 12

•	The rotary idle air control actuator allows air to by-pass the throttle valve to govern the idle speed, under all conditions.

Fig.12

1. Rotary valve

2. Air inlet

3. Air outlet

4. Motor

Operation

•	The device consists of a rotary valve, in the by-pass air way, mounted on the shaft of a special electric motor.

•	The motor has two sets of armature windings and is limited to an angular movement of 90.

•	The ECM supplies an alternating voltage to the two windings, depending on signals from sensors such as engine RPM and coolant temperature.

•	The alternating voltages exert opposing forces on the armature resulting in the valve taking up a position corresponding to the voltage ratio.

Idle air control (IAC) valve - stepper motor type  Fig. 13

•	The idle air control stepper motor positions an air by-pass valve mounted in the throttle body and is used mainly with single point injection systems.

Operation

•	The stepper motor has four control windings, enabling it to be precisely positioned in either direction.

•	It is controlled by the ECM and changes idle speed irrespective of throttle valve position.

•	As an additional function the stepper motor may open the throttle valve mechanically to increase idle speed.

Fig. 13


Sensors

 Heated oxygen sensor (HO2S)  Fig. 14

•	Sometimes referred to as a Lambda sensor.

•	Fitted in the exhaust system upstream of the catalytic converter, where it can measure the exhaust gases evenly from all cylinders.

•	Where necessary (e.g. on a V-engine with a dual exhaust system) two sensors may be fitted.

Fig.14

1. Protective cap with slots

2. Heating element

3. Sensor element

4. Electrical connections

Operation

•	The sensor reacts to the oxygen content of the exhaust gasses and uses the voltage generated (200-800 mV) to signal the ECM to modify the mixture (air/fuel) ratio.

•	If the oxygen content is at the ideal value (Lambda = 1), the sensor signal voltage will be 500 mV.

•	If the mixture is too weak the voltage will be 200-450 mV (Lambda 1,10).

•	If it is too rich the voltage will be 550-800 mV (Lambda 0,96).

•	In practice the oxygen sensor continually signals the ECM to alter the fuelling by a small amount, to maintain the mixture strength as near as possible (± 1%) to the theoretical ideal (stoichiometric) ratio of 14.7 parts air to 1 part fuel, which is the optimum for the catalytic converter to work efficiently.

•	The sensor will not begin to operate until it has reached 482-572°C, but most now incorporate a heater element, which enables the closed loop system to come into operation soon after a cold start.

Engine coolant temperature (ECT) sensor  Fig. 15

•	The ECT sensor probe is immersed in the

•	engine coolant and changes its resistance in proportion to temperature changes.

Fig.15

1. Electrical connections

2. Housing

3. NTC resistor

Operation

•	In most cases the sensor incorporates a negative temperature coefficient (NTC) resistor, which decreases in resistance as the coolant temperature rises.


Engine speed and position sensors

•	Engine speed (RPM), crankshaft position (CKP) and camshaft position (CMP) sensors are of two basic types, inductive or Hall effect. Inductive sensors may be mounted on the camshaft housing, crankcase or in the distributor, but Hall effect sensors are usually mounted in the distributor.

 Inductive type  Fig. 16

•	These sensors contain a permanent magnet partly enclosed in a soft iron core and partly in a field winding.

•	The tip of the sensor is positioned close to the flywheel ring gear, a separate toothed trigger wheel, or a toothed wheel attached to the crankshaft pulley.

•	In certain applications the sensor(s) are incorporated in the ignition distributor or in a separate unit driven by the camshaft.

Fig. 16

1. Soft iron core

2. Field winding

3. Permanent magnet

4. Flywheel ring gear

5. TDC pin

Operation

•	As the teeth or pins of the trigger wheel pass the sensor the magnetic field changes, inducing a voltage signal which is sent to the ECM.

Hall effect  Fig. 17

•	The Hall effect generator consists of a semi-conductor material, through which a current flows, enclosed by a magnetic field at right angles to the current flow.

Fig. 17

1. Trigger vane

2. Hall generator

3. Distributor shaft

Operation

•	When the ignition is switched on, a small voltage is generated across the semi-conductor (the `Hall' voltage).

•	A soft iron trigger vane attached to the distributor shaft is positioned to pass between the poles of the magnet.

•	This diverts the magnetic field and has the effect of reducing the Hall voltage to practically zero Fig. 18.

•	As the vane passes the sensor, the Hall trigger is switched `on' when it is opposite a gap [1] and [2] and `off' when there is no gap [3] and [4].

Fig. 18

Throttle position (TP) switch  Fig. 19

•	The throttle position switch assembly is mounted on the throttle body and attached to the throttle valve shaft. It incorporates an idle switch and a wide open throttle switch.

Fig.19

1. Full throttle switch

2. Throttle butterfly shaft

3. Idle switch

Operation

•	The idle switch governs fuel shut-off on the overrun and the wide open throttle switch signals the ECM to provide full load enrichment.

Throttle position (TP) sensor  Fig. 20

•	Used as an alternative to a throttle switch assembly, the throttle position sensor is a variable resistance (potentiometer) connected to the throttle valve shaft.

•	In the case of single point injection systems, load sensing may be by means of this sensor, avoiding the need for a MAP sensor.

Fig.20

1. Throttle butterfly shaft

2. Wiper arm

3. Resistance tracks

Operation

 It is supplied with a reference voltage and the output signal voltage is used by the ECM to interpret the exact throttle position.


Valves

Evaporative emission (EVAP) canister purge valve  Fig. 21

•	A reduction in pollution of the atmosphere can be achieved by fitting an evaporative control system.

•	Fuel vapour from the tank is stored in a carbon filter when the engine is stopped.

Fig. 21

1. Vapour inlet

2. Non-return valve

3. Solenoid

4. Vapour outlet

Operation

•	When the engine approaches normal operating temperature, the solenoid valve on the carbon filter opens, allowing the fuel vapour to be drawn out of the carbon filter into the inlet manifold.

Exhaust gas recirculation (EGR) valve  Fig. 22

•	To reduce the oxides of nitrogen emitted by the exhaust, a proportion of the exhaust gas can be returned to the inlet manifold when the engine is on full load.

Operation

•	The exhaust gas recirculation valve is operated mechanically or by vacuum to control the flow of exhaust gas into the inlet manifold.

Fig. 22


Safety precautions

Air bags (Supplementary Restraint System - SRS)

 Many of the models covered by this manual 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 and the car should be taken to a new car dealer for the make of vehicle, for the fault to be corrected before any other work is carried out.

•	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 engine starting and to avoid damaging catalytic converter, disconnect fuel injector valve multi-plug 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 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.


General test procedures

Injector spray pattern and delivery rate

Injector spray pattern

KE-Jetronic/Motronic

Electronic systems

Injector leak rate

KE-Jetronic/Motronic

Electronic systems

Injector delivery rate

KE-Jetronic/Motronic

Electronic systems

Ultrasonic cleaning

Oscilloscope testing

Injector pulse (dwell)

HT Circuit tests


Injector spray pattern and delivery rate

WARNING: Some of the following tests involve a finely atomised spray of fuel, which is extremely flammable, and temporary electrical connections. The greatest care must be taken to avoid sparks when making or breaking connections, in order to avoid fire or explosion.

ALWAYS have a suitable fire extinguisher at the ready and ensure that you are familiar with its operation.

•	Common problems with injector valves are dribbling, poor spray pattern and insufficient atomisation, all of which can cause poor engine performance, high fuel consumption, excessive exhaust emissions, difficult starting and a variety of other engine problems.

•	Fuel flowing through the injectors leaves deposits which gradually restrict the flow.

•	Particular driving patterns, such as a regular journey sufficient to thoroughly heat up the engine (15-20 miles), followed by a period of inactivity, can result in fuel around the injector valve nozzle being baked by the heat-soak conditions after each such journey, resulting in carbon deposits.

•	These deposits can either distort the spray cone or prevent the valve closing completely.

•	In some cases these deposits can be removed by using one of the proprietary solvents, some of which are added to the fuel and some applied directly to the injector valves, however these are rarely completely satisfactory.

•	Whenever injector valves are removed, the O rings used to provide a gas tight seal in the inlet manifold should be carefully examined, and renewed if their condition is at all suspect. Any leaks will admit unmetered air, causing increased idle speed and a lean mixture.


Injector spray pattern - KE-Jetronic/Motronic

•	Remove the injectors from the inlet manifold and position them over a suitable container.

•	Operate the fuel pump.

•	Lift the air flow sensor plate and check the spray pattern for each injector.

•	Each injector should produce an even cone shaped spray of well atomised fuel Fig. 1, although a slightly one-sided spray is acceptable, providing the total spray angle is no more than 35°.

Fig. 1


Injector spray pattern - electronic systems

•	Remove the injectors from the inlet manifold, complete with the fuel rail and position the injector valve nozzles over suitable containers Fig. 2.

•	Ensure that fuel supply pipe, return pipe and pressure regulator are connected.

•	Ensure that injector multi-plugs are disconnected.

•	Secure the injector valves in the fuel rail with binding wire Fig. 3 or a suitable clamp Fig. 4.

•	Operate the fuel pump, usually achieved by removing the fuel pump relay and jumping the supply terminals.

•	If injector valves have a resistance of 1,0-3,0 ohms a resistor of 5,0-8,0 ohms must be connected in series with the supply, but if the injector valve resistance is

•	15-17 ohms a 12 volt supply can be connected directly.

•	Connect a 12 volt supply to each injector in turn.

•	Compare the spray pattern of each of the injector valves. If one is significantly different to the rest, this indicates a fault.

•	The spray pattern will in many cases be cone shaped Fig. 5 , but can be of the narrow jet or divided jet type, depending on the application, however the pattern from all the injectors for a particular engine should be approximately the same.

Fig. 2

Fig. 3

Fig. 4

Fig. 5


Injector leak rate - KE-Jetronic/Motronic

•	Remove the injectors from the inlet manifold, wipe the nozzles dry and position them over a suitable container. Operate the fuel pump.

•	Observe the injector valve nozzles during a two minute period.

•	There should be no leaks.


Injector leak rate - electronic systems

•	Arrange the injector valves over measuring containers - see Injector spray pattern.

•	Disconnect the multi-plug from each injector valve.

•	Operate the fuel pump and observe the injector valve nozzles.

•	No more than one drop of fuel should leak past the nozzles in 60 seconds.


Injector delivery rate - KE-Jetronic/Motronic

•	Remove the injectors from the inlet manifold and place in separate containers. Remove the ducting from the air flow sensor and lift the sensor plate until 0,020 litres (0.04 pts) of fuel has been injected into one of the containers. Compare with the quantity of fuel delivered by the other injectors.

•	The maximum variation should be less than 0,003 litres (0,005-0,00 6pts).


Injector delivery rate - electronic systems

•	Arrange the injector valves over measuring containers - see Injector spray pattern.

•	Ensure that fuel supply pipe, return pipe and pressure regulator are connected.

•	Secure the injector valves in the fuel rail with binding wire or a suitable clamp.

•	Operate the fuel pump, usually achieved by removing the fuel pump relay and bridging the supply terminals.

•	If injector valves have a resistance of 1,0-3,0 ohms a resistor of 5,0-8,0 ohms must be connected in series with the supply, but if the injector valve resistance is 15-17 ohms a 12 volt supply can be connected directly.

•	Connect 12 volt supply to each injector in turn and compare the quantity of fuel delivered.

•	Typical delivery rates are around 0,2-0,25 litres (0.4-0.5 pts)/60 seconds, but can be as high as 0.95 pts (0.45 l)/60 seconds for injector valves fitted to large capacity/high performance engines.

•	If the delivery rate is low carry out the fuel pressure checks described in the relevant chapter, before condemning the injector valves.

•	Note carefully any significant difference between delivery rates.


Ultrasonic cleaning

•	The most satisfactory method of restoring the efficiency of fouled injectors is to have them ultrasonically cleaned.

•	Several machines are available for this operation. These are capable of cleaning the interior of a set of injectors, using ultrasonics, in just a few minutes, followed by simultaneous spray testing of the complete set for comparative purposes.

•	At the same time the filter baskets and pintle caps should be changed.


Oscilloscope testing

All electronically controlled intermittent injection systems operate by varying the opening time of the injections 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 milli-seconds (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.

Typical oscilloscope traces are shown in Fig. 6 and Fig. 7.

Fig. 6

Fig. 7

•	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'.

•	The other variable is the pulse period.

•	The pulse period is the time between a particular injector opening and the next opening of the same injector. Normally the pulse period will be once per crankshaft revolution. 50% of the fuel required for one firing stroke of the engine is injected at each injector valve opening.

•	The relationship between pulse period and pulse duration, for an engine with a speed range from 600-6000 rpm, is diagrammatically shown in Fig. 8 and Fig. 9.

Fig. 8

Fig. 9

•	When the engine is idling at 600 rpm this is equivalent to 10 revs per second, therefore the crankshaft rotates once every tenth of a second, equivalent to 100 milli-seconds  this is the pulse period.

•	At 6000 rpm, equivalent to 100 revs per second, the crankshaft rotates 10 times per 100 milli-seconds and therefore the injectors open ten times during this period.

•	This means that if the injector dwell period reaches 10 ms, the injector valves are open continuously.

•	The dwell meter may also be used to indicate changes in the opening period of the injectors. Injection dwell provides a precise means of checking the operation of the ECM and its associated sensors.

•	The reading indicates the percentage time that the injector remains open and delivers fuel. Some dwell meters and oscilloscopes have a scale calibrated in degrees, the reading may be converted to % dwell using the following formula:

Where

D% = Dwell period %

C = No. of cylinders

D° = Dwell angle in degrees


Test procedure  injector pulse (dwell)

•	Using a thin probe, connect scope test lead to one terminal of injector and second test lead to earth.

•	Crank the engine and check either scope for trace or dwell meter for reading.

•	If reading is obtained start engine and observe trace at idle speed. Open 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.

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

•	Additional information may also be displayed, such as the TDC or reference sensor signal Fig. 7 and a series of smaller pulses, which hold the injector open after the initial negative pulse.

•	A sharp positive voltage spike will be displayed as the injector closes.

Fig. 7

Test values  typical

Injection period - warm engine

Idle speed	2.2-2.4 ms

2000-3000 rpm	1.5-2.2 ms

Full throttle	8.2-8.4 ms

Volume air flow sensor

•	The oscilloscope may also be used to check the air flow sensor voltage.

•	Connect the low voltage test lead of the oscilloscope to the potentiometer terminal (No.7 on Bosch LE-Jetronic) and make the earth connection to the engine. Manually operate the sensor flap and observe the oscilloscope trace, checking for hash or irregular voltage readings as the flap is moved Fig. 10.

•	Some systems are fitted with a stepper motor which operates an idle air control valve. This enables small adjustments to be made to the idle speed in reaction to variations of operating temperature and load.

•	The stepper motor responds to an oscillating voltage signal from the ECM.

•	This voltage can be checked by connecting the low voltage test lead to each of the four stepper motor leads in turn.

•	By turning the ignition on and off an oscillating voltage trace between 0 and 12 volts should be observed Fig. 11.

•	If no signal is obtained, repeat the test at the relevant  terminals of the ECM multi-plug.

Fig. 10

Fig. 11


HT Circuit tests

NOTE: The following checks cover all the HT components of a `conventional' ignition system.

•	Distributorless systems have HT leads for each plug, but no cap or rotor arm.

•	Direct ignition systems dispense with the HT leads as well, but have individual coils for each plug, which may be tested for insulation breakdown.

•	Use a high-voltage capacity tester of approximately 500V to test insulation of HT components.

•	Ohmmeter can be used for testing component insulation, but will not be as effective.

•	Components should be observed for signs of `tracking' during testing.

Checking HT lead resistance  Fig. 1

•	Connect ohmmeter between ends of HT leads.

•	Measure resistance of each HT lead.

•	Check resistance is 25000-30000 ohms, unless otherwise specified.

•	If leads show excessive resistance, or are mechanically damaged in any way, install replacement set.

Fig. 1

Checking distributor cap insulation  Fig. 2

•	Connect a high-voltage capacity tester between each HT terminal in turn and cap casing.

•	Tester should indicate open circuit.

•	If there is any sign of insulation breaking down, install replacement distributor cap.

Fig. 2

Checking rotor arm insulation  Fig. 3

•	Connect a high-voltage capacity tester between rotor arm HT terminal and rotor arm case.

•	Tester should indicate open circuit.

•	Alternatively, remove distributor cap and hold coil HT lead, using insulated pliers, approximately ¼ in. above rotor arm contact.

•	Crank engine with starter.

•	If rotor arm insulation is faulty spark will jump across gap between HT lead and rotor arm and ground on distributor shaft.

•	If there is any sign of insulation breaking down, install replacement rotor arm.

Fig. 3

Checking ignition coil  insulation  Fig. 4 and Fig. 5

 Connect a high-voltage capacity tester between coil tower HT terminal and coil casing.

 Measure resistance of the insulation.

 Tester should indicate open circuit.

 If there is any sign of insulation breaking down, install a replacement ignition coil.

Fig. 4

Fig. 5