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Chevrolet Silverado / GMC Sierra. 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
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