B257B

DTC B257B

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (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/System Description

The body control module (BCM) supplies a reference voltage to the backlight dimmer switch. When the backlight dimmer switch is placed in a desired brightness position, reference voltage is applied through the backlight dimmer switch rheostat to the BCM. The BCM interprets this voltage signal, then applies a pulse width modulated (PWM) voltage to 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 5.2 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 (Interior Lights Dimming Schematics)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

Interior Lighting Systems Description and Operation (Interior Lighting Systems Description and Operation)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Verification

Ignition ON, switch ON exterior lighting, vary the interior backlight from dim to full bright by using the dimmer switch. The interior backlighting should dim and then become full bright.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the S36 instrument panel dimmer switch.
2. Ignition ON, test for 4.8-5.2 V between the 5 V reference circuit terminal 5 and ground.

• If greater than the specified range, test the 5 V reference circuit for a short to voltage. If the circuit tests normal, replace the K9 BCM.
• If less than the specified range, test the 5 V reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 BCM.

3. Ignition ON, test for less than 0.3 V between dimming signal circuit terminal 9 and ground.

• If greater than the specified range, test the dimming signal circuit terminal 9 for a short to voltage. If the circuit tests normal, replace the K9 BCM.

4. Install a 3 A fused jumper wire between the signal circuit terminal 9 and the voltage reference circuit terminal 5. The interior backlighting should now be dimmed.

• If the interior backlighting is not dimmed, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 BCM.

5. If all circuits test normal, replace the S36 instrument panel dimmer switch.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Instrument Panel Lamp Dimmer Switch Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for BCM replacement, setup, and programming