Pinpoint Test B: The Trailer Brakes Are Always Engaged

WARNING: This page is about a different car, the 2021 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 Operation 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 TRM gain setting, the higher the duty cycle the more braking power available. Without a trailer connected, the TRM sends the PWM signal to the trailer tow connector every 4 seconds as a trailer connectivity test. The TRM 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 the ABS module to the BCM. The BCM then sends the signal over the HS-CAN to the TRM. The TRM 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 TRM, the TRM enters a limited operating mode. The TRM receives fused battery voltage from the BCMC.

A short to voltage on the PWM signal output circuit, or incorrect messages from other modules can cause the TRM to engage the trailer brakes.

REFER to: Auxiliary Brake System - System Operation and Component Description .

Possible Sources
- Wiring, terminals or connectors
- TRM
- Trailer
Visual Inspection and Pre-checks
- Make sure BCMC fuse 42 (30A) is OK.
- Make sure BCM fuse 21 (5A) is OK.
- Make sure the trailer brakes are electric-magnet actuated drum type brakes.
NOTE: Use the correct probe adapter(s) when making measurements. Failure to use the correct probe adapter(s) may cause damage to the connector.

B1 CHECK FOR BCM (BODY CONTROL MODULE) DIAGNOSTIC TROUBLE CODES (DTCS)

NOTE: The TRM is a combined TBM and Trailer Lighting Module.
- Ignition ON.
- Using a diagnostic scan tool, carry out the BCM self-test.
Are there any Diagnostic Trouble Codes (DTCs) present in the BCM?

Yes DIAGNOSE all BCM Diagnostic Trouble Codes (DTCs). REFER to: Body Control Module (BCM) .

No GO to B2

B2 CHECK FOR ABS (ANTI-LOCK BRAKE SYSTEM) MODULE DIAGNOSTIC TROUBLE CODES (DTCS)
- Using a diagnostic scan tool, carry out the ABS module self-test.
Are there any Diagnostic Trouble Codes (DTCs) present in the ABS module?

Yes DIAGNOSE all ABS module Diagnostic Trouble Codes (DTCs). REFER to: Anti-Lock Brake System (ABS) and Stability Control .

No GO to B3

B3 CHECK FOR TBM (TRAILER BRAKE CONTROL MODULE) DIAGNOSTIC TROUBLE CODES (DTCS)

Using a diagnostic scan tool, carry out the TBM self-test.

Does the TBM communicate with the diagnostic scan tool?

Yes	If there are Diagnostic Trouble Codes (DTCs) in the TBM, DIAGNOSE all TBM Diagnostic Trouble Codes (DTCs). GO to the DTC Chart: TBM in Diagnostic Trouble Code (DTC) Chart . If there are no Diagnostic Trouble Codes (DTCs) in the TBM, GO to B4

No	DIAGNOSE the TBM does not communicate with the diagnostic scan tool. REFER to: ETHERNET MODULE COMMUNICATIONS NETWORK .

B4 CHECK THE BCM (BODY CONTROL MODULE) BRK_PDL_SW2 PID (PARAMETER IDENTIFICATION)
- Using a diagnostic scan tool,
  Access the BCM and monitor the BRK_PDL_SW2PIDwhile pressing and releasing the brake pedal.
Does the BRK_PDL_SW2 PID display ON with the brake pedal pressed and OFF with the brake pedal released?

Yes GO to B5

No DIAGNOSE the vehicle stoplamps. REFER to: Stoplamps .

B5 CHECK THE TBM (TRAILER BRAKE CONTROL MODULE) BRK_PDL_SW2 PID (PARAMETER IDENTIFICATION)
- Using a diagnostic scan tool,
  Access the TBM and monitor the BRK_PDL_SW2PIDwhile pressing and releasing the brake pedal.
Does the BRK_PDL_SW2 PID display ON with the brake pedal pressed and OFF with the brake pedal released?

Yes GO to B6

No REFER to: Anti-Lock Brake System (ABS) and Stability Control .

B6 CHECK THE TRM (TRAILER MODULE) OUTPUT

NOTE: Every 4 seconds a voltage pulse is output by the TRM as a trailer connectivity test.

Ignition ON.

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

Measure:

Positive Lead	Measurement / Action	Negative Lead
C4099-3	Courtesy of FORD MOTOR COMPANY	Ground

Is a steady voltage present?

Yes

GO to B7

No	The condition that caused the concern is not present at this time. The concern is intermittent and may have been caused by a loose or corroded connector, or an excessive load on the HS-CAN3.

B7 CHECK THE TRM (TRAILER MODULE) PWM (PULSE WIDTH MODULATION) OUTPUT CIRCUIT FOR A SHORT TO VOLTAGE

Ignition OFF.

Disconnect TRM C2498C.

Ignition ON.

Measure:

Positive Lead	Measurement / Action	Negative Lead
C4099-3	Courtesy of FORD MOTOR COMPANY	Ground

Is any voltage present?

Yes	REPAIR the circuit.

GO to B8

B8 VERIFY ALL WIRING CONNECTIONS

Ignition OFF.

Using a good light source, inspect TRM C2498C and the 7 way trailer tow connector for the following:
- corrosion - install new connector or terminal and clean the module pins
- damaged or bent pins - install new terminals or pins
- pushed-out pins - install new pins as necessary
- spread terminals - install new terminals as necessary

Are the connectors free of corrosion, damaged pins, bent pins, pushed-out pins and spread terminals?

Yes

GO to B9

No	REPAIR the affected connectors or terminals. Refer to OEM CONNECTOR REPAIR PROCEDURES for schematic and connector information.

B9 CHECK FOR CORRECT TRM (TRAILER MODULE) OPERATION

Connect TRM C2498C, make sure it seats and latches correctly.

Operate the system and verify the concern is still present.

Is the concern still present?

Yes	CHECK OASIS for any applicable service articles: TSB, GSB, SSM or FSA. If a service article exists for this concern, DISCONTINUE this test and FOLLOW the service article instructions. If no service articles address this concern, INSTALL a new TRM module. REFER to: Trailer Module (TRM) .

No	The condition that caused the concern is not present at this time. The concern is intermittent and may have been caused by a loose or corroded connector, low battery voltage or an excessive load on the HS-CAN3.