The BCM supplies a voltage reference through the instrument panel (I/P) dimming voltage reference circuit to the interior lamp 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 I/P 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 lamp supply voltage circuit and the backlight lamp control circuit illuminating the lamps on these dimming circuits. At the same time the BCM sends a serial data message requesting the front door modules to illuminate the door switches. For more detailed information concerning interior lamp operation refer to Interior Lighting Systems Description and Operation.

Schematic Reference

Interior Lights Dimming Schematics

Connector End View Reference

Lighting Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

1. Ignition ON, observe the driver door switch while commanding the Driver Door Switch Integrated Sw. Backlighting Test ON and OFF with a scan tool. The switch backlighting should turn ON and OFF between commanded states.

⇒	If the backlighting does not turn On and OFF or remains always ON, replace the driver door switch.

2. Ignition ON, observe the driver heated and cooled seat switch while commanding the Driver Door Switch Non-Integrated Sw. Backlighting Test ON and OFF with a scan tool. The switch backlighting should turn ON and OFF between commanded states.

⇒	If the backlighting does not turn On and OFF or remains always ON, refer to Front Door Switch Backlighting Circuit Test.

3. Ignition ON, observe the passenger door switch while commanding the Passenger Door Switch Integrated Sw. Backlighting Test ON and OFF with a scan tool. The switch backlighting should turn ON and OFF between commanded states.

⇒	If the backlighting does not turn On and OFF or always remains ON, replace the passenger door switch.

4. Ignition ON, observe the passenger heated and cooled seat switch while commanding the Passenger Door Switch Non-Integrated Sw. Backlighting Test ON and OFF with a scan tool. The switch backlighting should turn ON and OFF between commanded states.

⇒	If the backlighting does not turn On and OFF or remains always ON, refer to Front Door Switch Backlighting Circuit Test.

5. 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.

⇒	If the scan tool dimmer control operates normal, refer to Interior Backlighting Circuit Test.

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 component with the inoperative backlighting.
2. Test for less than 5.0 ohms of resistance between the appropriate ground circuit terminal listed below and ground.

•	Headlamp Switch terminal 8

•	Hazard Switch terminal B

•	Ashtray Lamp terminal B

•	HVAC Control Module terminal 11 C1

•	Driver Information Display Switch terminal 6

•	Clock terminal 2

•	Radio terminal 8 C1

•	Left Middle Door Window Switch terminal 1 -- Limo

•	Right Middle Door Window Switch terminal 1 -- Limo

•	Traction Control Switch terminal A

•	Floor Shifter Assembly terminal D C2

•	Garage Door Opener Transmitter terminal 3

•	Sunshade Switch terminal A

•	Sunroof Switch terminal 5

•	Left Rear Door Window Switch terminal 1

•	Left Rear Heated Seat Switch terminal J

•	Left Rear Seat Lumbar Switch terminal E

•	Right Rear Door Window Switch terminal 1

•	Right Rear Heated Seat Switch terminal J

•	Right Rear Seat Lumbar Switch terminal E

•	Auxiliary HVAC Control Module terminal 8

•	Right Steering Wheel Control Switch terminal 1

•	Left Steering Wheel Control Switch terminal 8

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

3. Connect a test lamp between the appropriate supply voltage circuit terminal listed below and ground.

Incandescent Dimming

•	Hazard Switch terminal A

•	Ashtray Lamp terminal A

•	Traction Control Switch terminal D

•	Floor Shifter Assembly terminal C C2

•	Garage Door Opener Transmitter terminal 2

•	Sunshade Switch terminal D

•	Sunroof Switch terminal 3

•	Left Rear Door Window Switch terminal 2

•	Left Rear Heated Seat Switch terminal F

•	Left Rear Seat Lumbar Switch terminal D

•	Right Rear Door Window Switch terminal 2

•	Right Rear Heated Seat Switch terminal F

•	Right Rear Seat Lumbar Switch terminal D

•	Left Middle Door Window Switch terminal 2 -- Limo

•	Right Middle Door Window Switch terminal 2 -- Limo

LED Dimming

•	Headlamp Switch terminal 7

•	HVAC Control Module terminal 2 C3

•	Driver Information Display Switch terminal 5

•	Clock terminal 3

•	Radio terminal 7 C1

•	Auxiliary HVAC Control Module terminal 10

•	Right Steering Wheel Control Switch terminal 2

•	Left Steering Wheel Control Switch terminal 6

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 supply voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM

⇒	If the test lamp is always ON, test the supply voltage circuit for a short to voltage. If the circuit tests normal, replace the BCM.

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

Front Door Switch Backlighting Circuit Test

1. Ignition OFF, disconnect the (C4) harness connector at the appropriate driver or front passenger door control module.
2. Connect a 3A fused jumper wire between the low reference circuit terminal 4 and ground.
3. Connect a test lamp between the indicator supply voltage circuit terminal 13 and B+. The heated and cooled seat switch backlighting should illuminate.

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

4. If the switch backlighting tests normal, replace the door control module.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.