Troubleshooting Trailer Running Lights-Go Out When Trailer Is Hitched

Question:

When the ball hitch is hooked up the running lights were not working. When I disconnect the ball hitch and only plug in the seven pin connector the lights work. I’m confused.

asked by: Steve P

Expert Reply:

My best advice is to try the camper on a different vehicle if at all possible and see how it behaves. If it solves the problem, you know the wiring issue is very likely somewhere in the wiring on the original vehicle. If it doesn't fit the issue, you can be fairly certain that the problem lies in the wiring on the trailer somewhere. Trying another trailer on that vehicle would also be a good idea.

Typically the problem would be the exact opposite, where the lamps won't work unless the trailer is hitched, but for some reason this still feels like a ground problem to me. Usually trailer lamps are wired using the trailer's frame as a ground wire. You might have some luck by running a separate ground wire from the lamps up to the trailer tongue where the main ground is attached. This will assure a good ground connection.

A very remote possibility is that the running lamp circuit wire in the trailer connector on the vehicle or on the trailer has become frayed and although it hasn't broken yet, it's getting ready to. Try jiggling the trailer connector harness while the trailer is hitched to see if that running lamp circuit activates for you. If it does, you've confirmed a partially broken wire.

expert reply by: Mike L