Troubleshooting Trailer Brake Fuse Blowing When Truck Brakes are Applied

Published 11/22/2019

Question:

Hello, I am an owner operator and I’m having trouble with my trailer brake lights. I run Sandboxes in Texas for the oil fields and I use company/sand companies chassis. Recently I picked up this chassis and drove it off the sand box companies property. When I did my pre trip, the trailer brake light was working. I ran my shift not noticing the light until I did my post trip. I had their mechanics look at it at a job site and they said my fuse was blown in my fuse box on my kenworth T680. We replaced it and I worked when we tested it. When I got to the truck stop i checked it again and it was blown. I put another fuse in there, pumped my brakes a few times and it blew again. All other lights including my brake lights on my tractor are working. Never had an issue before now, and my pigtail is not frayed. More likely an issue on my tractor or an issue on their trailer? I understand electric problems can’t be diagnosed without working on the equipment, but any input is appreciated. Thank you

asked by: Nicholas C

Expert Reply:

I think that the cause will be on the trailer side. If the lights are all working properly on the tractor, and all of the other lights on the trailer are working properly, it sounds like there is short happening on the trailer side with the brake light wire. I would check all the wiring on the trailer going to the brake lights from the connector back. There may be a cut in the wire making contact with metal on the trailer causing a short, blowing the fuse.

I recommend isolating the trailer and the tractor so you can test each individually. You can use circuit tester part # PTW2993 to check the tractor's trailer connector.

When you test the trailer I recommend using a trailer tester like part # 8010. This gives you an external power source to cycle through the light functions on your trailer, and detect shorts. The tester comes in handy if you need to test trailers in the future and is a long term solution. However, if this is an isolated incident and you don't need to test anymore trailers in the future; another way to isolate the problem is to plug a different trailer into the tractor and run the lights and functions. If it does not blow the fuse then it most likely will be with the trailer side.

For testing the trailer without a tester, there are a few options. Plug a different tractor or truck into the trailer and see if the fuse blows. You can also use a car battery and hook a wire from the negative terminal to the ground on the 7 way connector. Then hook the positive wire to the remaining pins on the 7 way connector 1 at a time. This lets you know what wires are functioning properly by the lights coming on, functioning as they should. If any of the lights don't function properly then you know that the problem is somewhere between the 7 way and the lights you are testing.

expert reply by: Robert C