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Trailer Brakes Hum But Do Not Engage


I have replaced the trailer 7 pin connector. The truck is a 2012 silverado that has a 7 pin connecter. The trailer uses only 5 WIRES . All the lights work fine but the brakes hum but do not apply. brake lining checked , all good and adjusted. Ran a ground wire from ground side of the magnets to frame. Same result. have power coming to magnets from new controller. i have the ground wire from the magnets going to the ground connecter of the truck Just bought the trailer and believe it was not used for long time. When the magnets had a full 12 volts applied they locked up at that time i was trying to figure out the wiring. Thanks for your help. Does the wiring sound right. The wiring for the magnets is coming from the electric brake terminal of the truck. Thanks again.


Expert Reply:

First, put the magnet ground wires nearby the magnets on the trailer frame. Running them far away makes it harder for the system to complete the circuit. You mentioned that power is getting to the magnets but how much power? TO determine that you can use a multimeter, # PT89ZR. If the brake controller is sending back the full power (turn up the gain as far as it will go) but only minimal voltage is getting to the magnets, the brakes are not going to work properly.

Using the multimeter you can determine where the power is being lost. You can check it in several places starting with the blue wire at the back of the brake controller. Then test the power output at the truck side trailer connector, then just behind the trailer side connector. And last check it just before the magnet.

There will be some voltage loss due to resistance from the wiring. But if you are only getting a couple of volts at the magnet or anywhere else, something else is going on. Make sure that your trailer side connector is pinned out to match the truck connector.

If you are using an adapter to go from the truck connector to the trailer connector then there could be something wrong with it and a new one would be the way to go.

It does not sound like the magnets themselves are the issue since they locked up at 12 volts. Somewhere between the controller and the magnets there is a power loss. Make sure that the truck and trailer connectors are clean and free of corrosion inside and out. I have included a link to our trailer wiring FAQ that will also help.

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