Troubleshooting Trailer Taillights that All Go Out when Headlights and Brakes are Applied

Published 10/29/2015

Question:

All lights working fine when I dont have my running lights on. When I turn on running lights they work, right turn,left turn but when press breaks all lights go out

asked by: Steven

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.

expert reply by: Jameson C