2020 GMC AT4 Diesel Brakes lock up on their own. Extremely Dangerous!!!

[joeidaho]

I have a 2020 AT4 2500 Diesel with 37600 miles on it. Just under 2 weeks ago I was hauling my boat to the lake when the back truck brakes completely locked causing the vehicles behind me to veer off in to the side lanes to avoid rear ending the boat. I thought it was the boat trailer because the smoke coming off the back tires. I went another 200 yards and the truck did it again. This time everyone was back way way behind me. I couldn't pull over due to guard rails so I puttered up to the closest turn off and looked everything over. I disconnected the boat and made sure the brake away wasn't hung up and hooked it all back up again. I pulled out and went another 1/4 of a mile and it locked up again. This time I noticed the truck flashing the N for Neutral and the truck wouldn't go forward. So I had to put it in Neutral and then back to Drive so it would move forward again. I pulled over and disconnected all the hydraulics on the boat brakes and went on my way. Great weekend boating and no problem. On my way home, about a mile from the house the brakes lock up again; again causing all the vehicles behind me to slam on their brakes and go all over the place. Now, I had already called to make an appt for the trailer to get serviced. It was Sunday so I had to limp it home. Because I disconnected every logical part of the boat brake system I assumed no more issue. When I get home and I completely voided the brakes on the trailer of all fluid and took it to the boat dealer for service. The boat is new and this was the second time I towed it so I assumed it was some kind of anomaly with the boat trailer. 

Today I was pulling a fully loaded enclosed trailer and the truck locked the brakes again going 60 on a highway and all the vehicles behind me went to the sides and hurried around me to avoid a collision. Fortunately I was only a few hundred yards from my destination so I disconnected the trailer and took the truck to the closest service center. They have me scheduled to have it inspected tomorrow. 

I'm putting this on here so everyone can have the seed planted if there is some kind of computer glitch or something that comes up once the vehicle hits a certain mile mark etc. I honestly don't know but I have a fleet of other trucks for my construction business and this is the first GMC I have ever owned. I love the power and pulling capability but I would assume not many of you owners have reached this mile mark yet. But also to see if anyone else has had a similar issue. 

I'll keep everyone posted when I know more. 

[CRApex]

We have three with many more miles than yours, no issues with the brakes so far.  Sounds like a problem with the electronic parking brake perhaps.   Let us know what they find.

[greatmizzou]

You said Back truck brakes lock up and not the trailer brakes or your front brakes? 
 

I would assume its the trailer, right?  
 

What kind of aftermarket equipment or software modifications do you run? Do you have any collision detection options?

[joeidaho]

Update: 

I took the truck in on Monday and they kept it Tuesday to run a series of tests. No faults found. They couldn't duplicate the issue so I was allowed to take the truck back that evening. Yesterday (Wednesday) I am driving the truck down another highway doing about 55 and Slam!!! Brakes lock up!!! A motorhome behind me went off in to the ditch and about took out a fence. Luckily he and his wife were fine with no serious issues. I pulled off to the side and put the truck in park. This time, no trailer. Because I wasn't pulling anything, the truck almost came to a complete stop thus why the motor home about completely wrecked. 

Now - I did notice that this only occurs when I hit the acceleration and the transmission shifts down for more torque around the 50-55 mark. 

I slowly crawl the 8 miles back to the service center and now my truck is in jail. They are logging a ticket with the GM engineers and have all but certainly figured it's a transmission issue. No aftermarket modifications on the truck. Only a new air filter and tires. 

I took pictures of the skid marks on multiple areas to give verification. More to come. . . . .

[greatmizzou]

How bizarre.

 

I will give my 2 cents.  These trucks have hill hold brake assist and electric auto parking brakes.  If you are suddenly getting locked out wheels, all I can think of is your parking brake is engaging.  Perhaps they will find a shorted cable connected to the parking brake equipment that only causes intermittent parking brake engagement that normally wouldn't be triggered otherwise.
 

You said the truck wouldnt move and "N" was flashing so you moved transmission to N then back to D.  Perhaps that is what reset and releases the engaged parking brake.   
 

 

 

 

[It's Tim]

Flashing "N" could indicate a transmission fault.

[greatmizzou]

Yet another shocking issue reported with little follow up.

 

give me the juicy details!

[20L8TSierra24]

Wow. Curious to see what they figured out.

[MrMoe]

The same thing has happened to me twice now. The first time I did not have a trailer and the rear brakes locked up it felt like a tire blew but it did not. I thought maybe it was some type of forward crash alert or that maybe I hit a steel platform causing it to antilock. This time it just happened to me while I had my trailer and it happened all the sudden I was not accelerating and I was not near any other vehicles at all. I called my service advisor and will explain it to him and see what happens .

[redwngr]

Imho, it sounds a lot more like a transmission issue that a brake issue.. 

 

Flashing 'N' should help with diagnostics.

 

 

[HELPPLEASE]

We are currently experiencing this same issue. Did they ever find the culprit? What was the resolution??

[HELPPLEASE]

Are eithier of your trucks lifted? We have a 6" Apex factory installed lift. 

[Another JR]

I have a 2021 3500hd gas, and my neighbor has a 2021 3500hd diesel dually. We talk trucks whenever we see each other.  My neighbor’s truck has the stock suspension and tires. 
 

He experienced symptoms very much like these on the highway. He was driving along, then all of a sudden wham, the rear wheels locked up, then he could hear drive train whirring with no torque. He restarted it and was in some kind of limp mode and would move, but he decided to have it towed. 
 

His transfer case was destroyed and had to be replaced. The dealer said it was a fault in the four wheel drive control switch or computer (I can’t remember which one he said) that apparently had it try to switch to 4 low at high speed. 
 

It may be that others of you are having the same thing occur, but the traction situation at the time (srw vs dually, speed, tires, road conditions, etc) doesn’t generate enough torque to break the drive train, so the dealer doesn’t get that giant hint that it’s not brakes. 

Edited April 9, 2022 by Another JR

[mkirkland]

This is downright frightening...

[Another JR]

Same neighbor told me two weeks ago he had it happen a second time after the first repair. He was able to limp it home several miles, but in 4 low because it would not come out of 4 low.  He was talking about pursuing a buy back.  I saw yesterday he had it back, but I haven’t had a chance to ask him what was done by the dealer. It seems to have proved that the sudden lockup is caused by the transfer case shifting to 4 low though for some reason.