Trailer Lights Work with Battery But Do Not Work when Connected to Truck

Question:

My trailer lights wont work but when I hook them up to a small battery they turn on. I took my truck and hook it up to another trailer and it operated all lights correctly so I know the problem is in the trailer. Again the lights work when hook up to a battery. The side yellow lights work though.

asked by: Jordan T

Expert Reply:

Since the lights work on a different trailer then like you said the issue must be on the trailer. To determine the issue you will first want to check the trailers connector. If there is any dirt or corrosion you will need to clean it out on the pins and where the wires attach.

Next make sure the main trailer connector ground and the grounds for all of the lights are attached to clean and corrosion free bare metal surfaces.

Then you will want to inspect the trailer wiring from the plug to the lights looking for breaks in the wiring, copper strands touching bare metal, or loose connections and fix as needed.

If the trailer has an aluminum frame, tilt bed, or folding tongue, or if you recently added a plastic license plate bracket between a light and the frame then running jumper grounds from the lights to the main trailer connector ground can help.

Another possible issue could be that if the trailer has a large number of lights that draw a lot of power it could be too much draw for the tow package wiring on the truck. To check that you would need to disconnect some of the lights and if the others start working then you know that was probably the problem.

expert reply by: Michael H

Clement

5/15/2020

I too am having the same issue. I have a brand new trailer that I’m adding side markers to. The new lights work when connected to a battery or directly to the 7 way plug but not when spliced into the existing trailer wire. All lights on the trailer including existing side markers work, just not the new lights.

Chris R.

5/18/2020

The only thing I can think of is that the new lights aren't being properly grounded. They might have a dedicated ground wire or they may simply ground through the mounting hardware - in either case make absolute sure that they're connected to a clean, debris/rust/corrosion/paint free portion of bare metal on the trailer. If this doesn't resolve the issue please let me know and we'll go from there.

Clement

5/18/2020

@ChrisR I spliced the ground into the existing ground wire.