Pinpoint Test A: The Trailer Brakes Are Inoperative

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 volt and battery voltage, according to the TRM gain setting, the higher the duty cycle the more braking power available. 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. With the vehicle stationary and with the manual slider switch fully to the left with a gain of 10, there should be more than 11 volts supplied to the trailer tow connector. Without a trailer connected, the TRM sends a voltage pulse to the trailer tow connector every 4 seconds as a trailer connectivity test.

An open circuit or short to ground in the PWM signal circuit, an open or short to ground in the TRMBPP switch circuit, damaged or corroded connectors, open fuses, an internal failure of the TRM, an open or short to ground in the trailer wiring can all prevent the trailer brakes from operating.

The trailer brake system only works with electric-magnet actuated drum type brakes and electric over hydraulic brakes. If the trailer is equipped with a different style of brakes or if the trailer is not equipped with brakes, the system does not function.

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

Possible Sources
- Fuse
- 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.

A1 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 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 A2

A2 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 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 A3

A3 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. If there are no Diagnostic Trouble Codes (DTCs) in the TBM, GO to A4

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

A4 CHECK THE BCM (BODY CONTROL MODULE) PARK_BRK_SW PID (PARAMETER IDENTIFICATION)
- Using a diagnostic scan tool,
  Access the BCM and monitor the PARK_BRK_SWPID
Does the PARK_BRK_SW () PID display ON with the brake pedal pressed and OFF with the brake pedal released?

Yes GO to A5

No DIAGNOSE the vehicle stoplamps. REFER to: Stoplamps .

A5 CHECK THE TBM (TRAILER BRAKE CONTROL MODULE) PARK_BRK_SW PID (PARAMETER IDENTIFICATION)
- Using a diagnostic scan tool,
  Access the TBM and monitor the PARK_BRK_SWPID
Does the PARK_BRK_SW PID display ON with the brake pedal pressed and OFF with the brake pedal released?

Yes GO to A6

No REFER to: Stoplamps .

A6 CHECK THE TRM (TRAILER MODULE) OUTPUT

NOTE: Battery voltage must be greater than 10 volts.

Ignition ON.

NOTE: If the manual slider switch is held to the full left position for more than 5 minutes, the TRM automatically shuts down.

NOTE: Do not apply the brake pedal during this test.

Have an assistant press the manual slider switch fully to the left with a gain of 10 on the TRM.

Measure:

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

Is the voltage 10 volts?

Yes	The TRM is operating correctly. SEND the trailer to an authorized camper/trailer repair center.

GO to A7

A7 CHECK THE TRM (TRAILER MODULE) PWM (PULSE WIDTH MODULATION) OUTPUT CIRCUIT FOR A SHORT TO GROUND

Measure:

Positive Lead	Measurement / Action	Negative Lead
C2498C-14	Courtesy of FORD MOTOR COMPANY	Ground

Is the resistance greater than 10, 000 ohms?

Yes

GO to A8

No	REPAIR the circuit.

A8 CHECK THE TRM (TRAILER MODULE) PWM (PULSE WIDTH MODULATION) OUTPUT CIRCUIT FOR AN OPEN

Measure:

Positive Lead	Measurement / Action	Negative Lead
C2498C-14	Courtesy of FORD MOTOR COMPANY	C4099-3

Is the resistance less than 3 ohms?

Yes

GO to A9

No	REPAIR the circuit.

A9 VERIFY ALL WIRING CONNECTIONS

Using a good light source, inspect TRM C2498C.
- 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 A10

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

A10 CHECK FOR CORRECT TBM (TRAILER BRAKE CONTROL 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.