Troubleshooting Trailer Brake Lights not Working When Running Lights are On

Updated 08/26/2010 | Published 08/25/2010

Question:

I put all new trailer lights and wireing on my Boat trailer. Lights work, turns work. I am pluging into an adapter 7 pole to 4 pole and 5 pole. When I have the lights on and put on the brake the lights go out. When I turn off the lights and put the brake on the Lights work. What am i doing wrong? The only thing I did not do that I saw in your videos is I did not sand off the paint, and rust. Could that be it? Thanks for your time and Videos Please help

asked by: Dave M

Expert Reply:

It sounds like you might have a bad ground connection. When the taillights 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: Michael H