GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2004 🢒 SRX 🢒 Engine 🢒 Engine Electrical 🢒 Main Relay Diagnosis

Circuit Description

The main relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the main relay coil and the armature contact at all times. The engine control module (ECM) supplies the ground path to the relay coil control circuit, via an internal integrated circuit, called an output driver module. When the ECM commands the relay ON, the relay coil creates an electromagnetic field. This electromagnetic field overcomes the spring tension and pulls the armature contact into the stationary contact of the relay load circuit. The closing of the relay contacts allows current to flow from the battery to the following fuses and relays.

    • V8 ECM fuse
    • Post/MAF fuse
    • ODD coils fuse
    • EVEN coils fuse
    • CAM fuse
    • S/P fan relay
    • LO fan relay
    • HI fan relay

When the ignition switch is turned to the OFF position, power is interrupted to the output driver module in the ECM, and the main relay electromagnetic field collapses. This action allows the spring tension pulling on the armature to separate from the relay load circuit contact, which interrupts current flow to the fuses and relays.

If the main relay fails to close, the engine will crank, but will not run. The GMLAN and class II data communications will be available, with the use of a CANdi module and scan tool.

Diagnostic Aids
    • The main relay will remain powered up for 15 seconds after the ignition switch is turned OFF.
    • The main relay system diagnosis table assumes that the vehicle battery is fully charged. Refer to Battery Inspection/Test in Engine Electrical.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are DTCs P0686, P0687, P0689 or P0690 set?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3
  1. Turn ON the ignition, with the engine OFF.
  2. Remove the underhood fuse block cover.
  3. Probe the following fuses with a test lamp that is connected to a good ground. Refer to Troubleshooting with a Test Lamp in Wiring Systems.
    4. • V8 ECM fuse
    • Post/MAF fuse
    • ODD coils fuse
    • EVEN coils fuse
    • CAM fuse

Does the test lamp illuminate on at least one test point of each fuse?

Go to Step 6

Go to Step 4

4
  1. Turn OFF the ignition.
  2. Remove the main relay from the underhood junction block with the J 43244 Relay Puller Pliers. Refer to Relay Replacement in Wiring Systems.
  3. Connect a 20-amp fused jumper wire from the battery positive voltage terminal for the main relay armature at the underhood fuse block to the relay load circuit terminal at the underhood fuse block. Refer to Using Fused Jumper Wires in Wiring Systems.
  4. Probe the following fuses with a test lamp that is connected to a good ground:
    5. • V8 ECM fuse
    • Post/MAF fuse
    • ODD coils fuse
    • EVEN coils fuse
    • CAM fuse

Does the test lamp illuminate on at least one test point of each fuse?

Go to Intermittent Conditions

Go to Step 5

5

Replace the underhood fuse block. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 6

--

6
  1. Replace any open fuses.
  2. Turn OFF the ignition for 30 seconds.
  3. Attempt to start the engine.

Does the engine start and run?

Go to Step 7

Go to Intermittent Conditions

7
  1. Clear the DTCs with a scan tool.
  2. Operate the vehicle for 5 minutes.

Does a DTC set during this ignition cycle?

Go to Diagnostic Trouble Code (DTC) List

--