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.