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

Yyequipment · June 2, 2022

I am also experiencing the same issue right now with my work truck a 2020 Silverado 2500 Duramax with 82,000 miles and no modifications. Last week I was driving down the highway at 70 mph with a small trailer that has no brakes, I had the trailer tow feature as well as the exhaust brake on. I thought a strap had come loose and got under a tire causing the wheel to lock up or I had blown a tire. Very scary... everyone behind me slammed on their brakes and then about two hundred yards later it did it again. Both times I looked in the mirror and it the smoke appeared to be coming from the trailer, no brakes on the trailer so no possibility of that. I pulled over and both straps were tight and not damaged neither hub on the trailer was overheated and everything seemed to be fine. I slowed way down and it made it the 80 miles back home with no problems. I didn't no what to think and to be honest pretty much forgot about it over the Memorial day weekend. Tuesday comes around and I hook up a 20" cargo trailer I have with brakes and head out the 120 miles to make the same trip. About 90 percent of the way there at 70 MPH it does it again, locks up for maybe two or three seconds and then releases. Same exact feeling and sensation as the first time. Once again I slowed way down and turned the gain down on my controller to 0 and about ten miles down the road heading towards the dealership it locked up again and threw the tranny into some weird mode where it couldn't find a gear for about 5 seconds. Dealership said it would be at least a week for an appointment so I limped back home via the backroads at 45 mph and will try to get a service appointment here locally. I have never in almost 58 years of life experienced something like this happen with a relatively new vehicle. It is very dangerous not only for the operator but for the motoring public that happens to be around you when it happens.

swathdiver · June 2, 2022

@joeidaho @Another JR @HELPPLEASE@mrmoe

Any updates folks?

Another JR · June 2, 2022

I’ll keep an eye out for my neighbor and ask him

Another JR · June 4, 2022

My neighbor confirmed the dealer told him the second event was due to the transfer case suddenly shifting into 4 low. They do not know why it occurred. They also found some collateral damage to the transmission and repaired that. I would have thought they’d replace every part having to do with transfer case control, but apparently they didn’t.

He’s hoping for the best but if it happens again it will be a clear lemon law case under WA law and I suspect he’ll have them take it back.

shadow gray 2020 hd · June 6, 2022

Is this happening with trucks equipped with the auto transfer case?

Yyequipment · June 6, 2022

On 6/2/2022 at 8:24 AM, Yyequipment said:

I am also experiencing the same issue right now with my work truck a 2020 Silverado 2500 Duramax with 82,000 miles and no modifications. Last week I was driving down the highway at 70 mph with a small trailer that has no brakes, I had the trailer tow feature as well as the exhaust brake on. I thought a strap had come loose and got under a tire causing the wheel to lock up or I had blown a tire. Very scary... everyone behind me slammed on their brakes and then about two hundred yards later it did it again. Both times I looked in the mirror and it the smoke appeared to be coming from the trailer, no brakes on the trailer so no possibility of that. I pulled over and both straps were tight and not damaged neither hub on the trailer was overheated and everything seemed to be fine. I slowed way down and it made it the 80 miles back home with no problems. I didn't no what to think and to be honest pretty much forgot about it over the Memorial day weekend. Tuesday comes around and I hook up a 20" cargo trailer I have with brakes and head out the 120 miles to make the same trip. About 90 percent of the way there at 70 MPH it does it again, locks up for maybe two or three seconds and then releases. Same exact feeling and sensation as the first time. Once again I slowed way down and turned the gain down on my controller to 0 and about ten miles down the road heading towards the dealership it locked up again and threw the tranny into some weird mode where it couldn't find a gear for about 5 seconds. Dealership said it would be at least a week for an appointment so I limped back home via the backroads at 45 mph and will try to get a service appointment here locally. I have never in almost 58 years of life experienced something like this happen with a relatively new vehicle. It is very dangerous not only for the operator but for the motoring public that happens to be around you when it happens.

Update: Dealer said they will be installing a new Transmission valve body covered under powertrain warranty. I asked the dealer what component within the valve body failed and he did not know. I also inquired as to what collateral damage was caused by transmission locking up at 75MPH and he said they could not find any. Not what I would expect from a Allison.

Alien · June 16, 2022

I was pulling a 7500lb trailer with my 2020 GMC 3500 Crew Duramax 4WD with 78k miles at 55mph on a California freeway and the rear wheels locked up for approx 30ft then released then locked up again for approx 30ft then released. The trailer fishtailed both directions violently and I was able to get it pulled over to the shoulder. Put the tranny in park. Then I put in drive and reverse and clunking/grinding from 10 speed Allison tranny. It would move in either direction but clunking/grinding. Towed to dealer. All dealer has said so far is "internal transmission failure". In hindsight a few miles back when the tranny would upshift to a lower RPM below 2000rpm it would hard shift. This happened at approx 50-55mph. It did that a few times before it locked up. Very scary with a trailer. I almost had a bad wreck in heavy traffic. I'll keep you posted on what was found etc.

Alien · June 16, 2022

On 6/6/2022 at 4:00 PM, Yyequipment said:

Update: Dealer said they will be installing a new Transmission valve body covered under powertrain warranty. I asked the dealer what component within the valve body failed and he did not know. I also inquired as to what collateral damage was caused by transmission locking up at 75MPH and he said they could not find any. Not what I would expect from a Allison.

Just some info on the Allison. Its not actually an Allison built 10 speed transmission. It is build by GM inhouse and GM has a branding deal with Allison. Maybe thats why there are issues.

Yyequipment · June 17, 2022

On 6/6/2022 at 4:00 PM, Yyequipment said:

Update: Dealer said they will be installing a new Transmission valve body covered under powertrain warranty. I asked the dealer what component within the valve body failed and he did not know. I also inquired as to what collateral damage was caused by transmission locking up at 75MPH and he said they could not find any. Not what I would expect from a Allison.

Final update: Dealer installed a new transmission valve body and referenced bulletin #21-NA-275 on the repair order. The bulletin states ABS event. Not sure why you would need to replace a transmission valve body for a ABS event. Anyhow we will see how it performs in the next couple of weeks.

Another JR · June 17, 2022

The bulletin may have ABS event listed to lead techs diagnosing events to the SB even if they initially think it’s brake related.

The people who are having these events should report them to NHTSA. They have an on line reporting page.

HWHL · June 19, 2022

I have a 2020 1 ton denali. Pulled same trailer since purchasing with no problems. Left for Missouri with same trailer and I believe brakes applied very quickly on their own and then let of. Almost locked up. It happened about 6 times. I did everything to isolate the trailer to make sure it was not the problem. It still occured. Did not matter if cruse control was on or not. It would be 100’s of miles between occurrences. I am now seeing if being not in trailer mode makes a difference. I am heading to Branson Missouri and will drop off at dealer. The stops were so sudden and unpredictable. Everything in trailer was all over. Lucky we did not cause an accident. Anyone else have anything to add?

SJ2022 · August 8, 2022

Any update on if the issue still persists after repair has been made? We have had this happen two separate times for a total of 4 occurrences total. Our local Chevy couldn't replicate the issue the first time and told us "nothing is wrong, feel free to drive the vehicle as normal." The second time it happened, it locked up the brakes at 75 MPH and put us in the ditch and flat spotted our rear tires. Our local Chevy couldn't replicate the issue again, but said yeah you need a valve body, feel free to come take the truck and drive it until we can get the part....interesting. Opened a case with GM customer support, but seems like they are just as difficult to work with as our local dealership.

newdude · August 8, 2022

On 6/17/2022 at 4:28 PM, Yyequipment said:

Final update: Dealer installed a new transmission valve body and referenced bulletin #21-NA-275 on the repair order. The bulletin states ABS event. Not sure why you would need to replace a transmission valve body for a ABS event. Anyhow we will see how it performs in the next couple of weeks.

14 minutes ago, SJ2022 said:

Any update on if the issue still persists after repair has been made? We have had this happen two separate times for a total of 4 occurrences total. Our local Chevy couldn't replicate the issue the first time and told us "nothing is wrong, feel free to drive the vehicle as normal." The second time it happened, it locked up the brakes at 75 MPH and put us in the ditch and flat spotted our rear tires. Our local Chevy couldn't replicate the issue again, but said yeah you need a valve body, feel free to come take the truck and drive it until we can get the part....interesting. Opened a case with GM customer support, but seems like they are just as difficult to work with as our local dealership.

21-NA-275. Internal fault in the valve body. Its not the brakes that are causing the lockups, its the transmisison control valve body. Sounds like its an issue during a certain speed range and shifting events, its either losing vehicle speed signal or there's a pressure drop out.

Quote

"Some customers may comment on the following conditions while lightly to moderately accelerating, forcing an 8-7 or 8-6 downshift between 64-96 km/h (40-60 mph) with the transmission at operating temperature:

Rear wheel/tire lockup or an ABS event

Loud bang

Very hard 8-7 or 8-6 downshift

Feeling of hitting a large pothole or large speed bump

Flashing “D” on the DIC

SJ2022 · August 8, 2022

Thanks for the update. Wasn't sure if anyone else had the issue fixed and it started happening again...thinking we may get it fixed and get rid of the truck, too much of a safety issue for my liking.

Ianscape · September 3, 2022

My 2020 1500 with the 3.0 just started experiencing this. NB on I93 today when at 70 there was a loud slam in the transmission and felt like the brakes locked up....right on the 2 lane merge in Franconia Notch too. Truck had been in for a slamming sound the week prior that the dealership could not replicate and said to drive as normal. Back off to em, and thanks for the posts on the bulletin, I'll have em look at it!