Test 3: MOTRONIC Control Unit
- Check power supply to Motronic Control Unit (MCU): Connect BMW Service Test unit and perform L-Jetronic test 1). If values are correct, replace MCU.
- If voltage value between wire 18/35 and 5, or 4 and 5 is insufficient or incorrect, connect volt/ohmmeter between wire 4 on disconnected MCU plug and ground. Start engine. If volt/ohmmeter does not show battery voltage, trace circuit and repair wiring. See Fig 1 .
- Check wire 18 and 35 for breaks or poor connections. Connect volt/ohmmeter between wire 5 on MCU plug and ground. Resistance should be 0. Connect volt/ohmmeter between wire 18 or 35 on MCU plug, and wire 87 on plug of Relay 2. Resistance should be 0. If either test shows resistance, repair wiring.
- If resistance values are correct, check power supply to Relay 2: Disconnect Relay 1 and connect volt/ohmmeter between terminal 86 on relay socket and ground. Turn ignition on. Voltmeter should read battery voltage. If not, trace circuit and repair wiring. See Fig 1 .
- If voltage is correct, check ground of Relay 2: Pull off relay and check resistance between connector 85 and ground. Resistance should be 0. If not, trace circuit and repair wiring. See Fig 1 .
- If resistance is correct, check power supply of Relay 2: Pull off relay and connect volt/ohmmeter between connector 30 of relay socket and ground. Voltmeter should read battery voltage. If not, trace circuit and repair wiring. If voltage reading is correct, replace relay 2.
TROUBLE SHOOTING
| Condition | Possible Cause |
|---|---|
| Cold Engine Will Not Start | Fuel Pressure, Fuel Injector, Cold Start Valve, Motronic Control Unit, Speed Transmitter, Reference Mark Transmitter, Air Flow Sensor, Wire Connections and Plugs, Ignition Coil |
| Cold Engine Starts, But Stalls | Fuel Injector, Motronic Control Unit, Wire Connections and Plugs |
| Cold Engine Hard to Start | Fuel Pressure, Temperature Time Switch, Motronic Control Unit, Cold Start Injector, High Voltage Distributor, Spark Plugs, Temp. Time Switch |
| Warm Engine Will Not Start | Fuel Pressure, Fuel Injector - Motronic Control Unit, Speed Transmitter, Reference Mark Transmitter, Wire Connections and Plugs, Ignition Coil, Secondary Air of Engine |
| Erratic Idle/Warm-Up Phase | Throttle Switch, Coolant Temp. Sensor, Idle Valve, Motronic Control Unit, Idle Control Unit, Throttle Switch, Air Temp. Sensor, Temp. Switch 112° (45°C), Wire Connections and Plugs, High Voltage Distributor, Spark Plugs, Ignition Circuit, Intake System, Coolant |
| Engine Backfiring | Fuel Injector, Motronic Unit, High Voltage Distributor, Spark Plugs, Ignition Circuit, Exhaust System |
| Idle Speed Incorrect | Fuel Injector, Cold Start Injector, Motronic Control Unit, Intake System, Throttle Switch, Temp. Time Switch, Coolant Temp. Switch, High Voltage Distributor |
| Accelerator Hesitation | Fuel Injector, Motronic Control Unit, Wire Connections, High Voltage Distributor, Spark Plugs, Ignition Circuit |
| Accelerator Knock | Motronic Control Unit, Wire Connections, Spark Plugs, Ignition Circuit |
| Coasting Hesitation | Fuel Injector, Motronic Control Unit, Wire Connections and Plugs |
| Engine Misfiring All Conditions | Fuel Injector, Motronic Control Unit, Wire Connections and Plugs, Ignition Circuit, Intake System, Exhaust System, Secondary Air of Engine, Coolant, Insufficient Engine Power, Fuel Injector, Fuel Pressure, Motronic Control Unit, Air Flow Sensor, Throttle Switch, Coolant Temp. Sensor, Air Temp. Sensor, Ignition Circuit, Intake and Exhaust System |
| High Fuel Consumption | Fuel Injector, Cold Start Injector, Motronic Control Unit, Throttle Switch, Temp. Time Switch, Coolant Temp. Sensor, Air Temp. Sensor |
| CO Level Incorrect | Fuel Injector, Cold Start Injector, Oxygen Sensor, Wire Connections and Plugs, Intake System, Secondary Air of Engine |
| HC and NOx Excessive | Fuel Injector, Cold Start Injector, Motronic Control Unit, Oxygen Sensor, Wire Connections and Plugs, Intake System, Secondary Air of Engine |