B3903

DTC B3903

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 B3903 02
- Stop Lamp Relay Circuit Short to Ground

DTC B3903 05
- Stop Lamp Relay Circuit Short to Battery or Open

Diagnostic Fault Information





Circuit/System Description

The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5 V reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply voltage to the center high mounted stop lamp (CHMSL) relay coil. When the CHMSL relay receives voltage from the BCM, the relay coil is energized and the CHMSL relay switch contacts close applying battery voltage to the CHMSL, transmission control module (TCM), and engine control module (ECM).

Conditions for Running the DTC

* Battery voltage must be between 9-16 V.
* Brakes APPLIED.

Conditions for Setting the DTC

B3903 02

The BCM detects a short to ground in the stop lamp relay control circuit.

B3903 05

The BCM detects a short to voltage or an open/high resistance in the stop lamp relay control circuit.

Action Taken When the DTC Sets

* The Service Vehicle Soon indicator is commanded ON.
* The BCM disables the stop lamp relay control circuit.

Conditions for Clearing the DTC

* The condition responsible for setting the DTC no longer exists.
* A history DTC will clear once 100 malfunction-free ignition cycles have occurred.

Reference Information
Schematic Reference

Exterior Lights Schematics (Exterior Lights Schematics)
Connector End View Reference

Component Connector End Views (Connector End Views By Name)
Description and Operation

Exterior Lighting Systems Description and Operation (Exterior 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 CHMSL relay.
2. Ignition OFF, exterior lamps OFF, test for less than 5 ohm between the ground circuit terminal 2 and ground.

• If greater than the specified range, test the ground circuit for an open/high resistance.

3. Connect a test lamp between the control circuit terminal 1 and ground.
4. Ignition ON, turn the CHMSL relay control ON and OFF by applying and releasing the brake pedal. The test lamp should turn ON and OFF as the brake pedal is applied and released.

• If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.
• If the test lamp is always OFF, test the control 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, test or replace the CHMSL relay.

Repair Instructions

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

* Relay Replacement Service and Repair
* Control Module References (Programming and Relearning)for BCM replacement, setup, and programming