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Shifting issues


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I have a 2015 ALL TERRAIN EDITION. It’s been a good truck but lately I’ve experienced shifting issues. It hesitated to shift into overdrive and sometimes shifts hard when the engine switches back and forth from 4cyl, to 8cyl. I hear a noise when it shifts and GMC won’t even look at it. They say it shifts normal. I hate FORDS but they shift like butter. Why to do? 

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Check your transmission fluid.  Could be over due for a change.  Wont fix the issue 100% but it will help.  I would suggest checking your front and rear differentials, and transfer case fluids too.  All that can play into the shifting.  Replaced all of mine with Amsoil.

 

Another option is to go for a custom tune.  For example, Black Bear does your ECM and TCM.  Many owners on here love the transmission after the tune.

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2 hours ago, J. Mendoza said:

I have a 2015 ALL TERRAIN EDITION. It’s been a good truck but lately I’ve experienced shifting issues. It hesitated to shift into overdrive and sometimes shifts hard when the engine switches back and forth from 4cyl, to 8cyl. I hear a noise when it shifts and GMC won’t even look at it. They say it shifts normal. I hate FORDS but they shift like butter. Why to do? 

mine was doing this 2 weeks ago, torque converter was slowly failing and sending metal into the valve body causing hard shifts. swapping it out this week, i had 103k

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Mine only has 24,000 and it’s acting up. GMC DOES MOTHING TO FIX IT. It’s always the same answer, we can’t teplicate the problem! If the transmission is under warranty just replace the dam thing. I don’t know if I would buy another GMC. My neighbor got a 2500 HD and said the same thing. His comes with a fancier Allison tranny but it’s all the same. 

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I live a few doors down from one of GM’s transmission engineers who was involved in the project for the 6 speed transmissions. I have had the opportunity to talk to him about my 6L80, risks of running a transmission tune, shift hesitation and other common observations about the trans. He is a pleasure to speak with and has been very helpful.

Regarding your problem, did the vehicle recently have service that updated the computer? The transmission continually learns your driving habits and when the computer is flashed it needs to relearn your driving habits. You may have become accustomed to the vehicle shifting a certain way and are being caught off guard by the different behavior while it re-learns.

Likewise, have any of your habits changed in how you drive or how far you press the throttle down when accelerating? My truck behaves differently running E85 vs gasoline. This impacts my throttle input. The engine shifts based on a matrix of throttle input, speed and engine RPM. When I run E85, I get a boost in horsepower. My vehicle gets up to speed faster with the same percentage of throttle input compared to gasoline. This has an impact on when the vehicle decides to shift. Perhaps you got a winter fuel mix or some lower octane gas? Just a thought.

As previously recommended by another member, I would check your transmission fluid. Read your owners manual about how to properly check it. It must be checked when it’s hot and the vehicle is level. The engineer couldn’t stress to me enough the importance of the transmission fluid being properly maintained. Not over filled, not under filled, not dirty, not burnt/degraded, etc... Pay close attention to the color of the fluid and while driving, monitor your trans fluid temp. There is at least one member on here who reported his thermostat was defective which resulted in premature degradation of his trans fluid and, if I recall correctly, significant transmission damage requiring a rebuild. He also had a lot of difficulty getting it properly diagnosed, even after it was repaired, because the cause of the transmission damage hadn’t been isolated to the thermostat. He was persistent and eventually the thermostat was replaced which fixed the problem before resulting in any more damage. If your trans fluid temp is lower than outside ambient temp, shows abnormally low temp readings during extended or strenuous driving or goes above 200 degrees frequently, you’ve got a problem that needs to be looked at.

For others interested regarding a tune. I asked a few questions about shift firmness adjustments on tuners. He pointed out the main concern would be glazing/burning the clutches and having compounding issues from that over a period of time. They have done quite a bit of research about the longevity of the clutches using stock programming, once you begin tweaking things, it’s anyone’s guess if or how it will hold up. He didn’t say bad or good, he just pointed out it’s an unknown.

If your truck was driving fine before now and recently experienced this problem, I would not try to fix the issue with a tune. The cause of the change must be identified to be sure it isn’t going to cause damage. If you tune it away, you may just be hiding a bigger problem.

We all drive our trucks differently and they experience varying levels of stress on different components. Perhaps you drive your truck like you stole it, or you tow a lot, which may have degraded some of your fluids?

I’m currently changing all of my trans, diff and transfer case fluids with Amsoil. My truck has 70k miles and I drive like a turtle. Preventative maintenance is the name of the game to me. Good luck, I hope you find what’s causing your issue.

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Find another dealer to look at it. If it all of the sudden starting acting different than what it used to, something is not right....You do not have to take a GMC to a GMC dealer for warranty repairs, you CAN go to a Chevy dealer...

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I changed the fluid in my pan and filter at around 65k and it was DIRTY. Almost like coffee looking. Since then I have changed one gallon of trans fluid every 15k miles. The transmission in my truck performs nicely for me. I agree with whomever said it above, do some maintenance on the trans to keep it happy. I am currently at 106K miles and very happy with my truck.

 

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