Chevrolet Silverado / GMC Sierra. Service manual — part 2491

bright to dim when changing between the commanded states.

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

If the test lamp is always bright, test the control circuit for a short to voltage. If the
circuit tests normal, replace the BCM.

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

LED Dimming Circuit Test

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

2. Ignition OFF, verify that a test lamp illuminates between the listed ground circuit terminal

and battery voltage.

Right steering wheel switch terminal 1

Left steering wheel switch terminal 8

DIC switch terminal 5

Manual HVAC control module terminal 10 X2

Multifunction accessory switch terminal 3

If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Connect a test lamp between the listed LED dimming control circuit terminal and ground.

Right steering wheel switch terminal 2

Left steering wheel switch terminal 6

Power take off switch terminal 6

Trailer brake controller switch terminal 7

IPC terminal 8

Radio terminal 7 X1

DIC switch terminal 4

Manual HVAC control module terminal 1 X2

Multifunction accessory switch terminal 4

4. Command the dimming test high to low with a scan tool. The test lamp should turn from

bright to dim when changing between the commanded states.

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

If the test lamp is always bright, test the control circuit for a short to voltage. If the
circuit tests normal, replace the BCM.

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

2008 Chevrolet Silverado 1500

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

Front Door Switch Backlighting Circuit Test

1. Ignition OFF, disconnect the harness connector X4 at the door lock and side window

switch.

2. Connect a 3A fused jumper wire between the low reference circuit terminal 4 and ground.

3. Connect a test lamp between the backlight supply voltage circuit terminal 13 and B+. The

heated seat switch backlighting should illuminate.

If the switch backlighting does not illuminate, replace the heated seat switch.

4. If the switch backlighting tests normal, replace the door lock and side window switch.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Front Seat Heater Switch Replacement (w/o KB6)

Steering Wheel Control Switch Assembly Replacement

Driver Information Display Switch Replacement (without RPO SLT) or Driver
Information Display Switch Replacement (with RPO SLT)

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

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

Control Module References for BCM, door lock and side window switch, IPC, radio,
HVAC control module, and rear seat audio controller replacement, programming, and setup.

INTERIOR BACKLIGHTING MALFUNCTION (W/O AN3)

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 ignition switch is turned to the RUN position, the radio VF display turns ON at
maximum brightness. When the park lamps are ON, all incandescent and LED back lighting turn
ON at the dimming level indicated by the instrument panel (I/P) dimmer switch. At the same time
all VF displays dim to match the indicated dimming level. The panel dimmer switch

2008 Chevrolet Silverado 1500

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

potentiometer is an input to the body control module (BCM). When the driver selects a dimming
setting by moving the I/P dimming switch potentiometer, all incandescent back lighting lamps are
provided with a specific voltage. When the I/P dimmer switch is moved from MIN to MAX, all
VF displays, as well as all incandescent back lighting respond from minimum intensity to
maximum brightness in response to the I/P dimmer switch

Reference Information

Schematic Reference

Interior Lights Dimming Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Interior 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, observe the scan tool BCM Dimming Control Signal parameter while operating the
dimmer switch from high to low. The reading should change from a high percent to a low percent
as the switch is rotated.

If the reading does not change, refer to Dimmer Switch Circuit Test below.

If the scan tool dimmer control operates normal, refer to the Interior Backlighting Circuit
Test or the LED Dimming Circuit Test below.

2008 Chevrolet Silverado 1500

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

Circuit/System Testing

Dimmer Switch Circuit Test

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

2. Ignition OFF, verify that a test lamp illuminates between the ground circuit terminal 8 and

B+.

If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Dimming Control Signal parameter is less than 0.1 volts.

If greater than the specified range, test the signal circuit terminal 13 for a short to
voltage. If the circuit tests normal, replace the BCM.

4. Ignition ON, test for B+ between the I/P dimming voltage reference circuit terminal 15 and

ground.

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

5. Install a 3A fused jumper wire between the signal circuit terminal 13 and the voltage

reference circuit terminal 15. Verify the scan tool BCM Dimming Control Signal parameter
is greater than 2.6 volts.

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

6. If all circuits test normal, test or replace the headlamp switch.

Interior Backlighting Circuit Test

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

2. Ignition OFF, verify that a test lamp illuminates between the listed ground circuit terminal

and battery voltage.

Passenger window switch terminal F

Headlamp switch terminal 8

Transfer case control switch terminal 6

Sunroof switch terminal 5

Door lock switch terminal B

Roof beacon switch terminal 1

Sliding rear window switch terminal D

Garage door opener switch terminal 3

Outside rear view mirror switch terminal G

2008 Chevrolet Silverado 1500

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