When the ignition switch is turned to the ON position, the vacuum fluorescent (VF) display, radio, turns ON at maximum brightness. When the park lamps are ON, all incandescent back lighting turn ON at the dimming level indicated by the indicated dimming level. When the headlamp switch is placed in the PARK position, the park lamp supplies voltage circuit provides an input to the body control module (BCM). The BCM then supplies voltage to the I/P dimming switch through the I/P dimming lamps control circuit. The setting of the I/P dimmer switch determines the amount of voltage that the I/P dimming switch supplies to the BCM through the I/P dimming lamps low reference circuit. The BCM than sends a pulse width modulation (PWM) voltage to all the interior lamps. All the VF the incandescent back lighting lamps are provided a specific voltage and are then grounded. When the I/P dimmer switch is moved from MIN to MAX all VF displays, as well as all incandescent back lighting responds from minimum intensity to maximum brightness in response to the I/P dimmer switch.

Schematic Reference

Interior Lights Dimming Schematics

Electrical Information Reference

Scan Tool Reference

Ignition ON, turn the park lamps ON, observe the back lighting in the instrument panel (I/P). The back lighting should turn ON and the intensity will change with the operation of the dimmer switch.

1. Ignition OFF, disconnect the harness connector at the dimmer switch.
2. Test for less than 1 ohm of resistance between the low reference circuit and ground.

⇒	If greater than 1 ohm, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

3. Ignition ON, verify the scan tool I/P dimmer input is greater than 4.5 volts.

⇒	If less than 4.5, test the signal circuit for a short to ground. If the circuit tests normal, replace the BCM.

4. Install a 3-amp fused jumper wire between the signal circuit and the low reference circuit. Verify the scan tool dimmer switch is less than .5.

⇒	If greater than .5, test the signal circuit for a short to voltage or an open/high resistance.

5. Reconnect the dimmer switch.
6. Ignition ON, turn the headlamp switch to the HEAD position.
7. Rotate the I/P dimmer switch to the high and then low illuminations settings. The I/P back lighting should brighten and dim with each rotation.

⇒	If the back lighting does not change intensity, test the instrument panel lamp supply voltage circuit for a short to voltage.

8. If the circuit tests normal, replace the BCM.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.