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Main Relay Circuit: Discovery

  1. Main relay may also be called the Multiport Fuel Injection Load relay. Main relay is an integral component of the multifunction relay unit, which is located in right rear of engine compartment.
  2. Using a backprobe set, measure voltage between ground and multifunction relay unit 8-pin connector terminal No. 6 (Brown/Light Green wire). See WIRING DIAGRAMS . If battery voltage exists, go to step  4. If battery voltage does not exist, check fuse No. 7 (20-amp). If fuse is okay, go to next step. If fuse is not okay, repair short in Brown/Light Green wire and replace fuse.
  3. Measure voltage on battery side of fuse No. 7 (20-amp). See WIRING DIAGRAMS . If battery voltage exists, repair open in Brown/Light Green wire. If battery voltage does not exist, repair power supply circuit from battery. See appropriate wiring diagram in POWER DISTRIBUTION article in ACCESSORIES & EQUIPMENT.
  4. Disconnect negative battery cable. Disconnect ECM Black 36-pin and 18-pin connectors. Check for continuity in Blue/Red wire between ECM Black 18-pin connector terminal No. 17 and multifunction relay unit 6-pin connector terminal No. 3. See Figure. See WIRING DIAGRAMS . If continuity exists, go to next step. If continuity does not exist, repair open or short in Blue/Red wire.
  5. Using a backprobe set, check for continuity between multifunction relay unit 8-pin connector terminals No. 6 (Brown/Light Green wire) and No. 3 (Brown/Orange wire), and between terminals No. 6 and No. 8 (Brown/Orange wire). If continuity does not exist, go to next step. If continuity exists, remove main relay and recheck relay unit for continuity between specified terminals. If continuity does not exist, go to step  8. If continuity exists, repair or replace multifunction relay unit.
  6. Reconnect negative battery cable. Using a backprobe set, connect ECM Black 18-pin connector terminal No. 17 (Blue/Red wire) to ground. See Figure. See WIRING DIAGRAMS . Measure voltage between ground and multifunction relay unit 8-pin connector terminals No. 3 (Brown/Orange wire) and No. 8 (Brown/Orange wire), individually. See WIRING DIAGRAMS .
  7. If battery voltage exists at both terminals, substitute a known-good ECM and retest for original symptom or indication. If symptom or indication goes away, replace original ECM. If battery voltage does not exist, replace main relay and retest. If battery voltage still does not exist, check multifunction relay unit internal circuits. See WIRING DIAGRAMS . Repair or replace multifunction relay unit as necessary.
  8. Remove main relay. Check for continuity between main relay coil terminals. See WIRING DIAGRAMS . If continuity exists, go to next step. If continuity does not exist, replace main relay.
  9. Using a fused jumper wire, connect battery voltage and ground to main relay coil terminals. Check for continuity between main relay switch terminals. See WIRING DIAGRAMS . If continuity exists, main relay is operating properly. If continuity does not exist, replace main relay. Remove battery ground and check for continuity between main relay switch terminals. If continuity exists, replace main relay.
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