Troubleshooting Trailer Wiring

Question:

I had to replace the cord for my gooseneck horse trailer after it got jerked from the wiring box. All the lights work on the trailer except the interiors now. Also, I don’t think the trailer brakes are working. The truck identifies when a trailer is attached, but it is no longer doing that either. Could I have blown some fuses? Could it be an incorrect wiring of the plug end? On the plug end, I put the red and the black wires together on their blade.

asked by: Lisa F

Expert Reply:

It seems like the wiring could be incorrect and you will have to do a bunch of troubleshooting in order to figure out exactly what the problem might be. In many cases a bad ground is to blame for this type of issue, but in order to determine the specific problem you'll need to use a circuit tester such as # PTW2993 to test the connector on your truck after disconnecting it from your trailer. Check the wiring going into your harness from your taillights to see if those wires are carrying the correct signals, if not then the issue is likely with the vehicle wiring. If the signals going to your harness are okay, you'll need to test the signals coming out of the harness, and if they are correct then your issue is likely going to be with the trailer side wiring.

The issue may also be the 12 volt circuit, so you need to check that to see if it's active. It possibly popped the fuse when it became disconnected.

As I mentioned above, an important issue you will want to take a look at are the ground connections, as they can be the cause of numerous lighting problems. You'll want to make sure your grounds are securely attached to bare metal surfaces free of any paint, dirt or corrosion of any kind. Also check your wiring thoroughly to ensure there are none that are crimped, broken or otherwise damaged.

I've added links to a pair of help articles on trailer wiring that you may also find useful.

expert reply by: Kyle S