Trailer Brakes Lock Up When Connected to Truck

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

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

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..

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.

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.

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 :-/

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

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?

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.

Jmendana4@gmail.com

3/14/2020

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

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?

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.