Main Relay (Computer/Fuel System): Testing and Inspection
Voltage Supply, Checking
The Motronic Engine Control Module (ECM) Power Supply Relay (J271) supplies voltage to the Motronic Engine Control Module (ECM) (J623) and the Engine Control Module (ECM) 2 (J624).
Special tools, testers and auxiliary items required
• Multimeter.
• Wiring diagram.
Test requirements
• The fuse (S133) fuse OK.
• Battery voltage at least 12.5 volts.
• All electrical consumers such as, lights and rear window defroster, switched off.
• Vehicles with automatic transmission, selector lever in position "P" or "N".
• A/C switched off.
• Ground (GND) connections between engine/transmission/chassis OK.
• Ignition switched off.
Test sequence
- Remove the Motronic Engine Control Module (ECM) Power Supply Relay (J271).
Checking voltage
- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 1/30 to Ground (GND).
Specified value: battery voltage.
If the specified value was not obtained:
- Check the wiring connections from the Battery to the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 1/30 for an open or short circuit.
- Check the wiring connections for damage, corrosion, lose or broken terminals.
- If necessary, repair the faulty wiring connection.
If the specified value was obtained:
- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 4/85 to Ground (GND).
- Switch the ignition off.
Specified value: battery voltage.
If the specified value was not obtained:
- Check the wiring connections from the Battery to the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 4/85 for an open or short circuit.
- Check the wiring connection for damage, corrosion, lose or broken terminals.
- If necessary, repair the faulty wiring connection.
If the specified value was obtained:
Checking activation
- Switch the ignition on.
- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 3/86 to Ground (GND).
- Switch the ignition off.
Specified value: 0.5 V Max.
If the specified value was obtained:
- Replace the Motronic Engine Control Module (ECM) Power Supply Relay (J271).
If the specified value was not obtained:
- Check the wiring from the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 3/86 to the Motronic Engine Control Module (ECM) (J623) electrical harness connector T94a terminal 92 for a short to Ground (GND) or an open circuit.
- If necessary, repair the faulty wiring connection.
If no malfunction is found in the wiring and voltage supply was not OK:
- Replace the Motronic Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.
Checking Engine Control Module (ECM) 2 (J624) voltage
- Switch the ignition on.
- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 2/87 to Ground (GND) for voltage.
- Switch the ignition off.
Specified value: battery voltage.
If the specified value was not obtained:
Checking wiring
If the manufacturers test box is being used. Perform the following step.
- Install the test box.
If the manufacturers test box is not being used. Perform the following step.
- Remove the Motronic Engine Control Module (ECM) (J623) or the Engine Control Module (ECM) 2 (J624). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.
- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 2/87 to the Motronic Engine Control Module (ECM) (J623) electrical harness connector T94 terminals or the Engine Control Module (ECM) 2 (J624) electrical harness connector T94a terminals for resistance.
Specified value: 2.0 ohms Max.
If the specification was not obtained:
- Check the wiring for a short circuit to each other, Battery (+), Ground (GND), or an open circuit.
- Check the electrical harness connector for damage, corrosion, lose or broken terminals.
- If necessary, repair the faulty wiring connection.
If no malfunction is found in the wiring and the voltage supply OK:
- Replace the Motronic Engine Control Module (ECM) (J623) or the Engine Control Module (ECM) 2 (J624). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.
Final procedures
Assembly is performed in the reverse order of removal.
After repair work, the following work steps must be performed in the following sequence:
1. Check the DTC memory. Refer to => [ Diagnostic Mode 03 - Read DTC Memory ] Diagnostic Mode 03 - Read DTC Memory.
2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Mode 04 - Erase DTC Memory.
3. If the DTC memory was erased, generate readiness code. Refer to => [ Readiness Code ] Monitors, Trips, Drive Cycles and Readiness Codes.
- End of diagnosis.