Troubleshooting Left Side Stop and Turn Signal Light Not Working on a Flatbed Trailer

Updated 06/06/2011 | Published 06/01/2011

Question:

I am using a 2002 F250 4x4 towing a flat bed trailer. The left stop / turn light stopped working. The running light works. Thought it was the bulb. This is the strangest thing Ive ever seen. Tried several lights, none worked. Switched with the right side and bulb works as expected. Putting a meter on the lead to the light I have 12 v steady for the parking light and the up and down 12v for the turn. Yet when I plug the light back in nothing, except the parking light. The running lights across the back of the trailer work as well as the right side. These are all down the line pass the left light. If Im getting power why does the light not work? Im really stumped on this. Any ideas?

asked by: Fred L

Expert Reply:

My first reaction is a weak ground connection for the left side light. If the left turn and brake signal are on a separate circuit but a ground connection that is not sufficient, a number of things can go wrong, including one of the lights functions not working, while testing shows 12 volts power.

I had this same issue on a utility trailer some time ago also, and it ended up being a frayed wire that was just barely making contact to the trailer frame. In my case the issue was intermittent and took quite a while to run down.

Check the grounds for your trailer lights, and make sure that they are connected to clean corrosion free parts of the frame. Then, go over the wiring for any rubbed or nicked spot. Finally, check the trailer and vehicle side connectors for corrosion that may be bridging the gap between two or more circuits causing the issue.

expert reply by: Robert G