B3894

DTC B3894

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provide an overview of each diagnostic category.



DTC Descriptor
DTC B3894 5A
- Stop Lamp Switch Circuit Plausibility Failure




Circuit/System Description

The TCC Brake Switch signal hard wired to the TBCM is normally open and closes to battery voltage when the brake pedal is applied. A separate brake switch signal is hard wired to the Body Control Module (BCM) that is normally closed to battery voltage and opens when the brake pedal is applied. The BCM sends its brake switch information to the TBCM over GMLAN serial data. The TBCM compares the states of the two brake switch signals after a Master Cylinder Brake Pressure threshold is exceeded. If the brake switch signal states are not in agreement or if the serial data brake state is not valid, a DTC will set and the ITBC system will be disabled.


Conditions for Running the DTC

* The system voltage is in the range of 9.5-17 volts.
* The module initialization has taken place.
* The master cylinder brake pressure exceeds threshold



Conditions for Setting the DTC

The actual brake switch input to the TBCM, and the serial data information for the brake switch do not match.


Action Taken When the DTC Sets

* The trailer brake control will be disabled for the remainder of the ignition cycle.
* Service Trailer Brake System, will be displayed on the driver information center (DIC).



Conditions for Clearing the MIL/DTC

Clear DTCs with a scan tool.


Diagnostic Aids

If there are any other DTCs set for the stop lamp or lighting systems, or GMLAN communication such as U0100, U0121, U0140, or U2099, diagnose them first before to resolve DTCs described in this section.


Reference Information
Schematic Reference

Trailer Brake Control Schematics ([1][2]Electrical Diagrams)
Description and Operation

Trailer Brake Controls Description and Operation (Trailer Brake Controls Description and Operation)
Connector End View Reference

Component Connector End Views
Electrical Information Reference

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

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference

Control Module References for Scan Tool Information


Circuit/System Verification

Ignition ON, press the brake pedal. The brake lights should illuminate, and scan tool data list Brake Switch Status should indicate ON.


Circuit/System Testing

1. Disconnect the stop lamp switch and test for short to ground between terminal A of the stop lamp switch connector and terminal 37 of the TBCM.

• If the circuit test normal, replace the stop lamp switch.

2. Test for short to battery positive between terminal A of the stop lamp switch connector and terminal 37 of the TBCM.

• If all circuits/connections test normal, replace the TBCM.




Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.


Control Module References for TBCM replacement, setup, and programming