Let’s start by me saying I’m an idiot.

I have a 2019 f150 STX 2.7. I had a trailer brake installed and it worked perfect. I pulled my camper and when I got to the spot, I plugged in the camper to shore power while the 7pin was still hooked up. I have been camping for years, and this was a bonehead move and I just forgot. The cab began to smell like burnt plastic and wire. After using a multi meter, we found that the TCM is sending a constant 12v to the brake controller which locks up the brakes. So I took the brake controller off and replaced it. I am still getting 12v to the 7pin when the car is running and the camper is not hooked up. The fuse never blew, but the controller was smoked. The fuse is still good, and I have a new controller, what do I do? There must be a short somewhere but how do I find it?

Fried TCM

 

Clarification, because there's no module called a TCM...

The BCM (Body Control Module) sends brake level information to the TBC (Trailer Brake Controller), also referred to as the ITBC. The BJB (Battery Junction Box) supplies power to the TBM via a 30A fuse.

I'd disconnect the TBC at the dash and do a cross-continuity check off all seven pins at the bumper. It's possible you fried the wiring and earned a short before you fried that output transistor.

 

So would this be something as simple as just running new wires? Or would it just need a new Blue wire ran?

 

I'm guessing he really means the TRM since that is what the 7-way plug is attached to.



Edit: Maybe he means the TBM.

 

So the other part of this is the 7pin still charges the camper, blinkers still all operational, but the brake is 100% engaged. That’s why I feel like it may be a simple rewire? All the fuses are 100%. #24 under the hood and #4 by the passenger right foot.

 

Remember there is no power to the Trailer unless the brakes are applied, turn signal on, 4 ways on, parking/head lights are on or in reverse, power is applied to the center pin of the connector.
Battery charge may always present on one lead with ignition on.
What lead is it for the Brakes and test for battery into the truck harness and where is comes from.
Is the issue the trailer battery power switch operated or faulty or wiring activating the Trailor brakes?
You have to know this before any repair is made.

 

As Bluegrass said (this is not an exact quote) there should be no voltages on any of the pins if the ignition is off and all of the truck lights are off. I am assuming your using all the factory wiring harness including to a Ford's OEM trailer connector and a Ford OEM brake controller. If you see voltage on one pin all the time it's not wired as my 2018 truck is for sure.

Now if your using an after market trailer connector could there be a wiring reversal between pin 2 & 4 in the truck or trailer wiring? In my 1973 Airstream trailer with the flat pins, not the old round pin connector, Pin 4 is the 12V charging connection and pin 2 is the trailer brake connector. If those were reversed the truck's 12V would be applying the brakes and the Trailer's 12V battery (or trailer's 120V powered charging system) could be frying the brake controller. Think about that for a while, make some measurements. It kind of sounds like the rest of your lighting is OK, running lights, left & right turn signals, back up light if tested.. The only other wires left is the ground wire in my trailer's case on pin 1 and the backup lights are on pin 7. Just a theory, my head is starting to spin . Good luck in your search

 

I did mean the TBM, since the issue is he's got 12v on the brake wire at all times.

He should disconnect both the TRM and the TBC, then cross-check all seven pins at the bumper. If they comes up clean, he know the issue lies in the TBM or BCM.

Disconnecting the BOO input at the TBM would then rule out or verify that module being the issue.