For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Rear Fog Lamp Relay B+ | 1 | 1 | 1 | --
Rear Fog Lamp Relay Control | 1 | 1 | 1 | --
Rear Fog Lamp Voltage Supply | 1 | 1 | 1 | --
Rear Fog Lamp Switch Signal | 2 | 2 | 2 | --
Rear Fog Lamp Ground | -- | 1 | 1 | --
Rear Fog Lamp Switch Ground | -- | 2 | 2 | --
1. Rear Fog Lamp Malfunction 2. Rear Fog Lamp Switch Malfunction

### Circuit/System Description

When the rear fog lamp switch is placed in the ON position, ground is applied through the rear fog lamp switch signal circuit at G300 to the body control module (BCM). The BCM applies ground through the rear fog lamp relay control circuit to the coil side of the fog lamp relay. Battery voltage is applied at all times from the REAR FOG LAMP fuse to the coil and switch sides of the fog lamp relay. When the relay coil is energized, the relay switch contacts close and allow battery voltage to be applied from the REAR FOG LAMP fuse, through the relay switch contacts, to the rear fog lamp. Ground for the rear fog lamp is applied at all times at G402. For more detailed information concerning fog lamp operation refer to Exterior Lighting Systems Description and Operation.

### Reference Information

#### Schematic Reference

Fog Lights Schematics

#### Connector End View Reference

Lighting Systems Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Output Controls

• | Scan Tool Data List

• | Scan Tool Data Definitions

### Circuit/System Verification

Ignition ON, observe the scan tool rear fog lamp switch parameter while pressing and releasing the rear fog lamp switch. The reading should toggle between ACTIVE and INACTIVE.

⇒ | If the reading does not change between the commanded states, refer to Rear Fog Lamp Switch Malfunction.

### Circuit/System Testing

#### Rear Fog Lamp Switch Malfunction

1. Ignition OFF, disconnect the harness connector at the rear fog lamp switch.
2. Test for less than 1 ohm of resistance between the ground circuit terminal A and ground.

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool rear fog lamp switch parameter is INACTIVE.

⇒ | If not the specified value, test the signal circuit terminal C for a short to ground. If the circuit tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the signal circuit terminal C and the ground circuit terminal A. Verify the scan tool rear fog lamp switch parameter is ACTIVE.

⇒ | If not the specified value, test the signal circuit terminal C for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

5. If all circuits test normal, test or replace the rear fog lamp switch.

#### Rear Fog Lamp Malfunction

1. Ignition OFF, disconnect the rear fog lamp relay.
2. Verify that a test lamp illuminates between the relay switch B+ circuit terminal 87 and ground.

⇒ | If the test lamp does not illuminate, test the relay switch B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the relay coil B+ circuit fuse is open, test the relay controlled output circuit terminal 30 for a short to ground.

3. Verify that a test lamp illuminates between the relay coil B+ circuit terminal 86 and ground.

⇒ | If the test lamp does not illuminate, test the relay coil B+ circuit for a short to ground or an open/high resistance.

4. Connect a test lamp between B+ and the rear fog lamp relay control circuit terminal 85.
5. Command the rear fog lamps ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒ | If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the BCM.

⇒ | If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

6. Disconnect the harness connector at the rear fog lamp.
7. Ignition OFF, test for less than 1 ohm of resistance between the rear fog lamp ground circuit terminal 2 and ground.

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

8. Connect the harness connector at the rear fog lamp.
9. Connect a 10A fused jumper wire between the relay switch B+ circuit terminal 87 and the relay controlled circuit terminal 30. Verify the rear fog lamp is activated.

⇒ | If the rear fog lamp does not activate, test the relay controlled circuit for an open/high resistance. If the circuit tests normal, test or replace the rear fog lamp.

10. If all circuits test normal, test or replace the rear fog lamp relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Rear Fog Lamp Switch Replacement

• | Relay Replacement

• | Control Module References for BCM replacement, setup, and programming