Trailer Brake Lights Do Not Work when Headlights Are On
Updated 10/16/2024 | Published 06/12/2013
Question:
All lights function properly except when the headlights are on and I brake, then there are no trailer lights. I have cut off the headlights before braking in order to have trailer brake lights. Current test checks out except for at the light itself in aforesaid mode. Please tell me what is wrong? thanks
asked by: Stephen B
Expert Reply:
It sounds as if there is a weak ground connection somewhere on the trailer. Ground issues normally show up when there is more load on a system. Your situation is a perfect example of that. With the running lights on already drawing power, stepping on the brakes adds even more draw on the system.
First, to isolate the issue, disconnect the trailer from the truck. Then use a circuit tester, like # PTW2993 if you do not already have one, to test the connector on the truck. Have someone run through the functions with and without the headlights on while you test the connector with the circuit tester.
If the issue occurs on the truck without the trailer connected then you will want to look at the ground connection to make sure it is attached to a clean and corrosion free bare metal surface. Also make sure the pins are clean and free of corrosion.
On the trailer side, make sure the main trailer connector ground is attached to a clean metal surface and clean out the connector to get rid or dirt and corrosion. You will also need to make sure the ground for each light is attached to a clean bare metal surface.
If the trailer has an aluminum frame, tilt bed, or folding tongue you will want to run jumper ground wires from each light to the main trailer connector ground. I have included a link to our FAQ article on wiring for you.
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Dwayne
10/16/2024
Thank you, sir, for the advice. You helped me solve a problem that I have been working on for two days. I had one turn signal /brake light that was not working while the tow vehicles headlights were on. After no change with cleaning the ground, I drilled out the screw holding in the ground wire and replaced it. The drilling exposed clean trailer metal and I attached the ground wire. The lights work perfect now.