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 instrument panel lamp dimmer switch is used to increase and decrease the brightness of the interior backlighting components. The instrument panel lamp dimmer switch provides a voltage signal that will increase as the brightness of the lights are increased and decrease as the brightness of the lights are decreased. The body control module (BCM) provides a low reference signal and a B+ voltage reference to the instrument panel lamp dimmer switch. When the instrument panel lamp dimmer switch is placed in the desired position, the dimmed voltage setting is applied from the instrument panel dimmer switch through the instrument panel lamp dimmer switch signal circuit to the BCM. The BCM interprets the signal and applies the dimmed voltage to the instrument panel lamp lamps supply voltage circuit and the interior backlighting components dim to the requested level.
Conditions for Running the DTC
* Battery voltage must be between 9-16 V.
* Park lamps ON.
Conditions for Setting the DTC
B257B 03
The BCM detects a short to ground in the instrument panel dimmer switch signal circuit.
B257B 07
The BCM detects a short to voltage in the instrument panel dimmer switch signal circuit.
Action Taken When the DTC Sets
The BCM will not DIM or illuminate any instrument panel lamps supplied by the instrument panel lamps dimmer switch signal circuit.
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 End Views By Name)
Description and Operation
Interior Lighting Systems Description and Operation (Interior Lighting Systems)
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 Testing
1. Ignition OFF, disconnect the harness connector at the S30 headlamp switch.
2. Ignition ON, test for B+ between the B+ reference circuit terminal 1 and ground.
• If less than the specified range, test the B+ reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K9 BCM.
• If greater than the specified range, test the B+ reference circuit for a short to voltage. If the circuit tests normal, replace the K9 BCM.
3. Verify the scan tool Incandescent Dimming Duty Cycle Command parameter is greater than 45%.
• If less than the specified range, test the signal circuit terminal 12 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 12 and the B+ reference circuit terminal 1. Verify the scan tool Incandescent Dimming Duty Cycle Command parameter is less than 8%
• If greater 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 K9 BCM.
5. If all circuits test normal, test or replace the S30 headlamp switch.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Headlamp, Instrument Panel Lamp Dimmer, and Fog Lamp Switch Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for BCM replacement, setup, and programming