Troubleshooting Trailer Wiring with 4 Pins but 5 Wires With Trailer Lights Not Working

Question:

My mechanic installed this Curt 4-wire system on my 2012 Volkswagen CC. My utility trailer also has a 4-pin connector, but 5 wires are coming out of it. Juice is definitely coming out of the car tested with a wire tester, but none of the lights work on the trailer. I attached pictures of each. Do I need a converter of some kind?

asked by: Attila

Expert Reply:

Thanks for the photo. The harness on your trailer is known as a wishbone harness and simply has the running lights split from side to side so you don't have a jump wire from one side of the trailer to the other. This allows a clean wiring from your trailer tongue to each tail light.

Because you are able to get power from the vehicle but the trailer lights don't work, there is some simple troubleshooting you can do. First, I recommend cleaning the terminals on your trailer connector with a little sandpaper to ensure you have a clean connection to your vehicle. Then use some dielectric grease # 11755 to keep moisture and debris out of your connection.

Next, I recommend checking your trailer ground wire to make sure you have a bare metal to metal connection to your frame that is free of corrosion or debris. Then use a circuit tester like # PTW2993 to check each function at the connector and then at your tail lights. If you have power for each function running the entire way, then check your trailer bulbs and the ground on your lights as they often ground through the mounting hardware. Clean the ground just like the frame ground at those locations. Finally trace the wiring using a the circuit tester to find a short in the wire and replace as needed.

Finally, if you need to replace your trailer wiring, use the 35 ft. Wishbone 4-Way Trailer Wiring Harness with 42" Ground Wire # A35W42B. I've attached a video and a wiring article to assist.

expert reply by: Jason S