2012 GMC tranny issues

[tegu69]

I have a 2012 Sierra 2500 hd, gas 6.0, auto transmission. I tow an 8000 lb fifth wheel. The truck pulls the trailer well and generally shifts through the gears fine, with an exception. When I stop in a rest area on the Interstate for a few minutes I shut the engine off. I then go to leave and accelerate on the entrance ramp. The transmission seems to hang in (I think) third gear. Most of the time, it is on a level surface and the truck is accelerating as I would normally get on the Interstate. The tachometer is at 5000 rpm, I let off the gas some to let it shift and it doesn't. I then get back on the gas to about 5500 rpm and it finally shifts. Since this only happens as described and otherwise drives fine, it has just been a minor concern to me. A couple of months ago, I got my oil changed at the dealership and I mentioned this to the service writer. He told me that since it only happens intermittently, that they probably couldn't duplicate the problem and he didn't know what it could be.
I am now within several hundred miles of being out of warranty and I will be making an appointment with a different dealership. I was wondering if anyone else had similar problems.

[tegu69]

To update. A few weeks ago, I took the truck to a local dealership. Since the transmission was almost due for service, I had them replace the filter and fluid and they also reprogrammed it. I just got back from a 400 mile round trip. I stopped in two rest areas and accelerated as I did previously (not full throttle). Both times the transmission shifted at 4000 rpm's or less and did not hang up as it did before. It seemed to shift appropriately for the amount of acceleration and so far I am pleased. I probably should have done one thing at a time, but the service writer seemed to think the reprogramming might help and wants me to call and let him know.

[Dadoffourgirls]

I have had this experience entering the freeway in my previous 2014 Impala and my current 2014 CTS. It happened last night entering the freeway. I think the software is designed to hesitate on the throttle lift, in case it is only momentarily.

[tegu69]

Thanks for the responses. I did a search and didn't find my issue, mostly hard shifting and slipping and such. I always use the tow/haul when trailing. My mileage is 35400 miles and I will make sure it is documented. I figured if it was some kind of computer problem, they would say it wouldn't be power train. The temperature idea is really interesting. I don't think I ever looked at it when starting. A lot of things get warmer after turning off the engine, maybe sending a message to computer to keep higher rpms? Have to get dw to get the video.

[ggardne2]

With a gas engine and an 8000 lb fifth wheel merging onto the freeway this sounds like normal operation. Since you are merging onto a freeway when this occurs I would assume you are heavy into the pedal. In a full (or near full pedal condition) the transmission will be calibrated to not upshift until near the maximum rated speed of your engine (5500 to 5800 RPM). Your engine and transmission were designed to do this, it shouldn't hurt anything.

[1oldtruck]

Have you looked at your transmission temperature before stopping and after restarting? I had a similar issue with my GMC 6.0 when towing. I found the temps would rise while parked for a short time and shifting issues would be present. I let it idle for a couple of minutes to lower temps before shutting down. Seemed to help,

[BurbMan]

Are you running with the tow/haul switch on when towing the trailer? Have you tried the same conditions with tow/haul off? Does the truck do this when not towing? The PCM determines the shift points based on inputs from various sensors and is raising the shift point for OD under certain conditions. As was noted, have the situation documented at the dealer while the warranty is in effectbut it doesn't sound like its even a problem as much as an annoyance.

[ScottG]

If you can, take a video of it misbehaving. Hard to argue with pictures!
I would make sure it gets fixed.

[2003silverado]

I didn't see where you listed a mileage of the truck, but I'm assuming you are coming up on 100,000? Just making sure you know the power train warranty is 5 years/100,000 miles. The 3/36,000 is bumper to bumper. If you are in fact coming up on 100,000 miles, as other have said take it to a dealer and insist they look at it whether or not they think they can replicate it. That will start a history so if shortly after the warranty expires you have a problem you will have a leg to stand on.

[nevadanick]

I had a 2011 6.0 and now a 2015 6.0 for work and it runs up to 5500 rpm Alot before shifting. It doesnt do it in every gear or every time and i dont worry about it. 100k on the last truck.

[jalichty]

I wouldn't care about whether the mechanics can replicate the problem. Take it in and tell them to find it before the warranty expires. Document the problem and keep hounding them. I had a GM product that would indicate on the gauges that the truck was going faster and faster until it pegged out at maximum speed, but never sped up one bit. When it pegged out, it would drop into "limp" mode for a period of time and then recover, but I was powerless to affect this performance. Had it in to the dealer about a half a dozen times and they couldn't replicate it either, hooked up a computer to it a couple of times but it didn't do it then either. After constantly griping for about two years and 20,000 miles the the dealer told me to write out a timeline of the problems and they would work on trading me out for a newer truck. After two plus years and 20,000 miles, it cost me $.10 per mile to give them that truck and get a new one with some upgrades. So, don't give up on this, make sure they do something for you.

Also, do you have the tow/haul in your truck? If so, is it possible that it gets stuck in tow/haul without you knowing it? Just a guess.

[sk734]

Type it into the net and see what threads pop up. Go to the GM site and search there or post your question.

[tegu69]

The transmission is due for service in about 6000 miles (severe duty). I thought I would have it done now when it is at the dealer. My last truck developed a chatter when turning at times. I was told that the rear needed an additive which seemed to help, but it turned out to be a temporary fix and started wearing out the right rear tire. I just don't want to have to pay for something like a new transmission (or rear-end) when the problem could have started while the vehicle was still under warranty. Thanks, Ron.
Thanks I'll look into it. (I must write slow)

[ScottG]

I actually had the same exact issue with a 93 Dodge/Cummins. I know that doesn't help but eventually the problem became so common on those trucks that there was a TSB issued for it.
I would research whether the same has happened for your truck.

[KD4UPL]

Have you had the transmission serviced regularly with fluid and filter changes? If not I'd start there.