How to Troubleshoot and Test Wiring on 2015 Ford F-150 When Trailer Brakes Do Not Work

Updated 05/21/2025 | Published 09/20/2017

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LCD Digital Circuit Tester

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Question:

2015 Ford F150 7 x 18 cargo trailer, electric brakes do not work! Also dome light inside trailer doesnt work. Both work fine when hooked to other trucks.

asked by: Ron

Expert Reply:

Since you have used the trailer with another vehicle and the trailer electrical functions worked properly with the other vehicle this tells us that the issue is most likely on the Ford. The two affected circuits you mentioned, the trailer brake circuit and the 12V power circuit that feeds the trailer's interior light, are totally separate, but they share the requirement of having a good ground connection in order to operate so this is the first thing I would check on the truck.

The wiring harness that connects to your 7-way socket will have a white ground wire. This white ground wire may be loose, or it may be attached to the frame at a point where there is now a bit of rust that could interfere with the connection. Primer, paint, grease and dirt also can interfere with a good ground. Bare metal-to-metal contact, protected with an application of dielectric grease like # 11755, ensures a reliable ground.

You can test your 7-way after making the new ground connection by applying a circuit tester like # PTW2992 to each of the circuits on the 7-way (one at a time). The linked video shows you use of the tester and the linked article will show you which pins to test for each function.

expert reply by: Adam R

Rob

5/18/2025

I have tested the socket on the truck and there is no continuity between the ground ( white wire) to the frame of the truck. I am going to check the wire harness ground to the frame above the spare tire. I am hoping that is the problem and clean up the connections and test again. Frustration but I think I am getting closer to the resolution to the problem.

Jameson C.

5/20/2025

@Rob Troubleshooting trailer wiring is a pain but it does sound like you are pretty close to figuring it out!

Rob

5/21/2025

@JamesonC I found the problem. The wires that connect the hubs to the electric brake are broken. That happened as a result of a blowout. Trying to find 1812 connectors but I am on the road. I am just going to use butt splices and use heat shrink on them. Took a bit but I found the problem.