Chevrolet Silverado / GMC Sierra. Service manual — part 2489

1. Ignition OFF, disconnect the headlamp low/HID relay.

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

circuit terminal 87 and ground.

If the test lamp illuminates, test the headlamp 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 headlamp.

6. Test for less than 1 ohm of resistance between the headlamp 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 headlamp.

8. Connect a 20A fused jumper wire between the B+ circuit terminal 30 and the headlamp

control circuit terminal 87. Verify the low beam headlamps are on.

If the headlamps do not turn on, test the control circuit for an open/high resistance. If
the circuit tests normal, test or replace the affected headlamp.

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

terminal 86.

10. Command the Low Beams 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 relay 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 headlamp low/HID relay.

High Beam Relay Control Circuit Test

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

2. Low beam headlamps ON, verify the high beam headlamps are not on.

If the high beam headlamps are on, test the high beam 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 16 and ground.

Verify the high beam headlamps are on.

2008 Chevrolet Silverado 1500

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

If the high beam headlamps are not on, test the high beam 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.

High Beam Headlamp 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 High Beams 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.

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

Headlamp Switch Replacement (without RPO SLT) or Headlamp Switch
Replacement (with RPO SLT)

Turn Signal Multifunction Switch Replacement

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

HEADLAMPS FLASH TO PASS INOPERATIVE

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

2008 Chevrolet Silverado 1500

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

The high beam headlamps are controlled by the BCM through the high beam relay. The relay is
supplied battery voltage and is energized when the BCM grounds the low side drive control
circuit. The flash to pass switch signal circuit is referenced from battery voltage by the BCM and
when the switch is activated the signal circuit is grounded, and the BCM determines the switch is
active when the signal circuit voltage is low.

Reference Information

Schematic Reference

Headlights/Daytime Running Lights (DRL) 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

With the headlamps ON, observe the scan tool Flash to Pass Switch parameter while pulling and
releasing the flash to pass switch. The parameter should cycle between Active and Inactive.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector X2 at the turn signal/multifunction switch.

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

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

2008 Chevrolet Silverado 1500

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

3. Ignition ON, verify the scan tool Flash to Pass Switch parameter is Inactive.

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

terminal D. Verify the scan tool Flash to Pass 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 signal/multifunction switch.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Turn Signal Multifunction Switch Replacement

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

HIGH BEAM INDICATOR 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 instrument panel cluster (IPC) illuminates the high beam indicator when the BCM detects
that the high beams are requested. The IPC receives a message from the BCM requesting
illumination.

Reference Information

Schematic Reference

Instrument Cluster 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