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I have a 2020 AT4 2500. At 375 miles, I pulled out onto a 3 lane road an went to give it the throttle and the truck jerked an bucked and wouldn't go. It limped home at about 35-40 mph because the transmission wouldn't shift. I got an email from OnStar to have it serviced immediately. I connected my  Diagnostic tool and it gave me 4 errors, all transmission related. 

 

I took it to the dealer and they said the transmission fluid was low. They also fixed the Wireless charging pad, (factory didn't plug it in) I drove the truck for another 2 weeks and at around 1100 miles on the odometer the same thing happened. I haven't even towed anything yet!

 

Took it in again. Dealership said GM is going to replace the transmission. Then called back and said they are not going to replace the transmission, but instead the "valve body". I asked for the part# of what they are replacing and he didn't know. I will get that information when I pick the truck up next week.

 

This is super frustrating to say the least. I got a text from the salesperson where got the truck, and he said ther rep told him that 4 out of 5 trucks are having this or similar transmission problems. 

 

Has anyone else had this problem?

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2 hours ago, gpraceworks said:

 ther rep told him that 4 out of 5 trucks are having this or similar transmission problems. 

Has anyone else had this problem?

Welcome to this forum!   Sorry to hear about your issue and thank goodness for warranties.  I think the "4 out of 5"  trucks story was misheard/misunderstood.  Perhaps 4 out of 5 trucks that have a transmission problem, are having this or a similar issue.    Enjoy your new truck once this hiccup is history. The '20 2500/3500's are currently the nicest trucks on the road, imo!

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3 hours ago, Donstar said:

Welcome to this forum!   Sorry to hear about your issue and thank goodness for warranties.  I think the "4 out of 5"  trucks story was misheard/misunderstood.  Perhaps 4 out of 5 trucks that have a transmission problem, are having this or a similar issue.    Enjoy your new truck once this hiccup is history. The '20 2500/3500's are currently the nicest trucks on the road, imo!

I'm sure he  Exaggerated a little bit...lol.

 

I'm looking forward to getting this behind me and just enjoying it. 

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When I bought my 2015 brand new I had the transmission take a crap after 200 miles. It would only shift into reverse and park. I had a great picture of it on a flatbed the day after I brought it home. They ended up replacing a bad electronic part, and back in business. For the next 94k miles I replaced a def tank heater, and a seat buzzer for the parking sensors. I got a nice bedrug out of the transmission problem when GM customer care called me to apologize about the transmission issue, unprompted.

 

My point is, that was a true Allison. Things happen, that's what warranties are for. I really doubt the 4/5 statement. Mine is smooth as silk, very nice. They'll get it fixed.

 

I drove the first 10k on my 2015 with a mild bit of nausea over the transmission and eventually forgot about it.... I wish I hadn't tiptoed those first 10k, there was no need for it.

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11 hours ago, jpgcrna said:

When I bought my 2015 brand new I had the transmission take a crap after 200 miles. It would only shift into reverse and park. I had a great picture of it on a flatbed the day after I brought it home. They ended up replacing a bad electronic part, and back in business. For the next 94k miles I replaced a def tank heater, and a seat buzzer for the parking sensors. I got a nice bedrug out of the transmission problem when GM customer care called me to apologize about the transmission issue, unprompted.

 

My point is, that was a true Allison. Things happen, that's what warranties are for. I really doubt the 4/5 statement. Mine is smooth as silk, very nice. They'll get it fixed.

 

I drove the first 10k on my 2015 with a mild bit of nausea over the transmission and eventually forgot about it.... I wish I hadn't tiptoed those first 10k, there was no need for it.

 

My 2020 Gasser with the 6 speed has had a couple of weird feeling shifts, but overall very nice.

 

I did see a 2020 2500HD on a flatbed Saturday at a corner and saw that they followed me and pulled into the local dealership. Curious what the problem was with that truck... didn't see if it was a gas or diesel, but I'm guessing a diesel as they mainly sell diesels there.

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Well, I spoke too soon.  Mine goes in after thanksgiving for a leak inside the bell housing.  Not a major one but leaking. Wiped off inspection plate 3 times and it keeps accumulating on it after a drive.  Probably pump or front seal in the bell housing.  We will see..I hate to have transmission dropped already.  Oh well!

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Im in the dealership now with what sounds like the same situation (same symptoms except it only felt like i lost 1st gear, after a few days the check engine light turned off and it seemed like the symptoms went away).  Service Tech is saying they are going to replace the tranny.

 

I've got about 6000 miles on it.  Pulled my 5th wheel a few hundred miles.

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  • 1 month later...

So I have what appears to be a transmission issue at just over 500 miles.  The first time I went up to 74MPH I heard a hollow howling type noise that seemed to come from the front right of the truck. My passenger stated he thought he felt the floor vibrating and it sounded like it was coming from the rear.  

 

What I have learned is if I put the transmission in manual shift, then do 74, I hear the noise in 10th gear.  If I downshift from 10th to 9th the noise goes away.  If while in 9th gear I slow down to 68, the noise comes back until I downshift into 8th.  When hearing this noise, both times the tach was exactly at 1500RPM.  

 

Also, at times of acceleration if I've heard the noise at lower speeds and when looking at the tach, it is at 1500RPM.  Anyone else having this issue?

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