Engine Component Power Supply Relay (J757), checking

Engine Component Power Supply Relay (J757), checking

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirements

• Fuse (SA4) OK.

• Fuse (SB5) OK.

• Fuse (SB7) OK.

• Motronic Engine Control Module (ECM) Power Supply Relay (J271) 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.

Start diagnosis

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






Specified value: 12.5 V

If the specified value was not obtained:

- Using a Multimeter, check the Engine Component Power Supply Relay (J757) socket 1/30 to the Generator (GEN) (C) B (+) 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 activation

- Switch the ignition on.

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






- Switch the ignition off.

Specified value: 12.5. battery voltage.

If the specified value was not obtained:

- Using a Multimeter, check the Engine Component Power Supply Relay (J757) socket 3/86 to the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 2/87 for an open circuit or short to Ground (GND).

- 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:

Checking Ground (GND)

- Using a Multimeter, check the Engine Component Power Supply Relay (J757) socket 5/85 to the Engine Control Module (ECM) (J623) electrical harness connector terminal T94/32 for resistance.

Specified value: 1.5 ohms.

If the specified value was not obtained:

- Check the Engine Component Power Supply Relay (J757) socket 5/85 to the Engine Control Module (ECM) (J623) electrical harness connector terminal T9432 for an open circuit or a short to Battery (+) or to Ground (GND).

- 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 Engine Component Power Supply Relay (J757).

- Perform a road test to verify repair.

If the fault does not return:

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

- End 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 (ECM) (J623), replacing ] Engine Control Module (ECM) (J623).

- Assembly is performed in the reverse of the removal.

Final procedures

After the 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 ] Engine.

End diagnosis.