Troubleshooting Trailer Lighting that Stops Functioning When Headlights are On

Updated 06/03/2019 | Published 05/31/2019

Question:

I have a 2009 landrover discovery 4 and when towing caravan it is fine till lights turned on and the brakes stop vehicle from moving . The caravan has std flat 7 pin male plug into shop bought flat to round adaptor to the car. I have disconnected the blue serv. brake wire in the caravan plug and this has fixed the problem but I assume I now have no trailer brakes. Can you help with my next move.

asked by: Chris S

Expert Reply:

Your main problem is an insufficient ground for the trailer wiring. Try improving the connection of the main ground wire of the trailer wiring harness. Make sure it is attached to a clean metal surface. Or you can run all of the trailer wires back to the main ground. This is actually the ideal solution. But you can also check the current grounds for the lights to make sure they are making a solid connection to a clean metal surface. When the vehicle headlights and trailer running lights are on you are asking the wiring of the setup to draw it's most amperage.

What you described is the classic insufficient ground scenario so finding a way to improve the grounds will fix the problem.

expert reply by: Jameson C