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 B3903 02: Stop Lamp Relay Circuit Short to GroundDTC B3903 05: Stop Lamp Relay Circuit Short to Battery or Open
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
Brake Pedal Position Sensor 5-Volt Reference | C0277 06, C0870 03 | C0277 06 | 1 | -- |
Center High Mounted Stop Lamp (CHMSL) Relay B+ | 2 | 2 | -- | -- |
Brake Pedal Position Sensor Signal | C0277 06 | C0277 06 | C0277 07 | C0277 09 |
Center High Mounted Stop Lamp (CHMSL) Relay Control | B3903 02 | B3903 05 | B3903 05 | -- |
Center High Mounted Stop Lamp (CHMSL) Control | 2 | 2 | 2 | -- |
Left Rear Stop/Turn Lamp Control | 1 | 1 | 1 | -- |
Right Rear Stop/Turn Lamp Control | 1 | 1 | 1 | -- |
Brake Pedal Position Sensor Low Reference | -- | C0277 07 | 1 | -- |
Center High Mounted Stop Lamp (CHMSL) Ground | -- | 2 | 1 | -- |
Left Rear Stop/Turn Lamp Ground | -- | 1 | 1 | -- |
Right Rear Stop/Turn Lamp Ground | -- | 1 | 1 | -- |
Center High Mounted Stop Lamp (CHMSL) Relay Ground | -- | 2 | -- | -- |
1. Stop Lamps Malfunction 2. Center High Mounted Stop Lamp (CHMSL) Malfunction | ||||
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-volt 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 volts. |
| • | Brakes applied. |
Conditions for Setting the DTC
B3903 02
The DTC will set when the BCM detects a short to ground in the stop lamp relay control circuit.
B3903 05
The DTC will set when 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
Connector End View Reference
Description and Operation
Exterior Lighting Systems Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Testing
- Ignition OFF, disconnect the CHMSL relay.
- Ignition OFF, exterior lamps OFF, test for less than 5 ohms between the ground circuit terminal 86 and ground.
- Connect a test lamp between the control circuit terminal 85 and ground.
- Ignition ON, command 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 all circuits test normal, test or replace the CHMSL relay.
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
| ⇒ | 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. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for BCM replacement, setup, and programming |
