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 from the CHMSL/BCK-UP fuse through the instrument panel lamp supply voltage circuit, the backlight lamp control circuit, and the LED dimming supply circuit illuminating the LED and I/P panel lamps.

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 scan tool 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.

2. 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 0.2 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 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 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 C3 harness connector at the BCM.
2. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal 2 and ground.

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

3. Ignition OFF, connect the C3 harness connector at the BCM and disconnect the harness connector at the component with the inoperative backlighting.
4. Verify that a test lamp illuminates between the appropriate ground circuit terminal listed below and B+.

•	Headlamp Switch terminal 8

•	Instrument Panel Cluster (IPC) terminal 6

•	Driver Window Switch terminal F - Coupe

•	Driver Window Switch terminal B C1 - Sedan

•	Driver Door Lock Switch terminal F - Coupe

•	Driver Door Lock Switch terminal B - Sedan

•	Outside Rearview Mirror Switch terminal B

•	Passenger Window Switch terminal C

•	Passenger Door Lock Switch terminal F - Coupe

•	Front Passenger Door Lock Switch terminal B - Sedan

•	Radio terminal 8 C1

•	HVAC Control Module terminal 10 C1

•	Driver Information Display Switch terminal 5

•	Left Steering Wheel Controls terminal 8

•	Right Steering Wheel Controls terminal 1

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

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

Incandescent Dimming

•	Headlamp Switch terminal 7

•	Instrument Panel Cluster (IPC) terminal 8

•	Driver Window Switch terminal B - Coupe

•	Driver Window Switch terminal F C2 - Sedan

•	Driver Door Lock Switch terminal G - Coupe

•	Driver Door Lock Switch terminal A - Sedan

•	Outside Rearview Mirror Switch terminal E

•	Passenger Window Switch terminal B

•	Passenger Door Lock Switch terminal G - Coupe

•	Passenger Door Lock Switch terminal A - Sedan

LED Dimming

•	Radio terminal 7 C1

•	HVAC Control Module terminal 1 C1

•	Driver Information Display Switch terminal 4

•	Left Steering Wheel Controls terminal 6

•	Right Steering Wheel Controls terminal 2

6. 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.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.