Chevrolet Silverado / GMC Sierra. Service manual — part 2486

The daytime running lamps (DRL) are controlled by the body control module (BCM). The BCM
grounds the DRL relay control circuit in order to energize the DRL relay coil. The DRL relay
then sends voltage to the low beam headlamps.

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

1. Ignition ON, observe the scan tool Ambient Light Sensor parameter voltage while carefully

supplying sufficient light to simulate daylight conditions to the ambient light sensor, then
covering to simulate darkness. The voltage reading should change from approximately 0.69
to 4.52 volts.

If the reading does not change between light and dark, refer to the Ambient Light
Sensor Circuit Test below.

2. Connect a test lamp between the relay side of the DRL fuse and ground.

3. Verify that the test lamp turns on and off while commanding the daytime running lamps ON

and OFF with a scan tool.

2008 Chevrolet Silverado 1500

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

If the test lamp does not turn on and off while changing between the commanded
states, refer to the DRL Relay Control Circuit Test below.

4. If 1 or both of the daytime running lamps do not turn on and off, refer to the Daytime

Running Lamps Circuit Test below.

Circuit/System Testing

Ambient Light Sensor Circuit Test

1. Ignition OFF, disconnect the harness connector at the ambient light sensor.

2. Ignition OFF, test for less than 1 ohm of resistance between the listed ground circuit

terminal and ground.

Ambient light sensor terminal B

Ambient light/sunload sensor terminal C

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

3. Ignition ON, verify the scan tool ambient light sensor parameter is greater than 4.5 volts.

If less than the specified range, test the signal circuit for a short to ground. If the circuit
tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the listed signal circuit terminal and the low

reference circuit terminal. Verify the scan tool light sensor parameter is less than 1.0 volt.

Ambient light sensor terminal A to B

Ambient light/sunload sensor terminal D to C

If greater than the specified range, 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 ambient light sensor.

DRL Relay Control Circuit Test

1. Ignition OFF, disconnect the harness connector X4 at the BCM.

2. Ignition ON, verify the daytime running lamps are not on.

If the daytime running lamps are on, test the DRL relay control circuit for a short to
ground. If the circuit tests normal replace the underhood fuse block.

3. Install a 10A fused jumper wire between the relay control circuit terminal 23 and ground.

Verify the daytime running lamps are on. are on.

If the daytime running lamps are not on, test the DRL relay control circuit for an
open/high resistance. If the circuit tests normal replace the underhood fuse block.

4. If all circuits test normal, replace the BCM.

2008 Chevrolet Silverado 1500

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

Daytime Running Lamps Circuit Test

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

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

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

3. Connect a test lamp between the control circuit terminal A and the ground circuit terminal

B.

4. Command the daytime running 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 voltage. If the circuit
tests normal, replace the underhood fuse block.

If the test lamp is always off, test the control circuit for a short to ground or an
open/high resistance. If the circuit tests normal, replace the underhood fuse block.

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

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Ambient Light Sensor Replacement

Headlamp Bulb Replacement (GMC) or Headlamp Bulb Replacement (Chevrolet)

Underhood Electrical Center or Junction Block Replacement

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

EXTERIOR COURTESY 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 exterior courtesy lamps are supplied with the courtesy lamp supply voltage from the driver
and passenger door lock and side window switches. The lamps are controlled by the driver and
passenger door lock and side window switches through low side drive control circuits.

Reference Information

2008 Chevrolet Silverado 1500

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

Schematic Reference

Exterior 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, command the Puddle/Courtesy Lamp ON and OFF with a scan tool. Verify the
exterior courtesy lamps turn on and off.

Circuit/System Testing

1. Disconnect the harness connector at the affected courtesy lamp bulb.

2. Ignition ON, verify that a test lamp illuminates between the courtesy lamp supply circuit

terminal and ground.

If the test lamp does not illuminate, test the courtesy lamp supply circuit for a short to
ground or an open/high resistance.

3. Connect a test lamp between the low side control circuit terminal and the courtesy lamp

supply circuit terminal.

4. Command the Puddle/Courtesy Lamp 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

2008 Chevrolet Silverado 1500

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