Troubleshooting Trailer Running Light Circuit on 2007 Tundra That Keeps Blowing Fuses

Question:

I have a 2007 toyota tundra truck. Anytime I plug in one of my trailers it blows the fuse. I have brake lights and signal also 4 ways work. No running lights. I replaced the harness on the truck but didnt make a difference. So Im kind of stumped what to check next. Its just on my 4 pin not on my rounded connection trailer plug.

asked by: Terry V

Expert Reply:

Is this occurring with just ONE of several trailers, or with all of them? If it's just one of them- it sounds as if you've pretty much narrowed down the problem to the running lamp circuit on that one trailer. Somewhere in the wiring there's a short circuit on that running lamp feed. Best thing to do is start at the trailer's connector, and follow the running lamp circuit from the connector (should be a brown wire) all the way back to the taillights. Examine the wire closely, looking for cracks, loose connections or worn spots in the wire insulation that would allow the copper wire to short out to the trailer frame. Once the short is located and repaired, you'll be good to go. I'll link you to some of our trailer wiring articles that should prove to be helpful.

If this is occurring on all of your trailers, and you'v already replaced the wiring harness on the vehicle, the problem is most likely in the factory wiring past where the replacement harness is plugged in. Given the age of the vehicle, probably the easiest way to proceed is to find the brown wire feeding the trailer harness on the vehicle, cut it and tap it into the running light circuit for the truck's taillights. It'll save you the trouble of tracking down the short in the wiring on the vehicle, which could be just about anywhere.

expert reply by: Mike L