Originally Posted by DeclansDaddy
This seems to be exactly what happens I just don't know why.
Check the voltage coning out of the back of the controller when the trailer is hooked up and the brakes applied, usually this is the blue wire. If the voltage does not
drop there but is dropping at the trailer plug then you have a problem with your wiring between the controller and the plug at the rear of the truck.
If the voltage does drop on the blue wire at the back of the controller when the brakes are applied, check the voltage going into the back of the controller with the brakes applied, should be black. If the voltage on the black wire drops then you have a problem with the power supply wiring to the controller.
If the voltage does not drop on the black wire but drops on the blue wire when the brakes are applied then this is internal to the controller and may be by design. You may have to actually be moving and decelerating to a stop for the controller to sense deceleration and apply full power to the brakes.