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

Voltage Supply, Checking

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirements

• The Motronic Engine Control Module (ECM) (J220) fuses 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 procedure

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

Start diagnosis

- 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 circuit or a 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:

- Switch the ignition on.

- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 3/86 to Ground (GND).

Specified value: battery voltage.

If the specified value was obtained:

Checking wiring

If the manufacturers test box is being used. Perform the following step.

- Install the test box. Refer to repair information.

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

- Remove the Motronic Engine Control Module (ECM) (J220). Refer to repair information.

- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 3/86 to the Motronic Engine Control Module (ECM) (J220) electrical harness connector T94 terminal 92 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 (+), and Ground (GND).

- 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:

Checking activation

- Switch the ignition on.

- Using a Multimeter, check the Motronic Engine Control Module (ECM) Power Supply Relay (J271) socket 4/85 to the Motronic Engine Control Module (ECM) (J220) electrical harness connector T94 terminal 69 for voltage.






- 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): Refer to repair information.

If the specified value was not obtained:

- Check the wiring connection for an open circuit, short circuit to Battery (+) or Ground (GND).

- Check the wiring connection for damage, corrosion, lose or broken terminals.

- 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) (J220). Refer to repair information.

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 ] Readiness Code.