D. Components & Wiring
Fault TracingA simple, but very efficient, way to fault trace the LH-Jetronic system is to check all the electric circuits ending at the connector of the Electronic Control Unit (ECU). The tests will thus indicate condition of all components and wires leading to the tested terminal.
If there is a problem, yet all tests prove positive, this indicates a fault in the Electronic Control Unit. There are no procedures included on how to test the ECU. Install a correct Electronic Control Unit to see that the problem disappears.
CAUTION: First test all circuits according to the fault tracing procedures outlined. If the Electronic Control Unit is replaced first, a faulty circuit may destroy the new Electronic Control Unit the same way as the original one.
Note: The fault tracing procedure is intended to be a complete system test procedure and test steps should be performed in the order presented. Refer to the wiring diagram before proceeding with this test procedure.
D1 Access Engine Control Module
- Remove panel below right side of dashboard and panel in front of right side door.
- Check ground connections at control unit
- Make sure that all wires are securely connected.
D2 Disconnect Connector From Control Unit
- Turn off ignition
IMPORTANT Ignition must be off when disconnecting/connecting connector.
- Lift catch upward and fold connector downward.
D3 Making Test Connections
Removing Protective Cover:
- Remove plastic cover from connector.
IMPORTANT
- Do not insert instrument probe into front of connector. Experience has shown that this can introduce faults.
- Check terminals in holes through side of connector. Do not press too hard on terminals only enough to make good electrical contact.
- Terminal number is stamped on side of connector.
D4 Check Ground Connections
- Connect ohmmeter between ground and:
- terminal 5
- terminal 11
- terminal 19
- terminal 25
Resistance should be nearly 0.0 Ohms in all cases.
D5 Check Lambda-sond Shielding Wires
- Check that wires are connected to terminal 5.
D6 Check That Test Points Are Not Grounded
- Connect an ohmmeter between ground and:
- terminal 12
- terminal 22
Resistance should be infinite in both cases.
Note:
Constant idle speed system (CIS) = red/white (1)
Lambda-sond = green/white (2)
D7 Check Engine Speed Signal From Ignition System Control Unit.
- Connect a voltmeter between ground and terminal 1.
- Operate starter motor. Voltmeter should deflect.
D8 Check Voltage Supply
- Turn on ignition.
- Connect a voltmeter between ground and terminal 18. Voltmeter should show battery voltage.
- Check for voltage when operating starter motor.
- Turn off ignition.
D9 Check Fuel Injection Relay
- Connect a voltmeter between ground and control unit terminal 9.
- Connect a wire between ground and control unit terminal 21.
- The main relay should now engage and voltmeter should show battery voltage.
- Do not disconnect ground wire from terminal 21.
D10 Check Pump Relay
- Ground terminal 17. Pump relay should now switch on fuel pumps.
- 1986
- Connect a voltmeter between ground and terminal 18 in 20-pole connector at right side A-post.
1987-
- Connect a voltmeter between ground and terminal 8 in 35-pole connector at right side A-post.
Voltmeter should show battery voltage.
- Disconnect ground wire from terminal 17.
D11 Check Air Mass Meter
- Fold back dust cover from air mass meter connector to uncover wiring.
D12 Connect A Voltmeter
- Connect a voltmeter between ground and air mass meter terminal 5.
- Then connect voltmeter across terminals 5 and 1.
Voltmeter should show battery voltage in both cases. Disconnect wire between ground and control unit terminal 21.
D13 Connect An Ohmmeter
- Connect an ohmmeter between air mass meter terminals 6 & 7.
Resistance should be approx. 3.7 Ohms.
- Connect ohmmeter between terminals 6 & 14.
Resistance should be between 0 and 1000 Ohms depending on position of CO adjustment screw.
D14 Check Injectors
- 1986
- Connect an ohmmeter between control unit terminal 13 & terminal 18 in the 20-pole connector at right side A-post.
1987-
- Remove relay.
- Connect a bridge wire across terminals 2 (BL-Y) and 3 (GN-R) in relay socket.
- Connect an ohmmeter between control unit terminals 13 & 9.
Resistance should be 2.25 Ohms.
If resistance is approx:
2.9 Ohms indicates one defective injector or lead
4.4 Ohms indicates two defective injectors or leads
8.5 Ohms indicates three defective injectors or leads
If injector is defective:
- Disconnect connectors from injectors and check each one separately.
Resistance should be approx. 3 0.
- Install new injectors as appropriate and connect electrical connectors.
Do not disconnect bridge wire at this stage.
D15 Check Ballast Resistor
- Disassemble connector and measure resistance of each resistor in turn.
- Connect an ohmmeter to centre terminal and each one of resistor terminals.
Resistance should be 5.5 - 6.5 0.
D16 Check Idle Valve
- 1986
- Connect an ohmmeter between control unit terminals 10 & 23, and terminal 18 in the 20-pole connector next to right side A-post.
Resistance should be approx. 20 Ohms in both cases.
1987-
- Connect an ohmmeter between control unit terminals 9 & 10, and then between control unit terminals 9 & 23.
Resistance should be approx. 20 0 in both cases.
- Remove bridge wire from relay, connector.
- Install relay (see D14 1987).
D17 Check Preheating Resistor In Lambda-sond
- 1986
NOTE! Terminals at A-post connector must be disconnected or else wrong reading will result. Check resistance at the 2-pole connector under the hood, with the A-post connector connected.
- Connect an ohmmeter between ground and terminal 18 in the 20-pole connector next to right side A-post.
1987-
- Connect an ohmmeter between ground and terminal 8 (740), 10 (760) in the 20-pole connector next to right side A-post.
Resistance of cold Lambda-sond (20°C - 68°F) = approx. 3 Ohms.
Resistance of hot Lambda-sond (350°C - 662°F) = approx. 13 Ohms.
D18 Check Coolant Temperature Sensor
- Connect an ohmmeter between ground and control unit terminal 2.
Resistance varies with temperature according to graph.
Resistance at:
-10°C (14°F) 8260 - 10560 Ohms
+20°C (68°F) 2280 - 2720Ohms
+80°C (176°F) 290- 364Ohms
If incorrect:
- Measure resistance at sensor to find out if it or wiring is defective.
- Also check ground wire at intake manifold.
D19 Check Throttle Switch
- Connect an ohmmeter between ground and control unit terminal 3.
Resistance should be approx 0.
- Depress accelerator pedal slightly.
Resistance should increase to infinity (switch opens).
If incorrect:
- Measure resistance at switch to find out if it or wiring is defective.
- Check ground wire and its connections to intake manifold.
D20 Check Wire From Climate Unit Control Panel.
- Turn ON ignition
- Connect an ohmmeter between ground and control unit terminal 16.
- Select AC.
Resistance should change.
- Turn off ignition.
- Select AC.
Resistance should change.
D21 Assemble Connector
- Reinstall plastic cover on connector
- Connect connector to control unit
IMPORTANT! Always switch off ignition before disconnecting/connecting connector.
D22 Start Engine
If engine does not start:
- Test with a new control unit.
D23 Connect CO-meter
- Connect CO-meter and adapter 5151 to test point in front of catalytic converter.
- Warm-up engine
D24 Check Lambda-sond
- Disconnect Lambda-sond connector.
- Ground the wire leading to control unit.
CO should increase to show that wire and control unit are OK.
- Connect a voltmeter to Lambda-sond.
Meter should deflect to show that Lambda-sond functions.
If CO is as specified, voltage should be approx. 0.5 V.
- Reconnect Lambda-sond connector.
D25 Check Burn-off Function Of Air Mass Meter Filament
NOTE! Engine must be warm. Coolant temperature must exceed 6O°C (14O°F).
- Remove rubber dust protector from air mass meter connector.
- Connector must be connected to air mass meter.
- Connect a voltmeter between terminals 1 & 4.
- Increase engine speed to 2100 rpm (35 r/s) and turn off engine.
- After 4 seconds the voltmeter pointer should swing for about 1 second to indicate that the filament burn-off function is operating.
- Remove voltmeter.
- Reinstall dust cover on connector.
D26 Check/Adjust CO
If no faults have been found and engine is still not running satisfactorily then:
- Test circuit with a new control unit.
- Remove test equipment.
- Reinstall central electrical unit, control unit, panels etc.