Troubleshooting Trailer Wiring Issue When Truck and Trailer Test Fine Independently

Question:

We have a 2006 Chevy Colorado w/ a 7 blade/4 plug. The T-connector has been installed. Lights on truck work. Both the truck and trailer test fine but when we connect truck to any trailer, we have no running lights on the trailer and only have brakes/turn signal on trailer if we shut lights on the truck off. Not only have we not been able to figure it out, two local businesses had the same issue with this truck. The second said it was fixed until we went to pick up and hooked it to a trailer to be sure.

asked by: Lynn L

Expert Reply:

It sounds like you have a bad ground connection either on the vehicle connection side or on the trailer itself. It is not uncommon for the truck to test okay and the trailer to test okay individually when this is the issue. The ground is good enough to function on it's own, but fails when connected together. I recommend checking the connection on your 7-way/4-way to ensure the ground is connected bare metal to metal on the frame and is free of corrosion or debris. Then also check the trailer ground connection to make sure it is bare metal to metal and also free of corrosion or debris. At times even a thick paint can cause a ground to not work properly.

Next check each connection point to make sure there is no debris in the connections and there are no bent pins. I recommend using dielectric grease # 11755 to seal these connection to keep moisture and debris out. Finally, put a little sand paper on each side of your trailer connections to make sure there is bare metal to metal contact there and add some dielectric grease.

Let me know if you have further issues and we can continue to troubleshoot.

expert reply by: Jason S