When the headlamp switch is turned to either the PARK or HEAD position, ground from G400 is applied through the switch and the tail lamp relay control circuit energizing the TAIL LAMP Relay. With the relay energized, battery voltage from Circuit Breaker 19 is applied through the switch side of the relay and the control circuit to the I/P dimmer switch. This voltage is then applied through the dimmer switch rheostat and the control circuit illuminating all instrument panel cluster (IPC) lamps and components.

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

Observe the operation of the I/P lamps while turning the park lamps ON and OFF. The I/P lamps should turn ON and OFF as the I/P dimmer switch is turned ON and OFF.

1. Ignition OFF, disconnect the harness connector at the I/P dimmer switch.
2. Test for less than 1.0 ohm between ground circuit terminal 2 and ground.

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

3. Connect a test lamp between the control circuit terminal 1 and ground.
4. Ignition ON, turn the park lamps ON and OFF. The test lamp should turn ON and OFF, when switching between the commanded states.

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

⇒	If the test is always ON, test the control circuit for a short to B+.

5. Ignition OFF, connect the harness connector at the dimmer switch and disconnect the harness connector at the appropriate component.
6. Test for less than 1.0 ohm between the ground circuit terminal of the appropriate component listed below and ground:

•	Instrument panel cluster (IPC) ground circuit terminal A8.

•	Air brake pressure gage lamp ground circuit terminal 2.

•	Marker lamp switch (RPO JE4 & RQ2 & U86 or JE4 & RQ3) ground circuit terminal 6.

•	Traction Control Switch (RPO NW9) ground circuit terminal 6.

•	PTO enable switch (RPO PTO) ground circuit terminal 6.

•	Rear axle driver control switch - two speed (RPO HPZ & A/T) ground circuit terminal 2.

•	Over-drive (OD) defeat switch (RPO LCT) ground circuit terminal 6.

•	Auto cruise switch (RPO K34 or PTO) ground circuit terminal 6.

•	Rear compartment lamp switch ground circuit terminal 6.

•	HVAC control assembly ground circuit terminal 4.

•	Outside heated mirror switch (RPO DNN) ground circuit terminal 6.

•	Exhaust brake switch (RPO K40) ground circuit terminal 6.

•	High idle switch (RPO UF3) ground circuit terminal 6.

•	Cigar lighter lamp ground circuit terminal B.

•	Radio (RPO UM6 or UM7) ground circuit terminal A3.

•	Automatic transmission shift lever position indicator (RPO LCT) ground circuit terminal D.

•	Automatic transmission shift selector (RPO MD) ground terminal

•	Automatic transmission shift selector (RPO MD) ground terminal V.

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

7. Ignition ON, park lamps ON, verify that a test lamp illuminates between the control circuit terminal of the appropriate component listed below and ground:

•	Instrument panel cluster (IPC) ground circuit terminal A9.

•	Air brake pressure gage lamp ground circuit terminal 1.

•	Marker lamp switch (RPO JE4 & RQ2 & U86 or JE4 & RQ3) ground circuit terminal 5.

•	Traction Control Switch (RPO NW9) ground circuit terminal 5.

•	PTO enable switch (RPO PTO) ground circuit terminal 5.

•	Rear axle driver control switch - two speed (RPO HPZ & A/T) ground circuit terminal 4.

•	Over-drive (OD) defeat switch (RPO LCT) ground circuit terminal 5.

•	Auto cruise switch (RPO K34 or PTO) ground circuit terminal 5.

•	Rear compartment lamp switch ground circuit terminal 5.

•	HVAC control assembly ground circuit terminal 8.

•	Outside heated mirror switch (RPO DNN) ground circuit terminal 5.

•	Exhaust brake switch (RPO K40) ground circuit terminal 5.

•	High idle switch (RPO UF3) ground circuit terminal 5.

•	Cigar lighter lamp ground circuit terminal A.

•	Radio (RPO UM6 or UM7) ground circuit terminal B10.

•	Automatic transmission shift lever position indicator (RPO LCT) ground circuit terminal A.

•	Automatic transmission shift selector (RPO MD) ground terminal U.

⇒	If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace I/P dimmer switch.

8. If all circuits test normal, replace the appropriate component.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.