Troubleshooting Trailer Brakes Locking Up

Question:

When I apply the trailer brakes the brakes lock up. Brakes are magnet type and are in new condition. When I try to adjust brake controller in truck, it does the same thing. Its almost as if full power is being applied but I have the controller set to the lowest setting. I checked all wiring and brake adjustment and everything checks out good. I am stumped as to what would be causing this. I have checked grounds and wiring and like I said everything else works. Thanks,

Expert Reply:

The first thing I would check would be the voltage coming out of the brake output circuit on the 7-way of your tow vehicle. I would use a multimeter like our # BTMT15 to test for voltage on this circuit when you have the brakes applied on a low setting. You should see a number that is much less than 12 volts as 12 volts that should be the max output of the brake controller.

If you see 12 volts there then you know something is not wired up right on the vehicle side of the equation. You will want to sever the brake output wire from the back of your brake controller (typically blue) and repeat the same test with the multimeter. If you get the same full 12 volts with the brake controller set at a low setting then you know the problem is in the brake controller. If you see a lower voltage then the problem is somewhere between the trailer connector and the brake controller. I would recommend you test the wiring from where you severed the wire back to the trailer connector to find the problem and then fix as necessary.

There may be an issue on the wiring of the 7-way, the connector could either be mis-wired so that the brake light circuit is on the brake controller output, or the circuits could be shorting to each other and the 12 volts are being sent down the brake output circuit as well. There also could be corrosion in the plug that could cause these problems. If so, clean as necessary.

I attached an FAQ on trailer wiring for you to check out also.

expert reply by:
Jameson C

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