Trailer Brake Controller Shows 3.5 Volts Output with No Trailer Connected

Expert Reply:

If no trailer is connected and you test the brake pin on the 7-Way, there should be only be a tiny amount of voltage because that is how the controller senses if there is a trailer. But 3.5 volts sounds too high. If this is occurring without the brake or manual override engaged then there is something drawing power on the blue wire (short to blue wire).

If this happens when you use the manual override, then the power sent back will only go as high as it is set. To determine if the brake controller is the issue, use your circuit tester to test the blue wire on the controller. Sever the blue wire a few inches from the back of the controller and test the wire on the brake controller end of the cut wire. It should not have voltage unless you apply the manual override. If it does, then the controller is bad and it will need to be replaced.

If it does not show voltage unless you apply the manual override then there is a short to the blue wire somewhere between the controller and the trailer connector. The most likely culprit is a dirty and/or corroded connector. Clean out any corrosion in the connector but also check the back of the connector where the wires attach because corrosion can build up there.

expert reply by: Michael H