Trailer Brakes Lock Up When Connected to Truck

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Question:

hello, i have a 16ft trailer with 2 axels and the front tires are locking up. then when i unplug the trailer from the truck i can hear the breaks release.

asked by: Sean W

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Expert Reply:

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 through the various functions to make sure everything is working.

If any of them are showing a constant 12 volts, and the matching pin on the trailer side goes to the brakes, that would be why the brakes lock up. This is especially an issue on 6-Way trailer connectors because there is 2 different ways they could be wired: the center pin will either be brakes or 12 volt power. If the trailer does not match the brakes will lock up.

There could also be a short caused by corrosion build up on the trailer connectors pins (vehicle or trailer side) or corrosion where the wires attach. Cleaning off corrosion could solve the problem.

It could also be an issue with the brake controller. To test the brake controller sever the blue wire several inches from the back of the controller. Then use the circuit tester to test the end coming from the controller. It should only show power when the manual override is applied. If it shows power at any other time then the controller is bad and will need to be replaced.

The trailer brake magnets could have a short as well. Check the wiring for any frayed wires that might be touching bare metal. Make sure the grounds are attached to clean and corrosion free bare metal surfaces. Visually inspect the magnets and if you can see the windings coming through then the magnets need to be replaced.

To further test the magnets you can follow the FAQ article I have linked. I have also linked additional FAQs on brake controller troubleshooting and trailer wiring for you.

expert reply by: Michael H

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Joseph

3/13/2020

Down hill 40,000 lbs rear trailers axles lock up? What would be the best safety maneuvers during adverse weather conditions and no shoulders to pull out.

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Chris R.

3/13/2020

I'm not quite sure what you're asking. Are the brakes on the rear axle of your trailer locking up every time you travel downhill?

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[email protected]

3/14/2020

No, one incident in which the rear axle on my trailer locked up going down hill.

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Chris R.

3/14/2020

So even if it happened just once it's probably a good idea to give your trailer wiring a good inspection before the next trip, looking for anything that could cause a short (frayed/exposed wiring, loose connections, rust/corrosion inside the 7-Way connector, etc). I am assuming the brakes eventually released without intervention?

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Tim D.

7/12/2020

I am currently on a trip to upper peninsula. About 6 hours into a 9 hour trip my mileage dropped from 10mpg to 5 mpg. Definitely not the truck as when I go to KOA, I drove the truck 15 miles and I was back to 20 mpg on a 6.7 Dually which is about my max. Is it possible my trailer brakes are pressing slightly to cause this decrease? And how would I check that?

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Chris R.

7/17/2020

I suppose this is possible, but there are likely a number of other things that could cause that MPG drop as well. If you didn't physically feel the trailer dragging at all during travel, another way to check this is to actually test the brake output pin on your truck's 7-Way. See if you're getting constant power there. If so, there's likely a short somewhere on the wiring that you will need to track down.

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Brad

7/16/2020

Thanks for the advice. The breaks on my trailer just started locking up but only on a single wheel. The gain was set to 4.5, somewhat flat terrain, and stopped when I changed the gain to 2.0. I haven't pulled the wheel to check the breaks.

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Chris B.

10/8/2020

@Brad What did you find? I have recently started having the same problem. New box, New drums, new complete self adjusting backing plate kit and everything replaced 6 months or so ago. Everything fine until 1 day right front locks up. Turn the box down and it doesn't but then also doesn't stop well. Changed trucks same problem. Pulled the wheel last night found nothing, everything still looks new. No broken springs, shoes or grease. Everything is shinny where it is supposed to be and not where it isn't supposed to be. Hair puller :-/

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Timothy M.

5/22/2021

@ChrisB . Did you find your issue. I'm having the exact same problem with a Big Tex 14 ET trailer with my f250

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Jimmy K.

8/4/2020

I have a 2002 Ford F250 and a 2016 F250, Both Diesel. When I connect either trailer, my Brakes lock up. Unplugged the front axle unlocks the brakes. The trailer is a two axle 20' goose neck with 5' dove tail. Trying to forget out what is going on..

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Chris R.

8/13/2020

Since it happens with both of your trucks hooked up, we at least know the issue is somewhere on the trailer. I'm thinking somehow there's a short bridging the 12V circuit on the 7-Way with the brake output circuit. My best advice is to simply try and trace the wiring on the gooseneck and look for any exposed sections on these particular circuits that could be touching a nearby wire or even bare metal.

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Giovanni H.

7/29/2024

hello Michael I'm having an issue with my 2022 bigtex trailer 12k axle, installed new break assembly in front axle and cut drums out for a clean magnet surface, but front driver side is getting to hot, I ranit 12 mills and checked tempeeture it read 420f and the rest 170-200f can't figure out what is wrong

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Bryce D.

7/30/2024

@GiovanniH Giovanni, it sounds like the driver's side was adjusted too tightly. What you're going to need to do is loosen the adjuster until you don't hear the pad contact the drum, then tighten the adjuster until you hear the pad contacting the drum while spinning the wheel. If you do that and you're still having the same issue, then I'd suggest checking the castle nut and make sure it's not installed too tightly. We recommend tightening it down enough to make sure the drum is fully seated on the spindle then loosening it. From here, you'll hand tighten the nut and then tighten roughly 1/4 turn with a socket.

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S.m

5/26/2025

Hi, 3 month old car trailer has done few light trips no cars on it, wheels lock up with brake controller on 2 out of 10, used it for the first 3months not a problem on 3 out of 10. Now have to start out on 1.5 and as time goes by say 1hr I can turn them up and they work fine. Have bypassed all wiring on trailer and straight to brakes from controller still does it. Changed cars with different brake controllers still does it trailer has done 100klms it still new. The only thing is trailer went under water for 1hr due to flood fresh water. I have tested all electric magnets and they all ohms at 3.5. I have had all hubs off and inspected all look and move fine. Why does it lock up when cold but not when warm and why just start out of the blue? Ant help will be much appreciated.

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Jameson C.

5/27/2025

@Sm This sounds like a short between the brake light and brake output circuit. So basically any time the brake lights come on the trailer brakes are locking up. If you put on a blinker do they grab for a second and release?

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Answer 1

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 2

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?…

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Answer 3

The correct way to wire your breakaway battery, like the Hopkins Engager #20400, is to have the black wire from the battery connected to the 12V power on your trailer, and then the blue wire to your breakaway switch which, in turn, connects to the…

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Answer 4

There are a few things you can check to make sure the brakes are adjusted correctly, but it sounds like there may be a short in the system. I would start by checking the trailer wiring connector on the truck and trailer for corrosion. If there is…

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Answer 5

The reason your brakes are locking up when you use the 7 Pole to 6 Pole Adapter, #47549, is because there are two ways that 6-way round trailer connectors can be wired, as you may have already discovered. The difference is the pin for the electric…

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Answer 6

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 7

The issue you described sounds like a problem on the trailer side wiring. I have attached the wiring diagram of the Titan BrakeRite EHB Electric-Hydraulic Actuator part #T4813100 for you. The first thing you will want to check is the blue brake…

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Answer 8

I am almost certain that the issue is that the connector on your 2015 Chevy Silverado 2500 and the one on the trailer are pinned out differently. I have included a picture that shows how a vehicle and trailer side 7-Way are wired (ignore the colors…

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Answer 9

After doing some research it seems that your 2008 Shoreland'r boat trailer has a 3,500 pound weight capacity so The Dexter 10 inch Hub-and-Rotor Assembly #K08-435-05 would work for you. Your axle should have an axle sticker or plate on the axle…

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Answer 10

Trailer axle nuts don't have a torque rating as you don't want to run them down super tight. The trick is to tighten the nut till there's no play in the hub in and out with the wheel installed and then use the cotter pin to hold the nut…

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Answer 11

Since the trailer was off of the ground it would not take much voltage to stop the wheels from spinning so they may be getting power but they may not be getting enough power due to a short or wiring issue. But 9 times out of 10 weak trailer brakes…

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Answer 12

For a switch to supply or remove power going back to the gate you can use a simple on/off switch such as #PK34213. This switch is rated up to 50 amp applications. For a switch that has 3 positions you could use on/off/on switch #PK34218 which is also…

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Trailer Brakes Lock Up When Connected to Truck

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