All Trailer Lights Work Except Left Brake Light and Rear Marker Light

Question:

all lights work except the left rear brake light and the rear marker light

asked by: Jon H

Helpful Expert Reply:

Any time most of the lights are working but others are not it is indicative of a ground problem. The first step in testing the system is to disconnect the trailer. Then use a circuit tester such as # PTW2992 to test the trailer connector on the vehicle while someone sits in the cab and runs through each light function.

You will want to perform this test twice: once with just the vehicle side connector and again with any sort of adapter plugged in (such as # 37385).

If testing just the connector you find that something is not working there are a few common problems that could cause the issue. First if there is corrosion build up and/or dirt in the connector then it will not function properly. Clean out the connector inside and out and re-test.

If there are still problems then there is either an issue with the wiring to the connector of there is a blown fuse or relay in the tow package wiring. You can examine the wiring and fix any loose or frayed connections. Use the vehicle owners manual to find the locations of fuses/relays to check them. The best way to check a relay is to use a known good one in its place.

After testing the vehicle connector by itself you can then test with an adapter plugged in if you are using one. If the vehicle side connector is working but the adapter is not then there is a problem inside the adapter and it will need to be replaced.

On the trailer side make sure that its connector is clean and corrosion free inside and out. Then make sure the main ground is attached to a clean and corrosion free bare metal surface. Then you will want to do the same for each of the trailer lights.

You can also inspect the trailer wiring and fix any loose or frayed wires.

If the trailer has an aluminum frame, tilt bed, folding tongue, or if you recently added a plastic license plate holder between a light and its ground then these things can interrupt the ground connection. Running jumper wires from the light grounds to the main trailer connector ground can help.

I have included a couple of helpful articles on testing wiring for you.