Troubleshooting Trailer Brake Lights not Working When Head Lights are On

Question:

During the day the trailer lights work perfectly car headlights are off When the headlights are on the light rear brake light and turn signal go out on the trailer. I checked the voltage and there is a significant voltage at this spot. What is the issue?

asked by: Gary

Expert Reply:

It sounds like you might have a bad ground connection. When the headlights are on, that is when there is the most power draw on the system. Stepping on the brakes would then add more and this is when a ground problem will usually show up.

The taillights usually ground through the mounting bolts on trailers. Check for a bad ground, loose connection, and dirt and corrosion. Any of those can cause this problem. Also check the light bracket connections to the trailer frame as lights can ground through the mounting studs.

If you used a plastic license plate frame or bracket behind the left taillight instead of a metal one, that could be causing the problem as well.

I have included a video on the procedure for checking for a proper ground.

There is also an FAQ article on trailer wiring. Near the end of the page, after the plug diagram section, there is a section on weak grounds. I have included a link to this article below.

expert reply by: Jameson C