Main Relay (Computer/Fuel System): Testing and Inspection
Engine Control Module Power Supply Relay, Checking
Special tools, testers and auxiliary items required
• Multimeter.
• Wiring diagram.
Test requirements
• The Engine Control Module (ECM) (J623) 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 connections between engine/transmission/chassis OK.
• Ignition switched Off.
Test procedure
- Observe safety precautions. Refer to => [ Safety Precautions ] Generic Scan Tool
- Observe Clean Working Conditions. Refer to => [ Clean Working Conditions ] Clean Working Conditions
- Perform a preliminary check to verify the customers complaint. Refer to => [ Preliminary Check ] Preliminary Check.
Test sequence
- Terminal 5 (relay pin 87) provides a 12 V output to fuse box SB for fuses 2,3,4,5,6,7 and 8. With the ignition switch On, check these fuses with a 12 V test lamp connected to ground. The lamp should be on for each fuse.
If the fuses are not powered:
Checking voltage
- Remove the Engine Control Module (ECM) Power Supply Relay (J271).
- Turn the ignition switch On.
- Using a multimeter, check the Engine Control Module (ECM) Power Supply Relay (J271) (terminal 3) socket 30 to ground.
- Switch the ignition Off.
Specified value: Battery voltage.
If the specified value was Not obtained:
- Check the wiring connections from fuse 3 in Fuse Panel F to the Engine Control Module (ECM) Power Supply Relay (J271) (terminal 3) socket 30 for high resistance, an open or short circuit.
- Check the wiring connections for damage, corrosion, loose 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 Engine Control Module (ECM) Power Supply Relay (J271) (terminal 1) socket 86 to ground.
- Switch the ignition Off.
Specified value: Battery voltage.
If the specification was Not obtained:
- Check the wiring connections from fuse 14 in Fuse Panel B to the Engine Control Module (ECM) Power Supply Relay (J271) (terminal 1) socket 86 for high resistance, an open or short circuit.
- Check the wiring for an open, high resistance or short circuit to voltage or ground.
- Check the electrical harness connector for damage, corrosion, loose or broken terminals.
- If necessary, repair the faulty wiring connection.
If the specified value was obtained:
Checking wiring
- Remove the Engine Control Module (ECM) (J623). Refer to the Repair Information.
- Using a multimeter, check the Engine Control Module (ECM) Power Supply Relay (J271) to the Engine Control Module (ECM) (J623) electrical harness connector T94 for resistance.
Specified value: 1.5 ohms Max.
If the specification was Not obtained:
- Check the wiring for an open, high resistance or short circuit to voltage or ground.
- Check the electrical harness connector for damage, corrosion, loose or broken terminals.
- If necessary, repair the faulty wiring connection.
If the specified value was obtained:
- Replace the Engine Control Module (ECM) Power Supply Relay (J271).
If no malfunction is detected in the wiring, the relay and the voltage supply was OK:
- Replace the Engine Control Module (ECM) (J623). Refer to the Repair Information.
- Assembly is performed in the reverse of the removal.
Final procedures
After repair work, the following work steps must be performed in the mentioned sequence:
1. Check the DTC memory. Refer to => [ Diagnostic Mode 03 - Read DTC Memory ] Diagnostic Modes 01 - 09.
2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Modes 01 - 09.
3. If the DTC memory was erased, generate readiness code. Refer to => [ Readiness Code ] Monitors, Trips, Drive Cycles and Readiness Codes.