Main Relay (Computer/Fuel System): Testing and Inspection

Engine Control Module Power Supply Relay, Checking

Special tools, testers and auxiliary items required

• Multimeter.

• wiring diagrams.

Test requirements

• The Motronic Engine Control Module (ECM) (J623) fuses OK.

• Fuse (SA18) OK.

• The Ground (GND) connections at the engine and transmission OK.

• The battery voltage at least 12.5 V.

• The generator OK.

Test procedure

- Perform a preliminary check to verify the customers complaint. Refer to => [ Preliminary Check ] Preliminary Check.

Start diagnosis

- Using a multimeter,check the following wiring connection for voltage.





Specified value: 12.5 V

If the specified value was not obtained:

- Check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 1/30 to the Battery (A) positive terminal for an open circuit or short to Ground (GND).

- Check the electrical harness connectors for damage, corrosion, loose or broken terminals.

- If necessary, repair the wiring connection.

If no malfunctions are found in the wiring:

- Turn the key on.

- Using a multimeter,check the following wiring connection for voltage.





Specified value: 12.5 V

If the specified value was not obtained:

- Check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 2/87 to Battery (A) positive terminal for an open circuit or short to Ground (GND).

- Check the electrical harness connectors for damage, corrosion, loose or broken terminals.

- If necessary, repair the wiring connection.

If no malfunctions are found in the wiring:

Checking Ground (GND)

If the manufacturers test box is being used, perform the following step.

- Turn key off

- Install the test box.

If the manufacturers test box is not being used, perform the following step.

- Remove the Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.

- Using a multimeter, check the following wiring connection for resistance.





Specified value: 1.5 ohms.

If the specified value was not obtained:

- Check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 4/85 to the Motronic Engine Control Module (ECM) (J623) electrical harness connector terminal 23 for an open circuit or a short to Battery (+).

- Check the electrical harness connector for damage, corrosion, loose or broken terminals.

- If necessary, repair the wiring connection.

If no malfunctions are found in the wiring and the voltage supply was OK:

- Replace the Motronic Engine Control Module (ECM) Power Supply Relay (J271).

- Perform a road test to verify repair.

If the fault does not return:

Repair complete, Generate readiness code. Refer to => [ Readiness Code ] Readiness Code.

- End of diagnosis.

If the fault does return and no malfunction is detected in the wiring and the voltage supply was OK:

- Replace the Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module, Replacing ] Engine Control Module, Replacing.

- Assembly is performed in the reverse of the removal.

Final procedures

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 ] Readiness Code.