Troubleshooting Trailer Lights not Working After Installing a New Wiring Harness

Updated 04/24/2010 | Published 04/22/2010

Question:

I just replaced an old wiring harness with a new one on a small utility trailer. WHen plugged into my existing 4 pole connector on a 2007 Honda Pilot, no trailer lights illuminate. I drilled a new hole and connected the ground wire but still no joy. To see if the new lights were faulty, I connected a battery independently to the left rear light wires, and all the lights came on. What should I look for to determine the problem? When I use a tester on the trailer, it illuminates even when placed on the metal sides of the trailer.

asked by: Kirk L

Expert Reply:

To me, it sounds like you still have a ground and/or short problem on the trailer. The taillights are usually grounded via the mounting studs on the assembly. Check to make sure you have a proper ground. I have included a link to a video showing the procedure you can follow below.

You will also need to check to make sure that the connections are clean and free of corrosion as this could also cause a short or ground problem.

Finally, you will also need to check that the new harness is functioning properly using a circuit tester like # PTW2993. You will also need to check the trailer wiring for any bad spots (like breaks in the wires). To do this, you will need to check the wires at various spots from beginning to end. I have included another video on circuit testing.

expert reply by: Michael H