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.

### DTC Descriptors

DTC B257B 03: Lighting Control Switch Signal Low Voltage
DTC B257B 07: Lighting Control Switch Signal High Voltage

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Dimming 12-Volt Reference | B257B 03* | B257B 03* | -- | --
Instrument Panel Lamps Dimmer Switch Signal | B257B 03* | B257B 03* | B257B 07* | --
* Interior backlighting at full intensity and cannot be dimmed

### Circuit/System Description

The body control module (BCM) supplies a voltage reference through the instrument panel cluster dimming voltage reference circuit to the instrument panel cluster lamps 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 cluster lamps dimmer switch signal circuit to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage through the instrument panel cluster lamps supply voltage circuit and the LED dimming supply circuit to all related interior lamps, illuminating them to the desired level of brightness.

### Conditions for Running the DTC

The ignition is ON.

### Conditions for Setting the DTC

#### B257B 03

The BCM detects a voltage that is lower than 0.3 V on the dimming voltage circuit.

#### B257B 07

The BCM detects a voltage that is greater than 12.2 V on dimming voltage circuit.

### Action Taken When the DTC Sets

The BCM will not dim or illuminate any LED backlighting supplied by the backlight lamp control circuits.

### Conditions for Clearing the DTC

• | The condition responsible for setting the DTC no longer exists.

• | A history DTC will clear once 100 consecutive malfunction ignition cycles have occurred.

### Reference Information

#### 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

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

Ignition ON, vary the interior lamp dimming switch from dim to full bright. The interior backlighting should dim and then become full bright.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the S30 Headlamp Switch.
2. Ignition ON, test for greater than 11 V between the 12 V reference circuit terminal 1 and ground.

⇒ | If greater than the specified range, test the 12 V reference circuit for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module (BCM).

⇒ | If less than the specified range, test the 12 V reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 Body Control Module (BCM).

3. Ignition ON, verify the scan tool Dimming Control Signal parameter is less than 0.3 V.

⇒ | If greater than the specified range, test the signal circuit terminal 12 for a short to voltage. If the circuit tests normal, replace the K9 Body Control Module (BCM).

4. Install a 3 SA fused jumper wire between the signal circuit terminal 12 and the voltage reference circuit terminal 1. Verify the scan tool Dimming Control Signal parameter is greater than 11.7 V.

⇒ | 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.

5. If all circuits test normal, replace the headlamp switch.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Instrument Panel Lamp Dimmer Switch Replacement

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