GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra - 2WD 🢒 Brakes 🢒 Trailer Brake Controls 🢒 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 brake switch is hard wired to the trailer brake control module (TBCM). The body control module (BCM) sends a brake switch status serial data message to the TBCM. The TBCM compares the states of the brake switch signal to the serial data message after a master cylinder brake pressure sensor threshold is exceeded. If the brake switch signal and the serial data message are not in agreement or if the serial data message is not valid, a DTC will set and the trailer brake control system will be disabled.

Conditions for Running the DTC
    • Battery voltage is 9.5-17 volts.
    • The module initialization has taken place.
    • The master cylinder brake pressure exceeds threshold

Conditions for Setting the DTC
    • The brake switch serial data message is not valid.
    • 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.
    • The driver information center (DIC) will display Service Trailer Brake System.

Conditions for Clearing the DTC
    • The condition for the DTC is no longer present.
    • The TBCM clears the history DTC when a current DTC is not detected in 40 consecutive drive cycles.

Reference Information

Schematic Reference

Trailer Brake Control Schematics

Description and Operation

Trailer Brake Controls Description and Operation

Connector End View Reference

    •  Trailer Brake Control Connector End Views
    •  Engine Control Module 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

    •  Scan Tool Output Controls
    •  Scan Tool Data List

Circuit/System Verification

Ignition ON, press the brake pedal. The brake lights should illuminate and the TBCM Brake Switch Status parameter should indicate ON.

Circuit/System Testing
  1. Verify that DTC B3445, C0277, C0297, or C0890 are not set.
    2. ⇒ If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Ignition OFF, disconnect the harness connector of the TBCM.
  3. Ignition ON, test for greater than 11 volts between the IGN 1 voltage circuit terminal 15 and ground.
    4. ⇒If less than the specified voltage, test the IGN 1 voltage circuit for a short to ground or an open/high resistance. If the circuit test normal and the MISC - IGN fuse listed below is open, test the signal circuit terminal 37 for a short to ground. If the circuit tests normal, test or replace the RUN/CRANK relay.
    • Fuse 43 (Gas)
    • Fuse 40 (Diesel)
  4. If all circuits 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