Trailer brakes work when trailer not on hitch but stop working when hitched

[Campcountry]

I am hoping someone can shed some light on this issue. We have been trying to get the trailer brakes working on a trailer that we salvaged. We installed a cheap brake controller in the truck and when everything is hooked up and the trailer is hitched to the truck the lights work but the brakes do not. If we unhitch the trailer from the ball (leaving the wiring connected) the brakes work. We have checked the grounds repeatedly and they seem fine.

Any ideas?

[Jerryv]

That is a strange one. Maybe one of the brake wires is shorted to the trailer frame. When the frame is not on the ball there is no path to ground except through the wires so the brakes work. When the trailer is on the ball, the frame is grounded to the truck so the current goes through the hitch bypassing the brake solenoids.

Can you measure the current in the brake wires? You should be able to unhook wires in different places and figure out where the current is going.

Jerry

[Campcountry]

The brakes work fine when hooking the brake wires at the hitch up to a battery. The brakes continue to work when the wiring harness is then plugged into the truck. Using the manual override on the controller, the light turns from green to red as it should. Then, when the trailer is hitched to the ball - the red light will not come on and the brakes do not work. The trailer lights continue to work without a problem

[Campcountry]

Support for the brake controller seems to think there may be a short at one of the brakes as well. I guess we will rewire the trailer.