Chevrolet Silverado / GMC Sierra. Service manual — part 2487

circuit tests normal, replace the door lock and side window switch.

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 door lock and side window
switch.

5. If all circuits test normal, test or replace the affected bulb.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Outside Rearview Mirror Courtesy Lamp Replacement

Control Module References for driver or passenger door lock and side window switch
replacement, programming, and setup.

FRONT 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.

Circuit/System Description

The front fog lamp switch signal circuit is referenced from battery voltage by the BCM. When the
fog lamp switch is activated, the signal circuit is grounded through the switch and the signal
circuit voltage goes low. The BCM controls the front fog lamps through the fog lamp relay in the
underhood fuse block. Battery voltage is supplied to the relay coil and switch, and the BCM
energizes the relay by grounding the fog lamp relay control circuit.

Reference Information

Schematic Reference

Fog Lights Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

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

1. Ignition ON, observe the scan tool Front Fog Lamp Switch parameter while pressing and

releasing the front fog lamp switch. The reading should change between Active and Inactive.

If not the specified value, refer to the fog lamp switch circuit test below.

2. Ignition ON, command the Fog Lamps ON and OFF with a scan tool. Verify the front fog

lamps turn on and off.

If the front fog lamps are always on or always off, refer to the fog lamp control circuit
test below.

Circuit/System Testing

Fog Lamp Switch Circuit Test

1. Ignition OFF, disconnect the harness connector at the headlamp switch.

2. Test for less than 1 ohm of resistance between the ground circuit terminal 8 and ground.

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

3. Ignition ON, verify the scan tool Front Fog Lamp Switch parameter is Inactive.

If not the specified value, test the signal circuit terminal 10 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 10 and the ground circuit

terminal 8. Verify the scan tool Front Fog Lamp Switch parameter is Active.

If not the specified value, test the signal circuit 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 headlamp switch.

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

Fog Lamp Control Circuit Test

1. Ignition OFF, disconnect the fog lamp relay.

2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal

87 and ground.

If the test lamp illuminates, test the fog lamp control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.

If the test lamp does not illuminate, replace the underhood fuse block.

4. Verify that a test lamp illuminates between the B+ circuit terminal 85 and ground.

If the test lamp does not illuminate, replace the underhood fuse block.

5. Disconnect the harness connector at the affected fog lamp.

6. Test for less than 1 ohm of resistance between the fog lamp ground circuit terminal B and

ground.

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

7. Connect the harness connector at the fog lamp.

8. Connect a 15A fused jumper wire between the B+ circuit terminal 30 and the control circuit

terminal 87. Verify the fog lamps are activated.

If the fog lamps do not activate, test the control circuit for an open/high resistance. If
the circuit tests normal, test or replace the affected fog lamp.

9. Connect a test lamp between the B+ circuit terminal 85 and the relay control circuit

terminal 86.

10. Command the 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 relay 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.

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

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Front Fog Lamp Bulb Replacement (GMC) or Front Fog Lamp Bulb Replacement
(Chevrolet)

Fog Lamp Switch Replacement (with RPO SLT) or Fog Lamp Switch Replacement

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

(without RPO SLT)

Control Module References for BCM replacement, programming, and setup.

HAZARD 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/System Description

The hazard switch signal circuit is referenced from battery voltage by the BCM. When the hazard
switch is activated, the signal circuit is grounded through the switch and the signal circuit voltage
goes low. The BCM then sends an ON-OFF voltage signal through the fuses to the appropriate
turn signal lamps or to all of the turn signal fuses for hazard operation.

Reference Information

Schematic Reference

Exterior Lights Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Interior Lighting Systems Description and Operation

Circuit

Short to

Ground

Open/High

Resistance

Short to

Voltage

Signal

Performance

Hazard Switch Signal

2

1

-

-

Hazard Switch Ground

-

1

-

-

1. Hazard flashers inoperative
2. Hazard flashers always on

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Lighting - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado