Are you testing the voltage when the vehicle is at a stand still? If so you should note that since the controller is proportional it needs to feel a change in inertia to send out a voltage increase. If you haven't tested to see if the trailer brakes are actually applying you might but you'll have to be moving. Does the manual override lever increase the voltage or change anything at all when applied?
You might check the position of the controller as well since the sensor requires it to... view full answer...