Troubleshooting Weak Ground Signal on Trailer Pulled By a 2006 GMC Truck

Question:

I have power to all the lights wire to the trailer. I have the two LED running/stop/ signal lights. The harness from the truck to the rrailer match. the only thing I noticed is that the tester show a very week ground when attached to the trailer....the trailer is hooked up to the truck. Now when I use the truck as a ground the tester is bright when I attach the tester to the trailer it shows a dimmer light on the tester. Why??? Im thinking I may not have a strong enough ground connection from the truck and the trailer. Your thoughts please. 206 GMC trailer lights not working Thanks

asked by: Joe

Expert Reply:

To determine why your tester is showing a dimmer light, start with a check of all your ground wires to verify that they are securely attached to a clean, bare metal surface. These checks include the ground connector on the tow vehicle, the ground connector on the trailer, and the ground connection at each tail light assembly. Also check the condition of the wiring in between. Inspect all of the connector contacts for signs of even slight greenish or whitish discoloration. Either of those colors can indicate corrosion and they will need to be cleaned.

My other thought is that your trailer may have a short in the wiring. If the short is small enough some signal could still be passing through which would also give you a dimmer reading on your tester. To check for the short you will need to visually inspect the wiring for visible copper strands, cracks or wear spots in the insulation. Pay particular attention to any points where the wires might be rubbing against the trailer frame. The problem could be as simple as a tiny nick in the wire that is not easily visible. To fix the short you can either splice out that section of wire and replace it or mend the damaged area with electrical tape or shrink tubing.

If your trailer happens to be either a tilt trailer or a foldaway tongue trailer, the hinges do not always make full contact which will affect your ground as well.

If your trailer is aluminum make sure that your grounds are connected to the main trailer ground. Aluminum is not a good conductor of electricity and therefore will not ground well. With aluminum trailers the ground wires should be run from the lights to the main trailer ground.

As a side note, if you put a dab of dielectric grease (such as # 11755) at your grounding points it can extend the longevity of the ground connection. Dielectric grease also helps keep moisture out of electrical connections and helps prevent corrosion of the contact surfaces.

I have attached three FAQ articles. One covers troubleshooting for wiring installations, the one titled trailer wiring diagrams has a complete section at the bottom referencing grounding issues and one provides a review of dielectric grease.

expert reply by: Lori W