Should Trailer Brake Output Wire on 2003 Ford F150 Only Show Voltage With Brake Pedal

Published 07/02/2016

Question:

Should the 12V blue wire be always hot or just when you apply the brakes?

asked by: Joe

Expert Reply:

The wire that carries the brake controller output is typically blue, and shouldn't show voltage unless the manual brake switch on the controller is activated. If your brake controller is the time-delayed type, it'll also show voltage when the brake pedal is pressed.

If you're getting voltage all the time, use a circuit tester on the blue wire as it exits the rear of the controller and check for voltage there. If there's no brake pedal pressed, or no manual brake switch activation, there shouldn't be power on that wire. Then, check the red wire.

If you're getting power on the red wire without the brake pedal, it indicates a problem with the brake stop lamp switch on the vehicle. If you're not getting power on the red wire, but are getting output on the blue wire, either the controller is faulty or the manual brake switch is somewhat stuck. Try 'snapping' the manual brake switch off and on several times to see if that makes the output voltage stop. Sometimes the contacts in the switch get dirty and snapping the switch several times cleans the contacts.

expert reply by: Mike L