Chevrolet Silverado / GMC Sierra. Service manual — part 2494

Control Module References for Scan Tool Information

Circuit/System Verification

1. Ignition ON, observe the scan tool BCM Stop Lamp Switch Status parameter while pressing

and releasing the brake pedal. The reading should change between 1 count and
approximately 1023 counts.

If not the specified value, refer to the Stop Lamp Switch Circuit Test below.

2. Ignition ON, observe the scan tool TCM Brake Switch parameter while pressing and

releasing the brake pedal. The reading should change between Pressed and Released.

If not the specified value, refer to the Brake Switch Circuit Test below.

3. Ignition ON, observe the left, and right stop lamps while pressing and releasing the brake

pedal. The stop lamps should turn on and off.

If the left and or right stop lamps are always on or inoperative, refer to the Stop Lamp
Circuit Test below.

4. Connect a test lamp between the relay side of the CHMSL fuse and ground.

5. Verify that the test lamp turns on and off while pressing and releasing the brake pedal.

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

6. If the center high mount stop lamp is always on or inoperative refer to the CHMSL Circuit

Test below.

Circuit/System Testing

Stop Lamp Switch Circuit Test

1. Ignition OFF, disconnect the harness connector at the stop lamp switch.

2. Ignition ON, test for battery voltage between the 12-volt reference circuit terminal C and

ground.

If less than the specified range, test the 12-volt reference circuit for a short to ground
or an open/high resistance. If the circuit tests normal, replace the BCM.

3. Verify the scan tool BCM Stop Lamp Switch Status parameter is approximately 1023

counts.

If not the specified value, test the signal circuit terminal D for a short to voltage. If the
circuit tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the signal circuit terminal D and the 12-volt

reference circuit terminal C. Verify the scan tool Stop Lamp Switch Status parameter is 1
count.

2008 Chevrolet Silverado 1500

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

If not the specified value, test the signal circuit for an open/high resistance. If the
circuit tests normal, replace the BCM.

5. If all circuits test normal, test or replace the stop lamp switch.

Brake Switch Circuit Test

1. Ignition OFF, disconnect the harness connector at the stop lamp switch.

2. Ignition ON, test for battery voltage between the ignition supply circuit terminal B and

ground.

If less than the specified range, test the ignition supply circuit for a short to ground or
an open/high resistance.

3. Verify the scan tool TCM Brake Switch parameter is Released.

If not the specified value, test the signal circuit terminal A for a short to voltage.

4. Install a 3A fused jumper wire between the signal circuit terminal B and the ignition supply

circuit terminal A. Verify the scan tool Brake Switch parameter is Applied.

If not the specified value, test the signal circuit for an open/high resistance.

5. If all circuits test normal, test or replace the stop lamp switch.

Stop Lamp Circuit Test

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

2. Ignition ON, verify a test lamp illuminates between the terminals listed below and ground.

Terminal 1

Terminal 2

Terminal 4

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

3. Connect the harnes connector X4 at the BCM.

4. Ignition OFF, disconnect the harness connector at the affected stop lamp.

5. Test for less than 1 ohm of resistance between the ground circuit terminal GRD and ground.

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

6. Connect a test lamp between the stop lamp control circuit terminal MJR and ground.

7. Ignition ON, press and release the brake pedal. 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.

2008 Chevrolet Silverado 1500

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

If the test lamp is always OFF, test the control circuit for a short to ground or an
open/high resistance.

8. If all circuits test normal, test or replace the affected stop lamp.

CHMSL Relay Control Circuit Test

1. Ignition OFF, disconnect the harness connector X5 at the BCM.

2. Ignition ON, verify the center high mount stop lamp is not on.

If the center high mount stop lamp is on, test the CHMSL relay control circuit terminal
5 for a short to voltage. If the circuit tests normal replace the underhood fuse block.

3. Install a 10A fused jumper wire between the CHMSL relay control circuit terminal 5 and

battery voltage. Verify the center high mount stop lamp is on.

If the center high mount stop lamp is not on or the fuse in the jumper wire opens, test
the CHMSL relay control circuit for a short to ground or an open/high resistance. If
the circuit tests normal replace the underhood fuse block.

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

CHMSL Circuit Test

1. Ignition OFF, disconnect the harnes connector at the center high mount stop lamp.

2. Test for less than 1 ohm of resistance between the ground circuit terminal 1 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. Ignition ON, press and release the brake pedal. 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 test lamp is always OFF, test the control circuit for a short to ground or an
open/high resistance.

5. If all circuits test normal, test or replace the center high mounted stop lamp.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Stop Lamp Switch Replacement (Non-Adjustable Pedal) or Stop Lamp Switch
Replacement (Adjustable Pedal)

2008 Chevrolet Silverado 1500

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

High Mount Stop Lamp Replacement (Regular Cab) or High Mount Stop Lamp
Replacement (Extended and Crew Cab)

Tail Lamp Bulb Replacement (GMC) or Tail Lamp Bulb Replacement (Chevrolet)

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

TURN SIGNAL LAMPS AND/OR INDICATORS 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

When the turn signal switch is placed in either the left or right position, a ground signal is
completed from the turn signal switch to the BCM. When the hazard flasher switch is activated, a
ground signal is completed from the turn signal/hazard switch to the BCM. The BCM then sends
an ON-OFF voltage signal through the fuses to the appropriate turn signal and side repeater
(export only) lamps or to all of the turn signal fuses for hazard operation. The driver door switch
and passenger door switch also receive voltage from the BCM turn signal supply voltage circuits
for the mirror turn signals. The door switches have no function or control of the mirror turn
signals other than a pass through connection for the circuits. The audio chime is also activated
when the turn signals are ON. The instrument panel cluster (IPC) receives the signals to activate
the turn signal indicators over the GMLAN serial data system.

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

2008 Chevrolet Silverado 1500

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