Troubleshooting Trailer Lights that Stop Functioning When Running Lights Come On

Updated 09/10/2014 | Published 09/09/2014

Question:

Ok so basically what is happening is I have a boat trailer and I got the blinkers and brakes to work, but the running lights dont work. Also if the running lights are switched on neither the blinkers nor the brake lights work. I just got this boat and the problem started with the previous owner who could only get the lights to work on his wifes trailblazer. Same happened when i went to get the boat from him. No lights from my 2010 Honda accord, or 2001 xterra. Harnesses show power and other trailers work. So now thats just some background. Also when I take my multimeter and try to test the wire for the running lights it blows the fuse. All I did to get the blinkers and brakes to work was put a new flat 4 end on my xterra, but the problem still persists that it blows the fuse for the running lights when I use my multimeter. The blinker circuits test fine without blowing.

asked by: Jacob B

Helpful Expert Reply:

Sounds like your trailer lights have weak grounds. The behavior you described is textbook insufficient grounding. What you will want to do is check the grounds for the trailer taillights, make sure they are attached to a clean metal surface. Also, make sure the main ground from the trailer's wiring connector is attached to a clean metal surface on your trailer frame.

I attached an FAQ article on diagnosing 4-way installs for you to check out plus an FAQ article on trailer wiring.

expert reply by: Jameson C