Trailer Brakes Actuate Immediately When Trailer is Plugged in to Vehicle

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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?

asked by: Ian

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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.

expert reply by: Mike L

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Answer 1

There are a few things that can cause the brakes to lock up on a trailer. First, disconnect the trailer from the vehicle. Then use a circuit tester like #PTW2979 if needed to test the connector on the vehicle. Have someone sit in the truck and run…

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Answer 2

The easiest first step here would be to try the trailer on a different truck, and see if the same problem occurs. If the trailer brakes are free until the trailer is plugged in, I suspect that something is mis wired. I would suggest examine the…

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Answer 3

It sounds like your trailer brakes are sticking, but I doubt the problem is the brake controller. However, you can test this by applying the brakes like you normally do while coming to a stop. Once the trailer and vehicle stop get out and unplug your…

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Answer 4

Hey Luke, yes this is typically indicative of a problem with the Curt trailer wiring harness you installed. To test this for sure you would want to use a circuit tester like the part #PTW2993 to test for voltage on the input side of the converter box…

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Answer 5

First of all did this problem just start happening? If the trailer has always worked fine before on the same truck then it's likely a short on the trailer's brake wiring. I would start by cleaning the connector and make sure it's free of…

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Answer 6

The issue you are describing comes up now and again with certain Nissan vehicles. Apparently, depending on which plant the vehicle was built in, the brake output wire and the 12 volt power wire are reversed on the vehicle wiring harness. Basically to…

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Answer 7

It could be a tow package fuse on the vehicle. Use the owners manual to find the locations of fuses/relays related to the tow package wiring to check them and replace as needed. If the issue is not a fuse or relay then you can test the vehicle side…

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Answer 8

When you test the 4-Way on the Curt T-Connector Wiring Harness, part #56016, at the rear of your vehicle, it should show now power on all 3 pins with the lights off. If you have the vehicle head lights on, you should have around 12 volts at the brown…

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Answer 9

Sorry to hear about that Michael. There are a couple of basic troubleshooting steps you can do but really the Curt T-Connector Vehicle Wiring Harness for Factory Tow Package #C56357 is plug and play. The first thing I'd check is the ground wire…

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Answer 10

For your 2024 Nissan Frontier I recommend the etrailer Trailer Hitch Receiver #e98869. This is a Class III hitch with a 2" receiver and a max gross trailer/tongue weight of 5,000/500 lbs. This hitch also features a matte black powder coat…

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Answer 11

Sounds like the 12 volt accessory circuit is shorting out to the brake output circuit on the trailer wiring. Go over the wiring and look for any cut or exposed wire between these two circuits and fix as needed. If the stock trailer had a 6-way…

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Answer 12

Since wire color codes are NOT universal, we always recommend wiring by function, rather than wire color. If you look at the Trailer Wiring FAQ article I have linked you to, you can see some diagrams showing the proper way to wire a vehicle and…

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Trailer Brakes Actuate Immediately When Trailer is Plugged in to Vehicle

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