GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 XLR 🢒 Body Systems 🢒 Lighting 🢒 Rear Fog Lamps Malfunction

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

--

--

Rear Fog Lamp Switch Signal

2

2

2

--

Rear Fog Lamp Relay Control

B2540 02

B2540 05

B2540 05

--

Rear Fog Lamp Control

1

1

1

--

Fog Lamp - LR Ground

--

1

1

--

Fog Lamp - RR Ground

--

1

1

--

Fog Lamp Switch Ground

--

2

2

--

Rear Fog Lamp Relay Ground

--

1

1

--

1. Rear Fog Lamps Malfunction

2. Rear Fog Lamp Switch Malfunction

Circuit/System Description

The rear fog lamp switch signal circuit is grounded momentarily by pressing the rear fog lamp switch. The body control module (BCM) energizes the rear fog lamp relay by applying voltage to the rear fog lamp relay control circuit. When the rear fog lamp relay is energized, battery voltage is applied via the REAR FOG/ALDL TOP SW fuse to the rear fog lamp supply voltage circuit which illuminates the rear fog lamps. The BCM sends a serial data message to the instrument panel cluster (IPC) to enable the rear fog lamp indicator.

Reference Information

Schematic Reference

Fog Lights Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

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 X1 harness connector at the turn signal/multifunction switch.
  2. Ignition OFF, verify that a test lamp illuminates between B+ and the ground circuit terminal 3.
    3. ⇒If the test lamp does not illuminate, test the ground circuit for an open/high resistance.
  3. Ignition ON, verify the scan tool Rear Fog Lamp Switch parameter is Inactive.
    4. ⇒If not the specified value, test the signal circuit terminal 2 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 2 and the ground circuit terminal 3. Verify the scan tool Rear Fog Lamp Switch parameter is Active.
    5. ⇒If not the specified value, test the signal circuit terminal 2 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 turn signal/multifunction switch.

Rear Fog Lamps Malfunction

  1. Ignition OFF, disconnect the rear fog lamp relay.
  2. Ignition OFF, test for less than 5 ohms between the ground circuit terminal B1 and ground.
    3. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  3. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal B5 and ground.
    4. ⇒If the test lamp illuminates, test the control circuit for a short to voltage.
  4. Verify that a test lamp illuminates between the B+ circuit terminal B3 and ground.
    5. ⇒If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal B5 for a short to ground. If the circuit tests normal, test or replace the rear fog lamp relay.
  5. Disconnect the harness connector at the Inoperative rear fog lamp.
  6. Ignition OFF, exterior lamps OFF, test for less than 5 ohms between the ground circuit terminal A and ground.
    7. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  7. Connect the harness connector at the inoperative rear fog lamp.
  8. Connect a 10A fused jumper wire between the B+ circuit terminal B3 and the control circuit terminal B5. Verify the rear fog lamps are activated.
    9. ⇒If the rear fog lamps do not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the inoperative rear fog lamp.
  9. Connect a test lamp between the control circuit terminal B2 and ground.
  10. Command the rear fog lamps ON and OFF with the rear fog lamp switch. The test lamp should turn ON and OFF when changing between the commanded states.
    11. ⇒If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.
    ⇒If the test lamp is always OFF, test the control circuit for a short to ground or open/high resistance. If the circuit tests normal, replace the BCM.
  11. If all circuits test normal, test or replace the rear fog lamp relay.

Rear Fog Lamp Indicator Malfunction

Ignition ON, with a scan tool perform the Display Test for the IPC and observe the rear fog lamp indicator.

⇒If the rear fog lamp indicator illuminates during the test, replace the BCM.
⇒If the rear fog lamp indicator does not illuminate during the test or remains illuminated at all times, replace the IPC.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Relay Replacement
    •  Turn Signal Multifunction Switch Replacement
    •  Control Module References for BCM or IPC replacement, setup, and programming