Troubleshooting Trailer Wiring That Stops Working with Vehicle Headlights On

Question:

When I hook up my boat trailer with headlights off, turn signals operate correctly. With headlights on the turn signals dont change/flash enough. I can see the second filament flashing on and off but the overall light change is barely visible.

asked by: Tom

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