C. 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.
Disconnecting Connector From Control Unit
- Turn ignition OFF
Remove panel beneath right side of dashboard, door step molding, protective molding and side panel.
IMPORTANT! Ignition must be off when disconnecting/connecting connector.
- Lift catch upward and fold connector downward.
- Disconnect connector from control unit.
- Remove Plastic Cover From Connector
IMPORTANT!
Do not insert instrument probe into front of connector. Experience has shown that this can cause damage and 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.
Check Ground Connections
- Connect an ohmmeter between ground and terminal "5"
Resistance should be nearly 0 ohms
- Connect an ohmmeter between ground and terminal "11"
Resistance should be nearly 0 ohms
- Connect an ohmmeter between ground and terminal "19"
Resistance should be nearly 0 ohms
- Connect an ohmmeter between ground and terminal "25"
Resistance should be nearly 0 ohms
- Check that resistance is 0 ohms even when the ohmmeter is connected between "11" and "19". Ground wires are connected to intake manifold.
Check Lambda Sond Shielding Wire
- Check that wire is connected to terminal "5".
- Check ground connection between valve cover and firewall.
Check That Test Point Is Not Grounded
- Connect ohmmeter between ground and terminal 12 (BL:W), 222
Resistance should be infinite.
- Connect ohmmeter between ground and terminal 22 (P), 149
Resistance should be infinite.
Check Engine Speed Signal From Ignition System Control Unit
- Connect a voltmeter between ground and terminal 1.
- Operate starter motor,
Voltmeter should give following readings:
Ignition Off = 0 volts
Ignition On = current from battery (12 V)
Starter motor in operation = at least half current from battery.
Check current supply
- Turn ignition ON.
- Connect a voltmeter between ground and terminal 18.
Voltmeter should show battery voltage.
- Check for voltage when operating starter motor.
- Turn ignition OFF.
Check Fuel Injection Relay
- Connect a voltmeter between ground and control unit terminal "9".
- Ground terminal "21".
Relay should make circuit and meter should show battery voltage.
- The ground connection at "21" should remain in place for next test.
Check Air Mass Meter
- Move rubber cover on air mass meter to one side.
- Connect voltmeter between air mass meter terminal "5", and terminal "1".
Voltmeter should show battery voltage.
- Ground terminal "17".
- Pump relay should activate fuel pumps and pumps should start operating.
- Connect voltmeter between ground and terminal 87/2 in main relay.
Meter should show battery voltage.
- Disconnect ground leads from terminal "17" and "21".
Check Injectors
- Disconnect main relay.
- Connect an ohmmeter between terminal 13 on control unit and terminal 87/2 on main relay connector.
Resistance = 4 Ohms.
If reading is approx. 5.3 Ohms One injector or lead defective
If reading is approx. 8 Ohms Two injectors or leads defective
If reading is approx. 16 Ohms Three injectors or leads defective
If injectors are defective
- Disconnect connector from injector and test each injector separately.
Injector resistance should be 16 Ohms.
Check Air Control Valve
- Connect ohmmeter between terminal 87/2 on main relay connector and terminals "23" and "10" on control unit.
Resistance should be approx. 20 Ohms in both cases, at 20°C (68°F).
Check Preheating Resistor In Lambda Sond
- Disconnect power supply wire from main fuel pump.
- Remove fuse 4 (tank pump power supply).
- Connect ohmmeter between ground and terminal 87/2 on main relay connector.
Resistance of cold Lambda Sond (20°C = 68°F) = 3 Ohms.
Resistance of warm Lambda Sond (above 350°C = 662°F) = 13 Ohms.
- Reconnect power supply wire to main fuel pump.
- Reinstall fuse 4 (tank pump power supply).
- Reconnect main relay connector.
Check Air Mass Meter
- Connect ohmmeter between terminals "6" and "7".
Resistance should be approx. 3.7 Ohms.
- Connect ohmmeter between terminals "6" and "14".
Resistance should be between 0 and 1000 Ohms depending on position of CO-screw.
Check Coolant Temperature Sensor
- Connect ohmmeter between ground and terminal "2".
Resistance is temperature.dependent.
Resistance at:
-10°C (14°F) 8260-1O56O Ohms
+20°C (68°F) 2280-272O Ohms
+80°C (176°F) 290-364 Ohms
If incorrect:
- Check resistance at sensor to see if fault is due to sensor or wiring.
- Check ground wire connected to intake manifold.
Check Throttle Switch
- Connect ohmmeter between ground and terminal "3".
Resistance should be 0 Ohms, i.e. switch closed.
- Depress accelerator pedal slightly.
Resistance should increase to infinity, i.e., switch opens.
- Connect ohmmeter between ground and terminal 12.
Resistance should be infinite. i.e. switch open.
- Depress accelerator pedal completely.
Resistance should decrease to 0 Ohms, i.e. switch closed.
If incorrect:
- Measure resistance at switch to see if fault is due to switch or wiring.
- Check ground wire connected to intake manifold.
- Reassemble control unit connector.
- Reconnect to control unit.
IMPORTANT! Ignition must be off when disconnecting/connecting connector.
- Start engine
If engine does not start, test with a new control unit.
Check Lambda Sond
- Connect meter to adapter next to catalytic converter. Use 5151.
WARNING: CO-meter adapter can be extremely hot.
- Warm-up engine
- Disconnect Lambda Sond connector.
- Ground wire leading to control unit.
CO should increase to show that wire and control unit are OK.
- Connect voltmeter to Lambda Sond.
Meter should deflect to show that Lambda Sond functions. If CO is as specified, voltage should be approx. 0.5-1.0 volts.
- Reconnect Lambda Sond connector.
Check Burn-Off Current Of Air Mass Meter
NOTE: Engine must be warm. Coolant temperature above 60°C (140°F).
- Connect voltmeter between terminals "1" and "4" at Air Mass Meter.
- Increase engine speed to above 2100 rpm.
- Turn off engine.
After 4 seconds meter pointer should deflect for 1 second.
- Disconnect voltmeter.
- Refit rubber cover.
- Disconnect CO-meter.
WARNING! CO-meter adapter can be extremely hot.
If no faults are found during fault tracing procedures and engine is still rough:
- Connect a new control unit.
- Reinstall all panels, etc.