Troubleshooting Trailer Brakes That Stay Engaged with Tekonsha P3 Brake Controller  

Question:

Expert Reply:

There are two main potential causes for your trailer brakes suddenly locking up.

One is that the P3 controller # 90195 is damaged internally (less likely) and the other is that there could be a corrosion issue somewhere in your trailer wiring, either on the truck or on the trailer. Given the truck's age the corrosion issue becomes the more probable cause than a bad P3.

What can happen is that corrosion can develop on the vehicle socket, even inside the connectors where it is very tough to see it. This can even happen inside the molded trailer-side plug. Moisture can over time work its way inside the socket or plug and allow corrosion to form. Eventually this corrosive build-up can cause adjacent circuits to become bridged together. Sometimes even trapped water or foreign material can cause the circuits to become bridged. Also check the wiring/plug that connects to your 7-way socket; that too can be a place where corrosion occurs.

The best way to go is to test the truck and the trailer individually when they are not connected to each other.

First, apply a circuit tester like # 3808 to the truck socket contacts for ground and for trailer brakes. These are the two contacts at the bottom of the 7-way socket as you look at it. See the linked photo, please. The ground contact is at the 7-o-clock position and the trailer brake circuit contact (which carries the P3 output) is at the 5-o-clock position. Ground the tester and apply it to the brake circuit pin while a helper activates the manual override on the controller (with it set to full power and boost). You should detect a signal and that signal should cease when the manual control on the P3 is released. If you prefer you can do this test at the back of the controller by cutting the blue output wire a few inches from the controller and testing it in the same way, using the manual override lever. You should find a signal on that blue wire only when the manual lever is pressed.

If you detect a brake power signal without the manual control lever being pressed then either the controller is damaged or there is a short elsewhere in the brake circuit wiring on the truck.

Testing the trailer is simple enough too. Use the truck battery or a well charged spare to apply power to the trailer plug contacts for ground and brakes; these will again be the ones on the bottom side of the 7-way plug, as on the truck socket. With this direct 12V power applied the brakes should engage fully; you should be able to hear them hum and feel the drag on the wheel as you try to rotate it.

Please let me know the result of your testing and I will be glad to work with you further.

expert reply by: Adam R

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