With the ignition in the ON position, battery voltage from the GAUGES Fuse 10 (C4/C5) or Circuit Breaker 10 (C6/C7/C8) located in the I/P-1 fuse block is supplied to the I/P dimmer switch, which is part of the headlamp switch. When the dimmer switch is placed in a desired brightness position, voltage is applied through the dimmer switch rheostat, the I/P lamp dimmer switch signal circuit, the ILLUM Fuse 17 (C4/C5) or Circuit Breaker 17 (C6/C7/C8), and the I/P lamps supply voltage circuit illuminating all I/P backlighting components.

Battery voltage from the PRK LAMPS Fuse 3 (C4/C5) or Circuit Breaker 3 (C6/C7/C8) located in the I/P-1 fuse block is supplied at all times to the headlamp switch. When the headlamp switch is in the PARK position, battery voltage is applied through the park lamp supply voltage circuit, the LT and RT PARK Fuses (C4/C5) or Circuit Breakers (C6/C7/C8) located in the I/P-2 fuse block, and the left and right park lamp supply voltage circuits illuminating the park lamps, tail lamps, and the interior non-dimming backlighting components.

Schematic Reference

Headlights/Daytime Running Lights (DRL) 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, park lamps ON, observe the operation of the IP lights while operating the dimmer switch from high to low. The IP lights listed below if equipped, should illuminate and dim as the IP dimmer switch is rotated.

•	Instrument Panel Cluster (IPC)

•	Inflatable Restraint I/P Module Disable Switch

•	Automatic Transmission Fluid Temperature Gage

•	Air Brake Pressure Gage Lamp

•	HVAC Control Assembly

•	Automatic Transmission Shift Selector

•	Auxiliary HVAC Control Assembly

⇒	If one or more of the component lamps do not perform as specified, refer to I/P Lamps Dimming Malfunction.

2. Ignition ON, observe the park/tail lamps while turning the PARK lamps ON and OFF using the headlamp switch. The park/tail lamps should turn ON and OFF when changing between the commanded states.

⇒	If the park/tail lamps do not turn ON and OFF as specified, refer to Park Lamps Malfunction.

3. Observe the operation of the component lamps listed below if equipped, while turning the park lamps ON and OFF. The lamps should turn ON and Off when changing between commanded states.

•	Transfer Case Shift Control Switch

•	Two Speed Rear Axle Shift Actuator

•	High Idle Switch

•	Traction Control Switch

•	Power Take-Off (PTO) Switch

•	Marker Lamp Switch

•	Air Suspension Dump Switch

•	Exhaust Brake Switch terminal B

•	Differential Lock Switch Lamp, Inter-Axle Differential Lock Switch Lamp, Two Speed Rear Axle Driver Control Switch Lamp

•	Outside Heated Mirror Switch

•	Overdrive Defeat Switch

•	Driver Window Switch

•	Driver Door Lock Switch

•	Passenger Window Switch

•	Passenger Door Lock Switch

•	LR Window Switch

•	RR Window Switch

⇒	If one or more of the component lamps do not perform as specified, refer to Non-dimming Backlighting Malfunction.

I/P Lamps Dimming Malfunction

1. Ignition OFF, disconnect the harness connector at the headlamp switch.
2. Test for less than 1 ohm between the ground circuit terminal R and ground.

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

3. Connect the harness connector at the headlamp switch and disconnect the harness connector at the component with the inoperative backlighting.
4. Test for less than 5 ohms between the appropriate ground circuit terminal listed below and ground.

•	Instrument Panel Cluster (IPC) terminal A8

•	Inflatable Restraint I/P Module Disable Switch terminal F

•	Automatic Transmission Fluid Temperature Gage terminal D

•	Air Brake Pressure Gage Lamp terminal B

•	HVAC Control Assembly terminal B X1

•	Automatic Transmission Shift Selector terminal 5

•	Auxiliary HVAC Control Assembly terminal E

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

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

•	IPC terminal A9

•	Inflatable Restraint I/P Module Disable Switch terminal D

•	Automatic Transmission Fluid Temperature Gage terminal F

•	Air Brake Pressure Gage Lamp terminal A

•	HVAC Control Assembly terminal C X1

•	Automatic Transmission Shift Selector terminal 3

•	Auxiliary HVAC Control Assembly terminal H

6. Ignition ON, I/P dimmer switch on HIGH, turn the park lamps ON and OFF using the headlamps switch. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, test or replace headlamp switch.

⇒	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, test or replace the headlamp switch.

7. Ignition ON, park lamps ON, rotate the I/P dimmer switch between the high and low settings. The test lamp should illuminate from a high to a low brightness.

⇒	If the test lamp illumination does change as specified, test or replace the headlamp switch.

8. If all circuits test normal, test or replace the bulb or component.

Non-dimming Backlighting Malfunction

1. Ignition OFF, disconnect the harness connector at the component with the inoperative backlighting.
2. Test for less than 5 ohms between the appropriate ground circuit terminal listed below and ground.

•	Transfer Case Shift Control Switch terminal D

•	Two Speed Rear Axle Shift Actuator terminal B

•	High Idle Switch terminal B

•	Traction Control Switch terminal A

•	Power Take-Off (PTO) Switch terminal E

•	Marker Lamp Switch terminal B

•	Air Suspension Dump Switch terminal E

•	Exhaust Brake Switch terminal B

•	Differential Lock Switch Lamp, Inter-Axle Differential Lock Switch Lamp, Two Speed Rear Axle Driver Control Switch Lamp terminal 2

•	Outside Heated Mirror Switch terminal E

•	Overdrive Defeat Switch terminal B

•	Driver Window Switch terminal G - without crew cab

•	Driver Window Switch terminal E - with crew cab

•	Driver Door Lock Switch terminal B

•	Passenger Window Switch terminal B

•	Passenger Door Lock Switch terminal B

•	LR Window Switch terminal B

•	RR Window Switch terminal B

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

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

•	Transfer Case Shift Control Switch terminal C

•	Two Speed Rear Axle Shift Actuator terminal E

•	High Idle Switch terminal E

•	Traction Control Switch terminal E

•	Power Take-Off (PTO) Switch terminal B

•	Marker Lamp Switch terminal E

•	Air Suspension Dump Switch terminal B

•	Exhaust Brake Switch terminal E

•	Differential Lock Switch Lamp, Inter-Axle Differential Lock Switch Lamp, Two Speed Rear Axle Driver Control Switch Lamp terminal 1

•	Outside Heated Mirror Switch terminal B

•	Overdrive Defeat Switch terminal E

•	Driver Window Switch terminal D - without crew cab

•	Driver Window Switch terminal A - with crew cab

•	Driver Door Lock Switch terminal C

•	Passenger Window Switch terminal C

•	Passenger Door Lock Switch terminal C

•	LR Window Switch terminal C

•	RR Window Switch terminal C

4. Ignition ON, turn the PARK lamps ON and OFF using the headlamp switch. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage.

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.