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Trailer Brakes Actuate Immediately When Trailer is Plugged in to Vehicle

Question:

I have a question for you that has me a little stumped. We have a trailer with a 7-way plug. We have two trucks here that the trailer will work with just fine. However 4 trucks will not work. The four that do not work with the trailer all have the same issue as soon as you hook up the trailer the brakes engage. We have one of your testers and on all four trucks the BK light lights up even with the truck off and no one in it. All four trucks are using factory wiring with a brake controller plugged in under the dash at factory location 3 are fords one is a chevy. I also used a volt meter and between the ground and the brake terminals it shows 12.1-12.4 volts on each truck. To make it more confusing these 4 trucks all work with other trailers. Any suggestion? I would be willing to give more information if you need it or do more tests. Oh. Also the trailer was also recently worked on by a local electrical company new brakes they said they wired it back to code but we thought maybe they wired it to match the truck. Took the truck one of the 2 that it works with to a trailer dealer they checked the truck and said it was wired properly. How can all 4 factory wired trucks have the same problem?

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Expert Reply:

I have a suspicion about what might be causing the problem you mention. Do your trucks have 6-way trailer connectors? Or, do the trucks have 7-way connectors, but you are using a 6-way to 7-way adapters because the trailers use 6-way connectors?

There are two accepted ways of wiring a 6-way trailer connector. One uses the center pin as the brake voltage output circuit, the other uses the center pin as the 12-volt accessory circuit. If the truck is wired one way, and the trailer or the adapter is wired the other, the trailer brakes will immediately apply when the trailer is plugged in, because 12 volt power is being fed directly to the brakes. You can use a circuit tester on the trailer connectors to confirm this.

If the trucks that the trailer will not work correctly with have 6-way connectors, access the rear of the connector and swap the wires feeding the pin labeled S and A. If you are using an adapter, you can swap those wires or use an adapter like the Hopkins # HM47570 which offers a quick-change wiring plug, that allows you to swap those two wires without taking the connector apart.

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Mike L

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