All Trailer Lights Go Out when Brakes Applied and Running Lights On with a 1998 GMC Jimmy

Question:

I have a 98 gmc Jimmy with automatic lights at night. When my truck light are off during the day the lights on the trailer work fine. At night when my truck lights are on the trailer lights are on as well, but when I hit the breaks all if my lights on trailer shut off includ the side markers. I am using a four wire flat plug.

asked by: Kyle Y

Expert Reply:

This sounds like a short, overdraw situation, or a bad ground. The first thing I recommend is to check the vehicle and trailer side connectors and make sure they are clean and free of corrosion. On the trailer side check the wiring from front to back and fix any pinched or damaged wires.

On the vehicle side you can test the functionality of the 4-Way using a circuit tester suchPTW2993 as # 40376 and following the help article I have linked for you.

If the trailer has a lot of running lights, then that could be the issue. Too many lights can overdraw tow package wiring. To test this, you can disconnect some of the lights that operate off the running light circuit. If everything starts working normally then it was likely an overdraw situation and going to LED lights would fix that issue.

Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

expert reply by: Michael H