For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
BCM | 1 | 1 | -- | --
1. Interior Backlighting Does Not Dim

### Circuit/System Description

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.

### Diagnostic Aids

An open/high resistance in the dimmer switch circuit could cause the back lighting not to dim.

### Reference Information

#### Schematic Reference

Interior Lights Dimming Schematics

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Data Definitions

• | Scan Tool Data List

• | Scan Tool Output Controls

### Circuit/System Verification

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.

### Circuit/System Testing

1. Ignition ON, turn the headlamp switch to the HEAD position.
2. 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.

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

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Instrument Panel Lamp Dimmer Switch Replacement

• | Control Module References for replacement, setup, and programming