Trailer Connector Shows 12 Volts All of the Time on the Brake Output Wire on a 1997 Ford F-250

Question:

The vehicle wire harness on my 97 f250 has the blue wire that has 12 volts available at all times whether the truck is running or not. The voltage never changes when the brake pedal or controller are pressed. Is this correct or should the blue brake wire only have power while the tow vehicle is running?

asked by: Andrew S

Expert Reply:

On some vehicles, the key has to be on for voltage to go back to the trailer connector. On others, it does not. An electric brake controller will always send back some voltage to the trailer connector. This is how it senses if there is a trailer connected.

But it should not be showing 12 volts all of the time. If it is on the brake pin, 5 oclock position if looking at the face of the 7-Way, then that indicates that there is a short to the brake wire. If the 12 volt power is showing on the pin in the 1 oclock position, then that is the 12 volt circuit and it showing 12 volts all the time is not uncommon.

There can be a couple of things that can cause a short to the brake wire. If the brake controller was miswired, that can cause the problem. On a brake controller, the white wire is ground, the black is 12 volt power to the controller, blue is output to the trailer connector, and red is in from the stop light switch.

Depending on what brake controller you have, if the truck came with a factory 7-Way trailer connector, you could just use the wiring harness # 3035-P to plug into a factory connection point on the truck. This will eliminate any potential miswiring with the brake controller.

You can also test the blue wire on the back of the brake controller. If it shows constant output then chances are there is a problem inside of the brake controller and it will need to be replaced. If the controller was hardwired to the vehicle, make sure the ground is connected to the negative battery terminal.

You will also want to make sure that the trailer connector is free of corrosion inside and in the back of the connector. If a 7-Way was added as an aftermarket part, make sure that the white wire is grounded to the truck frame.

expert reply by: Michael H