Troubleshooting Cause for Truck Brake Lights to Fail After Plugging in Trailer

Updated 07/19/2017 | Published 07/17/2017

Question:

I hooked my truck up to the trailer and plugged in the power to my truck. everything works except my brake lights. I unhooked it and now I have no brake lights on my truck.

asked by: Chad

Expert Reply:

It sounds like there may be a short on the trailer or elsewhere in the truck's overall wiring that took out your brake light fuse. Check for a blown brake light fuse first thing.

Next inspect both the truck socket and the trailer plug for any signs of damage, corrosion or trapped foreign material. Corrosion can build up even inside the trailer plug or the truck socket so disconnect any connectors you can so you can check inside them for anything like corrosive build-up, trapped dirt, a strand of loose wire or anything else that could cause the brake light circuit to become bridged to another circuit.

On a 7-way set-up (see photo please) the 12V auxiliary power and the right brake/turn light circuit are right next to each other. If corrosion or another cause allowed those two circuits to become bridged together the result could be a blown brake light fuse.

Carefully check all trailer wiring, from the plug all the way back to each light, looking for signs of worn insulation or other damage to the wiring and repair or replace any damaged items.

expert reply by: Adam R