Trailer Brake Circuit Backfeeding Into Running and Left Turn Lights

Updated 09/01/2026 | Published 08/26/2026

Question:

Im working on a big tex heavy duty trailer all light work when tested with a lite tester . But when u test the breaks the running lite and left light circuit are also hot .

asked by: Douglas S

Expert Reply:

Hi Douglas,

It sounds like you have some power backfeeding into the other circuits when the brakes are activated. Since everything works normally when you test the lighting circuits, I would start by checking the trailer ground and the wiring connections at the trailer connector and any junction boxes. A weak or damaged ground can sometimes cause power to feed through other circuits and create some pretty strange results.

When you say you test the brakes, are you testing from the brake controller or applying power directly to the brake circuit? What is your year make and model? I'm assuming your testing the 7-way?

expert reply by: Heather A