Brake Controller Has Constant 12V Power But Trailer Brakes Do Not Engage
Updated 06/16/2025 | Published 01/17/2013
Question:
What would happen if the brake controller had constant 12 volt power? My car has constant 12 volt when not on the vehicle brakes, but when I step on the brakes, the brakes on the trailer are not applied.
asked by: Laura
Expert Reply:
First I suggest you double check your wiring from the controller. Typical brake controller wiring is: Red wire goes to the brake switch cold terminal, which gets power only when the pedal is pressed; Blue wire connects to the trailer brake circuit; White wire is ground; Black wire connects to the 12V power source. Please note that we do offer vehicle-specific wiring harnesses that plug into the OEM socket under your dash and provide bare wires on the other end to wire directly to your controller. (If you wish to advise your tow vehicle year/make/model I can recommend the appropriate harness, or you can search our site for the correct part.)
If the controller is wired correctly, next test for voltage on the black, red and blue wires at the back of the controller. Use a tester like # PTW2993 to confirm that you have constant 12V power on the black wire, a signal on the red wire only when the brake is pressed, and a signal on the blue output wire when the brake (or manual override) is pressed. I have included a troubleshooting video that shows how to do the testing. If it fails to provide output on the blue wire, then the controller needs to be replaced. If you detect constant 12V power on the blue output wire when the brake is not pressed, then the controller is damaged and must be replaced. If the controller passes these tests, then the problem could be in your connectors or trailer wiring.
Next disconnect the trailer and check the trailer socket connector on the back of your vehicle, and the mating plug connector on the front of your trailer. Thoroughly inspect both parts for signs of dirt, physical damage and corrosion, which can appear as either white or green discoloration, and clean as necessary.
If both connectors are clean and undamaged and all wiring is securely attached to the connector pins, then next test for output at the vehicle connector. With a helper on hand to press the brake pedal, ground the tester and check the pin that is attached to the blue wire in the 5:00 position; you should detect a brake signal voltage from the controller when the pedal is pressed. If the correct signal is detected at your vehicle connector the problem resides in your trailer wiring.
It is not uncommon for trailer wiring to develop a short due to vibration. Internal wires will rub against the trailer frame and can eventually wear off enough insulation to cause a short. When this occurs the trailer needs to be re-wired.
I have included two articles, one on troubleshooting brake controllers and one on general trailer wiring, that might help if you do need to troubleshoot your trailer.
In the event that you do need to replace your controller, I suggest the Tekonsha P2, part # 90885, which is among the most popular and best-performing units we offer.
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on Trailer Brake Controller
Bob M.
6/15/2025
2017 E450, Minnie Winnie. My brake controller has power all the time (light on) It is connected to factory harness. Is this normal ?