Pinpoint Test B : The Trailer Brakes Are Always Engaged: Notes

WARNING: This page is about a different car, the 2020 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.

Refer to Trailer/Camper Adapter for schematic and connector information

Normal Operations and Fault Conditions

The braking energy provided to the trailer is modulated with a PWM signal which varies between 0 volts and battery voltage, according to the TBM gain setting, the higher the duty cycle the more braking power available. Without a trailer connected, the TBM sends the PWM signal to the trailer tow connector every 4 seconds as a trailer connectivity test. The TBM varies the PWM signal based on various inputs such as vehicle speed, the brake torque message, BPP switch input, the manual slider switch and the gain buttons.

When the brake pedal is pressed, the normally-open BPP switch closes and sends voltage from BCM fuse 14 (10A) to BCM fuse 4 (5A). The voltage passes through the BCM fuse and continues to the TBM. The TBM receives additional information from the ABS module over the HS-CAN. If there is a concern in the ABS module preventing this information from being sent to the TBM, the TBM enters a limited operating mode. The TBM receives fused battery voltage from the BJB.

A short to voltage on the PWM signal output circuit, a short to voltage on the TBM BPP switch input circuit or incorrect messages from other modules can cause the TBM to engage the trailer brakes.

Possible Sources

Wiring, terminals or connectors
TBM
Trailer

Visual Inspection and Diagnostic Pre-checks

Make sure BJB fuse 49 (30A) is OK.
Make sure BCM fuse 4 (5A) is OK.
Make sure the trailer brakes are electric-magnet actuated drum type brakes.