Fuel Pump Relay: Testing and Inspection

Fuel Primer Relay J643, Checking

Special tools, testers and auxiliary items required
- Multimeter.
- wiring diagrams.

Test requirements
- Fuse SB42 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.

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

Fuel Primer Relay J643 socket Measure to
21/31 Engine Ground (GND)

Specified value: 12.5 V

If the specified value was not obtained:
- Check the Fuel Primer Relay J643 socket 1/30 to the Battery 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:
- Using a Multimeter, check the following wiring connection for voltage.

Fuel Primer Relay J643 Measure to
23/87 Engine Ground (GND)

- Crank the engine.

Specified value: 12.5 V

- Switch the ignition off.

If the specified value was not obtained:
- Check the Fuel Primer Relay J643 socket 3/86 to the Power Supply Relay (terminal 15) J329 socket 2/87 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.
- Install the test box.

If the manufacturers test box is not being used. Perform the following step.
- Remove the Motronic Engine Control Module (ECM) J220.
- Using a Multimeter, check the following wiring connection for resistance.

Fuel Primer Relay Motronic Engine Control Module (ECM) J220 electrical
J643 socket harness connector T121 or test box socket
16/MOT 65

Specified value: 1.5 Ohms.

If the specified value was not obtained:
- Check the Fuel Primer Relay J643 socket 16/MOT to the Motronic Engine Control Module (ECM) J220 electrical harness connector T121 terminal 65 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 Fuel Primer Relay J643.
- Perform a road test to verify repair.

If the fault does not return:
Repair complete, Generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes
- End diagnosis.

If the fault does return and no malfunction is detected in the wiring and the voltage supply was OK:
- Replace the Motronic Engine Control Module (ECM) J220.
- 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. Diagnostic Mode 03 - Read DTC Memory
2. If necessary, erase the DTC memory. Diagnostic Mode 04 - Erase DTC Memory
3. If the DTC memory was erased, generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes

End diagnosis.