Reason Why Turning On Headlights/Running Lights Causes Problems with Trailer Lights

Question:

So when the trucks running and the trailers plugged in the brake lights work the turning signals work same with the hazards. But if I turn on the running lights all the light light up full beam and and no brakes nor any signals turn the head lights of everything works again? What do you recon I should look at first?

asked by: Cory M

Expert Reply:

When a trailer's lights function properly until the headlights are turned on the cause is almost always a weak ground connection either on the vehicle or on the trailer. Since running lights remain on continuously, unlike turn signals and brake lights which are usually on only momentarily, these tend to show up any weakness in the ground.

Those other light functions will work since they don't draw as much current but once the additional continuous load of the running lights is added to the mix then often the ground just can't handle that extra current.

Find your trailer's main ground wire where it connects to the frame, usually with a ring terminal. Remove it and sand off that spot to remove any paint, primer, rust, grease or dirt that could be blocking the ground connection. Usually this takes care of it. You can protect that grounding point from future problems by applying dielectric grease like # 11755.

expert reply by: Adam R