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Thomcat

Much commented 8 speed transmission

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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?

 

https://gm-techlink.com/?p=11127

 

(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.???)

 

 

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I have 8500 miles on my 5/19 build and yeah if its cold or I do something stupid or too quick I will get a lil jerk, but any car or truck would and have had same thing happen on all my trucks so was never cause for alarm, just slow down warm it up a bit and smoothly throw into drive and smoothly take off, I can get a bit quick sometimes but that's my fault...….both of my Toyotas did same thing if I just got in started it slammed it into drive and took off with heavy foot.....they aren't made to do that

 

my transmission has been flawless, while driving its great and barely feel it shifting between gears....smooth as butter on freeways and around town....I would buy another in a heartbeat

 

I don't like fact that I cant check tranny is my only gripe...no dipstick, but whateves I just take it in when its time

 

would I get the 10 speed? OF COURSE, why not have more gears, but I certainly don't need one

 

 

Edited by Dunn
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3 hours ago, Thomcat said:

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?

I’d like to know the answer this as well.  Plan to keep the truck 10 years if I could.  

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I have 18,000 on my RST z71 with 5.3/8speed and I am very happy. The new 5.3 and 8 speed is much more responsive, stronger and smoother than my 2011 5.3 with 6 speed. I know many people love the 6.2 and 10 speed. I test drove the 6.2 and 5.3 and at my age (68 years young) I just did not feel enough difference to justify the coin. I spent the difference on my AMP steps and tool box. I live 1/2 mile from the 130 tollway in Texas and drive 85 mph a lot. I can’t believe how my truck runs at 85mph and only pulls about 1700 rpm’s. If money was no object I am sure the 6.2/10 speed is great but I love my truck.

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Really like mine also,  its the 5.3/8 speed,  only issue was a couple of hard shifts, which gm is said was part of the "learning" process.  Must have been right,  hasn't done that since about the 3000 mile mark,  has over 20k now.

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MY previous truck which was a 19 TB LT with the 5.3 and 8 speed tranny operated just fine without any problems. This was a 10/18 build so it would of had the Dexron fluid. Never seen no need to have the fluid changed as I never had any problems, had 21K on it when I traded it for my 2020 LT with the 3.0 and 10 speed.

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I love the 5.3/8 speed combination in my 19 sierra elevation 7/19 build date 3900 miles. Very smooth and responsive, can hardly feel it shifting and has plenty of power. Much better combination than the 5.3l/4 speed combo in my 2001 Tahoe I came from. I definitely dont regret this combination! Sure I would have sprung for the 6.2l/10 speed in a higher trim level if I could have afforded it, but I got a good deal on this 19 at the end of the year. If I want more power that's what the 383 stroker in my 86 k10 is for although this 19 may give it a run for its money.

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2 hours ago, Booger T said:

MY previous truck which was a 19 TB LT with the 5.3 and 8 speed tranny operated just fine without any problems. This was a 10/18 build so it would of had the Dexron fluid. Never seen no need to have the fluid changed as I never had any problems, had 21K on it when I traded it for my 2020 LT with the 3.0 and 10 speed.

DextronVI is hydroscopic, but apparently by itself does not have a tendency to cause shifting problem only it comes in contact with and absorbs water. GM never indicated how water may come in contact with the fluid...... air temp/humidity conditions, manufacturing/filling, inspection, puddle splash, or submersion of the vent.

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8 hours ago, Thomcat said:

DextronVI is hydroscopic, but apparently by itself does not have a tendency to cause shifting problem only it comes in contact with and absorbs water. GM never indicated how water may come in contact with the fluid...... air temp/humidity conditions, manufacturing/filling, inspection, puddle splash, or submersion of the vent.

Just throwing this out there.

 

If it is hydroscopic then we should see more problems, if it causes issues, in area's of high humidity like in the southeast, Florida and such and no so much any issues in the west like Arizona and southern California.  Just thinking out loud on this.

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16 hours ago, Dunn said:

 

....my transmission has been flawless, while driving its great and barely feel it shifting between gears....smooth as butter on freeways and around town....I would buy another in a heartbeat....

 

X2

 

 

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25 minutes ago, Black02Silverado said:

Just throwing this out there.

 

If it is hydroscopic then we should see more problems, if it causes issues, in area's of high humidity like in the southeast, Florida and such and no so much any issues in the west like Arizona and southern California.  Just thinking out loud on this.

Problem is there is no data to go on for failure, especially since the insult that caused the contamination may not have been in the owners area. Just brainstorming, more than likely the history of a p[articular vehicle - a submersion problem rather than atmospheric. One good dunk through high water with a hot tranny could cool it enough to draw a partial vacuum and suck some water in through the vent. GM is not spending all those bucks on warranty 100% refills for laughs.

 

A good example of a similar problem is my '18 Traverse........those equipped like mine with a black shields over the electronic parking brake module are susceptible to contamination with water (probably driving through high water?) and non brake release failure.......in response GM changed production using an upgraded  blue shield to prevent water intrusion which would be also used to repair older units that failed. Dealer will not retrofit units like mine that have not failed and I may be one puddle or rainstorm away from brake lockup....and I'm not about to charge any deep puddles to find out.

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Moisture can occur when the fluid does not reach normal operating temps for sustained periods of time.  Boiling point of water is 212ºF so if the ATF never get to that point, sooner or later moisture is going to accumulate.

Quote

 


GM Dexron HP



"The base stock of old fluid (19355656) has the ability to attract low levels of moisture, which can cause degradation of the coefficient of friction of the fluid. The new fluid (19417577) has a new GTL (Gas to Liquid) base stock that is able to tolerate more moisture. Shuddering transmissions and torque converters have been recovered with the new fluid. New transmission fluid tests have been developed due this issue to prevent it from occurring with new transmission fluids."


 

 

30,000 miles+ and no problems with my 2017 8-sp.  But then again, I replaced the oem fluid with 100% Synthetic AMSOIL early on, before 10,000 miles.  I don't trust any OEM fluids and have for decades drained it out and used what I want.   40 yrs of using AMSOIL ATF and never had a problem. 

You realize the same company who designs and implements the AFM system is the same group that designs filter and oil specs, right?

Edited by elcamino
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Originally the 8speeds were factory filled with Dexron HP fluid not Dexron VI  The 1-2 harsh shift everybody has in the morning or after cold soak is on purpose to heat the fluid!  The faster the fluid warms up the better.  The 8speeds are absolute torque monsters!  They love the input the more the better and behave exceptionally well when HOT. The shifts are very quick and almost seamless.  Driving around town with OLD MAN pedal input is the problem especially with the whole adaptive learning going on.  Then add the 4cly/8cyl cutting in and out while OLD MAN driving.  The overall robustness/integrity of the 8L90 is very solid.   

 

 

Edited by mookdoc6
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13k miles in my 2019 Sierra and the shudder has been constant. It’s often and hard enough that it completely ruins the driving experience for me. Oddly enough it seems to happen more when the trans is warm.

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11,000 miles on my 2019 made on 3/31/19

And, smooth as silk. I really love this truck!

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