The body control module (BCM) supplies a voltage reference 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 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 cluster lamps supply voltage circuit and the LED dimming supply circuit to all related interior lamps, illuminating them to the desired level of brightness.

The ignition is ON.

B257B 03

The BCM detects a voltage that is lower than 0.3 V on the dimming voltage circuit.

B257B 07

The BCM detects a voltage that is greater than 12.2 V on dimming voltage circuit.

The BCM will not dim or illuminate any LED backlighting supplied by the backlight lamp control circuits.

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

Ignition ON, vary the interior lamp dimming switch from dim to full bright. The interior backlighting should dim and then become full bright.

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 BCM.

5. If all circuits test normal, replace the headlamp switch.

Perform the Diagnostic Repair Verification