4 pin trailer lights. The tail lights work fine by themselves. Both turn signals also work just fine. Brake lights also work fine. The problem is when the tail lights are on and you apply the brakes the tail lights go out and no brake lights work. Everything use to work just fine. I'm lost at what could cause this. The only thing I can think of is possible bad ground.

Any help would be appreciated.

Thanks

 

I would say it is a poor ground.
It's ok passing return current of two lamps but not when four are on at the same time.
You will most likley see the tail lights are brighter by themselves after you find and fix the fault.

 

I'm having the same problem with my trailer brake lights not working. Would the short be on the trailer or on the truck or what? Same problem whether I use the 4-pin or 7-pin plug.

 

Had a similar problem with my boat trailer. It was a bad ground at the light.

 

I had the same issue with my trailer and it was a bad ground. Also check to make sure that none of your wires have split and are touching the metal of the trailer.

 

Before we all dive too deep into "olympic cause jumping," I'd suggest a little sanity. First, you have to find out if the problem is in the truck or the trailer. A test light will do, or you can buy plug in testers from your local Mac or Snap-on guy. (Some auto parts stores also cary them.) I built one myself many years ago that plugs into both the truck and trailer. (Think in-line tester.) It's saved my bacon hunderds of times. Yes, a bad ground is a fairly common problem with trailer wiring, but let's narrow the field a bit more first.

 

Tilt trailers are bad about not passing the ground from the tongue to the body of the trailer. Bond the two togeter at the hinge point.