Troubleshooting Vehicle Side Trailer Wiring Fuses and Relays on 2010 Ford F-150 XLT 4.6L

Question:

I am only getting right brake/ blinker light on my trailer. I replaced the wire harness and a blown fuse for trailer parking lights and still not working. I hooked it up to my dads pickup and they all work fine. I also switched out relays and that didn’t work either. Any ideas???

asked by: Dave V

Expert Reply:

I recommend starting where it appears you have, and in a specific order. There is one fuse location and two relay locations you will want to check in the power distribution box in the engine compartment. First, check the 20-amp fuse in location 45. This is the Trailer tow stop turn relay feed.

If that checks out fine, then check the left-hand stop/turn relay in location 37 and the right-hand stop/turn relay in location 38. The best way to test a relay is to replace the existing relays with relays that you know are in good working order (which it seems you already know).

If the fuses and relays seem okay, then I would use a circuit tester, like part # PTW2992 and test the power output at the trailer connector on your truck. I have attached a video that shows how to test a trailer connector. If you do not have voltage for the right turn signals and right brake at the trailer connector, then I recommend taking the truck to your local Ford dealer to have them inspect the tow package wiring.

If you have power at the trailer connector, then that tells us the issue is on the trailer. The most likely culprit at this point would be a poor ground on the trailer which may allow some vehicles to work and others not to base on the vehicle sensitivity to ground problems. I would inspect the main trailer ground and the grounds for each of the lights. Make sure they are rust and corrosion free. You may want to run the taillight grounds back to the main trailer ground for the best connection.

expert reply by: Jason S