1997 Ford F-350 Trailer Brakes Locking Up When Harness Is Plugged In

Updated 08/19/2021 | Published 08/18/2021

Question:

I have a Gooseneck horse trailer that we pull with a 97 F-350. Never had any trouble with the trailer brakes until yesterday. When I plugged in the harness the brakes immediately engaged. I found the ground wire on the truck battery was loose, which I tightened. Nothing has changed on the truck or trailer since the last use. Without the harness plugged in, the light on the controller comes on as normal when the brakes are applied and goes off when released. Plug the harness in and the light comes on and the trailer brakes engage. Any help would be appreciated.

asked by: James G

Expert Reply:

There are a few things you can do, to diagnose the problem. Start by disconnecting the trailer from your vehicle, then use a circuit tester # PTW2979 to test the connectors on the vehicle. Go through the functions with your tester and make sure everything is working. Inspect the connectors and connections, make sure there is not any rust or corrosion built up in these areas.

If any of the pins show a constant 12 volts, and the corresponding pin on the trailer side goes to the brakes, that would be why the brakes lock up. This is especially an issue on 6-Way trailer connectors because there are two ways they could be wired: the center pin will either be brakes or 12 volt power. If the trailer does not match the brakes will lock up.

It may also be a short with the brake magnets; inspect the wiring for frays and bare wire touching metal surfaces. Also ensure the ground wires have a solid ground on bare metal free from rust and debris.

I have included a link on trailer wiring and testing trailer brake magnets for you.