The body control module (BCM) supplies a voltage reference through the through the instrument panel cluster dimming voltage reference circuit to the instrument panel cluster lamps dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired brightness position, reference voltage is applied through the dimmer switch rheostat and the instrument panel cluster lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage through the instrument panel, the backlight lamp control circuit, and the LED dimming control circuit illuminating the LEDs and instrument panel cluster lamps.

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

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, observe the scan tool Indicator Backlight Command 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.

2. Command the Incandescent Dimming test to change intensity with the scan tool. The appropriate incandescent lighting items should change in intensity.

⇒	If any incandescent lighting items do not change in intensity, refer to Interior Backlighting Circuit Test.

Dimmer Switch Circuit Test

1. Ignition OFF, disconnect the harness connector at the S30 Headlamp Switch.
2. Ignition ON, test for greater than 11 V between the 12 V reference circuit terminal 1 and ground.

⇒	If greater than the specified range, test the 12 V reference circuit for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If less than the specified range, test the 12 V reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

3. Ignition ON, verify the scan tool Dimming Control Signal parameter is less than 0.3 V.

⇒	If greater than the specified range, test the signal circuit terminal 12 for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module (BCM).

4. Install a 3 A fused jumper wire between the signal circuit terminal 12 and the voltage reference circuit terminal 1. Verify the scan tool Dimming Control Signal parameter is greater than 11.7 V.

⇒	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 K9 Body Control Module (BCM).

5. If all circuits test normal, test or replace the S30 Headlamp Switch.

Interior Backlighting Circuit Test

1. Ignition OFF, disconnect the harness connector at the appropriate inoperative backlighting component.
2. Test for less than 5.0 Ω between the ground circuit terminal 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 and ground.
4. Ignition ON, command the appropriate Incandescent Dimming or LED Dimming 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 OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒	If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module (BCM).

5. If all circuits test normal, test or replace the appropriate component with the inoperative backlighting.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.