Hydraulic Trailer Coupler is Locking Trailer Brakes When Trailer is in Reverse  

Question:

Expert Reply:

Without any background information, it will be hard to say what you have going on.

Have you recently replaced the coupler or master cylinder? Did the brakes previously function correctly, but are recently locking up when the vehicle is placed in reverse?

Given the information available, my best guess is that either your reverse lock-out solenoid is faulty, one of its wires has become disconnected, or the signal from the reverse light circuit on the vehicle is not reaching the solenoid for some reason.

If your hydraulic coupler has a lock-out solenoid, check its connections. One lead should be tied into the wire that runs to the center pin of the 7-way trailer connector or the blue wire of a 5-pole connector. The other lead should be grounded to the trailer frame (it does not matter which of the two wires goes where). Make sure these connections are secure and are not corroded. You can use a circuit tester like part # 40376 and test the lead from the solenoid that attaches to the trailer connector. You should see power on that wire when the vehicle transmission is placed in reverse. If not, unplug the trailer and test the center pin on the 7-way on the rear of your tow vehicle. If you have a 5-pole connector, check the pin fed by the blue wire. Once again, it should show power when the vehicle is placed in reverse. Also give the connector on the vehicle and the trailer a close inspection. If you can see any green corrosion on the pins or sockets, it might be preventing the signal from getting through. Clean or replace the connectors as necessary.

If your trailer is equipped with a break away system, which is typically a cable ran from the vehicle to a lever on the coupler, it is also possible that it might have been engaged. If this is the case, consult the owners manual for your coupler for the break away reset procedure.

If you examine the areas mentioned above and cannot locate the problem, get back with me and we will look at some other possibilities.

expert reply by: Mike L

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