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Tired of reading dozens of posts on shifting issues with 8 speeds implying there is a common design problem with the tranny.....seems not a true design defect would affect all units. Just hit 6000 miles on my post 3/19 production 8 speed factory filled with Mobil 1 Synthetic LV ATF HP transmission fluid. First cold overnight (32F or below) 1-2 shift I sometimes delayed or harsh which can be avoided with a 3-5 warm up. Other than that quirk shifts smooth first time, every time, throughout the range in all temperatures.
Here's my .02 of opinion, guesses, and experience on the 8 speed........take it for what it is worth. I find it a fine tranny, smoother and more responsive than my previous 6 speed. Luck or something else? All 8 speed trannys are not equal.......might depend upon which version you own, how you treated it and what juice it is using.....appears there are at least 3 different versions:
Prior to 2019 production the 8 speeds came with a factory fill of DextronVI. Basically these trannys contained a mechanical pump to produce hydraulic pressure to operate all components in the system , valve body to direct fluid routing, corresponding servos apply pressure to the bands to brake or release clutches and drum sets of planetary gears, drumsets each contain two gear combinations, and multiple drumsets are simultaneously braked or released to get all 8 gear combinations.
Beginning with 2019 production a start stop version of the tranny was developed. In the previous years' the engine had to be running to drive the mechanical tranny pump to produce hydraulic pressure. This version would not work with start stop since when the truck is in auto stop the engine is off, the mechanical pump stops, pressure is lost to the servos and all the bands/clutches will release. A restart would require a significant delay to reapply pressure to get the tranny moving in the correct gear. These start stop trannys incorporate an accumulator (pressurized reservoir) to store pressure to maintain pressure on the bands while the engine is stopped until restarted and the mechanical pump resumes application of pressure.
All post 3/19 production, with the 3/19 door sticker or later build date, incorporate a factory fill of Mobil 1 Synthetic LV ATF HP transmission fluid instead of the previous DextronVI. As indicated in the tech article below seems Dextron VI is hydroscopic and operation with a hydrated solution of DextronVI would seem related the shudder problem. Otherwise why would GM spend tons of money equipping dealers with specialized equipment and instructions and drums of new Mobil1 stock for 100% replacement of all previous Dextron in the trannys and torque convertors? The Mobil1 is backward compatible and recommended for use in all 8 speeds instead of Dextron replacement. Just a thought but this could be related my first hard cold 1-2 shift if the accumulator bleeds down overnight might be the cause of the first 1-2 hard shift.....as far as I'm concerned a quirk not a defect.
Will any given tranny or torque convertor have a problem? Looks like a crap shoot biased by luck, historical use, and version of the build. Here's some questions that come to mind:
Will all production versions of 8 speeds develop a problem due to a design defect? Seems not since many work just fine and the only ones you hear about are complaints posted rather than the trouble free ones.
Are all DextronVI fill destined to have a shifting problem, or possibly just a small portion that develop hydrated fluid? How does it get hydrated?....absorption of atmospheric humidity alone or actual submersion of the vent from puddles or high water?
What damage is done by hydrated DextronVl....does it form surface rust on metals, i.e torque convertor clutch, drums or friction characteristics of the band/clutch materials, which can affect shift quality.......is it permanent?.....is it reversible?.........ill fluid change alone correct it or just mitigate it?
(Read that the 10 speeds use a different approach to handle the auto stop cycle.....instead of an accumulator they use an electrical hydraulic pump to pressurize the bands during the stop cycle. Will this change develop into an age related problem like DextronVI hydration in the 8 speeds.???)
My name is rich, I am an absolute newbie at troubleshooting like this. I have a 2007 GMC Sierra2000HD, it worked fine until yesterday night, someone pulled down hard enough on the shifting lever that the indicator will show as if it was in the R position.
The vehicle, will not start now, the key will not release the steering wheel or shifting lever.
The key was Not in the ignition when the damage occured.
It is my work truck and i have to have it fixed by tomorrow morning or the world stops spinning,
Please help me at least get started.
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