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What issues were you experiencing?
Did your 8 speed have the original fluid in it?
I think all the 8 speed prior to new body style t1xx came with the Mobil hydroscopic trans fluid which can accumulate moisture over time and cause shifting issues. 
 
My 2018 has original fluid, only 10k miles but no issues other than occasional light clunk if you let off gas in traffic and it upshifts. 
 
Did you mean transmission valve body was replaced? 


I had the exact same issues you’re now having. I had the dealership replace my fluid at 20k miles and just a few day’s ago they replaced the “Throttle Body” and now drives like a Cadillac, no bucking, clunk or searching for gears. The tech told me that’s going to fix everything, so far so good [emoji1303]


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On 3/7/2020 at 3:32 AM, tlaw91 said:


That will not help.
You can have a service fast learn done or have the valve body replaced or have the wiring harness replaced for a failing temp sensor. That’s it.


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Thank you for the input. Seeing as you work on these for a living I believe you.

 

What happens, does the valve body get gunked up inside? Or does the wiring harness for the temp sensor deteriorate?

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  • 3 weeks later...
On 3/7/2020 at 6:52 AM, TXGREEK said:

 


I had the exact same issues you’re now having. I had the dealership replace my fluid at 20k miles and just a few day’s ago they replaced the “Throttle Body” and now drives like a Cadillac, no bucking, clunk or searching for gears. The tech told me that’s going to fix everything, so far so good emoji1303.png


Sent from Above

 

Is this the function of a transmission throttle body?

"The transmission throttle, a mechanism that manages the flow of fluid, contains a butterfly valve. This valve opens or closes to either allow or obstruct the passage of air or fluid and converts line pressure, which is directed to valves in the transmission."

 

Maybe this has been an overlooked problem with the 8-speed the whole time. Maybe the new fluid and a new throttle body is what needs to be done (and a new torque converter if it got destroyed due to the old fluid).

 

My 2017 Sierra 6.2L 8-speed has just started to develop the "rumble strip" feeling at 60mph. It's always had a sporadic hard shift from 1-2 or 2-1 as well. I'm taking it in soon, but I'm afraid the fluid flush is going to dislodge some particles and make things worse. I wish I could figure out the best way to get all of the old fluid out and refill with 100% new, but everyone argues over the best way and there is no consensus.

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Quote

 I'm afraid the fluid flush is going to dislodge some particles and make things worse. I wish I could figure out the best way to get all of the old fluid out and refill with 100% new, but everyone argues over the best way and there is no consensus.

 

 

It’s a proven fact, most of the things we worry about, NEVER HAPPEN.

Edited by elcamino
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11 hours ago, Mileguru said:

 

My 2017 Sierra 6.2L 8-speed has just started to develop the "rumble strip" feeling at 60mph. It's always had a sporadic hard shift from 1-2 or 2-1 as well. I'm taking it in soon, but I'm afraid the fluid flush is going to dislodge some particles and make things worse. I wish I could figure out the best way to get all of the old fluid out and refill with 100% new, but everyone argues over the best way and there is no consensus.

You're waiting on a consensus? Interesting. No judgement just making a note.  :)

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15 hours ago, Mileguru said:

Is this the function of a transmission throttle body?

"The transmission throttle, a mechanism that manages the flow of fluid, contains a butterfly valve. This valve opens or closes to either allow or obstruct the passage of air or fluid and converts line pressure, which is directed to valves in the transmission."

 

Maybe this has been an overlooked problem with the 8-speed the whole time. Maybe the new fluid and a new throttle body is what needs to be done (and a new torque converter if it got destroyed due to the old fluid).

 

My 2017 Sierra 6.2L 8-speed has just started to develop the "rumble strip" feeling at 60mph. It's always had a sporadic hard shift from 1-2 or 2-1 as well. I'm taking it in soon, but I'm afraid the fluid flush is going to dislodge some particles and make things worse. I wish I could figure out the best way to get all of the old fluid out and refill with 100% new, but everyone argues over the best way and there is no consensus.

I have 120,000 on my 2015 8 speed and had the fluid replaced at 100,000 with Mobil 1 Synthetic LV ATF HP. It fixed the shudder and crappy shifting and now drives and shifts as smooth as butter.

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It completely fixed the shifting issues I had with the clunking and dropped the pan, added fresh fluid to it. IMO, Dropping the pan and draining it then adding fresh fluid isn’t as drastic as flushing. I believe the Torque converter will be looked at next, maybe. The good thing is that it’s not giving me any issues, very strong and tuned 6.2. But, if GM didn’t fix the issue, I’d have a brand new Waptor on da driveway!


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