Brake Light Fuse Keeps Blowing After Installing a Trailer Wiring Harness

Updated 08/13/2020 | Published 04/01/2015

Question:

Ok I have no brake lights I checked the fuses and found it was blown so I replaced it and every time you apply the brake it blows the fuse strait away what is this and how do I fix it please and what would make this happen all of a sudden as I havent been towing a trailer

asked by: Sonia

Expert Reply:

What you are describing is a classic short circuit. Two wires touching or a wiring touching bare metal, a wire attached to the wrong location, and even corrosion can cause over draw on the circuit and blow that fuse.

If it were to happen only when connected to a trailer then the short would be on the trailer side. You would have to check the trailer connectors to make sure they are clean and free of corrosion, check the wiring for any pinched, frayed, or otherwise damaged wires, and make sure that the wiring is going to the correct lights.

I have included a couple of links that should also help.

expert reply by: Michael H

Erik P.

6/20/2020

I have a 2013 F-150 that keeps blowing the trailer signal/brake fuse under the hood when I have my "brand new" trailer plugged in and apply the brake. It's the round 7 pin plug. Signals and running lights work fine until you touch the petal and it blows the fuse. Then no signal or brake lights on trailer. I even hooked up to a different brand new trailer and same thing happened and I've crawled all under my trailer checking for bare wires, etc... Replaced the fuse and checked plug on truck with test light and everything works as it shld. I'm stumped and wld appreciate any advice/help. Thanks!

Chris R.

6/24/2020

We at least know the issue is on the truck. Is the 7-Way harness new too or just the trailer? Have you recently installed a trailer brake controller? I'm just trying to figure out what could be shorting out that circuit.

Doug

8/4/2020

I'm having the same problem. A trailer I've used before without problem, when I attach it to a new (2nd hand) vehicle and hot the brakes a fuse blows and the brake lights no longer work on the vehicle - nor obviously on the trailer. Multimeter shows nothing amiss when the trailer is not attached. Thanks!

Chris R.

8/13/2020

There has to be a short somewhere with the wiring on the trailer. You will need to inspect the brake light wiring from the 7-Way all the way back to the tail lights and look for any exposed sections that could be touching a nearby wire or bare metal, fixing/replacing as needed.