Pinpoint Test C: The Trailer Brakes are Always Engaged

Auxiliary Brake System - Trailer Brake Control (TBC) Module

Pinpoint Test C: The Trailer Brakes are Always Engaged

NOTE: Carry out all pinpoint tests without a trailer connected to the vehicle.

Normal Operation

The braking energy provided to the trailer is varied with a Pulse Width Modulated (PWM) signal that switches between 0 volts and battery voltage, the higher the duty cycle the more braking power available. With the vehicle stationary and the manual slider switch fully to the left with a gain of 10, there should be more than 10-12 volts supplied to pin 3 of the 7-pin trailer tow connector. The Trailer Brake Control (TBC) module also receives information from the brake pressure switch (also called the speed control deactivation switch) and the stoplamp switch from the PCM over the High Speed Controller Area Network (HS-CAN).

This pinpoint test is intended to diagnose the following:

- Wiring, terminals or connectors

- TBC (Trailer Brake Control) module

- Customer trailer

PINPOINT TEST C : THE TRAILER BRAKES ARE ALWAYS ENGAGED

NOTICE: Use the Flex Probe Kit for all test connections to prevent damage to the wiring terminals. Do not use standard multi-meter probes.

C1 CHECK FOR ANY PCM DTCs

- Ignition ON.

- Enter the following diagnostic mode on the scan tool: PCM Key ON Engine OFF (KOEO) Self-Test.

- Retrieve and record any PCM DTCs.

Are any PCM DTCs retrieved?

Yes
REFER to Computers and Control Systems.
No
GO to C2.

C2 CHECK FOR ANY TBC (Trailer Brake Control) MODULE DTCs

- Enter the following diagnostic mode on the scan tool: TBC (Trailer Brake Control) Module Self-Test.

- Retrieve and record any TBC (Trailer Brake Control) module DTCs.

Are any TBC (Trailer Brake Control) module DTCs retrieved?

Yes
REFER to DTC Charts Trailer Brake Control Module.
No
GO to C3.

C3 OBSERVE THE VEHICLE STOPLAMP OPERATION

- Monitor the vehicle stoplamps while pressing and releasing the brake pedal.

Do the vehicle stoplamps operate correctly?

Yes
GO to C4.
No
REFER to Lighting and Horns to diagnose the stoplamp switch.

C4 CHECK THE TBC (Trailer Brake Control) MODULE OUTPUT
NOTE: Every 4 seconds a voltage pulse is applied as a trailer connectivity test.

- Ignition ON.

- Make sure the brake pedal is not pressed and the manual slider switch is not being applied.

- Measure the voltage between 7-pin trailer tow connector electric brake supply pin, component side and ground.






Is the voltage approximately 0.3-0.4 volt?

Yes
The TBC (Trailer Brake Control) module and vehicle wiring are operating correctly. SEND the trailer to an authorized camper/trailer repair center.
No
GO to C5.

C5 CHECK THE TBC (Trailer Brake Control) MODULE PWM (Pulse Width Modulated) OUTPUT CIRCUIT FOR A SHORT TO VOLTAGE

- Ignition OFF.

- Disconnect: TBC (Trailer Brake Control) Module C2142.

- Ignition ON.

- Measure the voltage between 7-pin trailer tow connector electric brake supply pin, component side and ground.






Is any voltage present?

Yes
REPAIR circuit CAT19 (BU). TEST the system for normal operation.
No
GO to C6.

C6 CHECK FOR CORRECT TBC (Trailer Brake Control) MODULE OPERATION

- Disconnect the TBC (Trailer Brake Control) module connector.

- Check for:
- corrosion

- damaged pins

- pushed-out pins


- Connect the TBC (Trailer Brake Control) module connector and make sure it seats correctly.

- Operate the system and verify the concern is still present.

Is the concern still present?

Yes
INSTALL a new TBC (Trailer Brake Control) module. REFER to Trailer Brake Control (TBC) Module . TEST the system for normal operation.
No
The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. TEST the system for normal operation.