6L80E thermostatic bypass valve replacement and Bulletin #21-NA-199

Andrew18 · April 11, 2022

So I went ahead and purchased the "updated" thermostatic valve for my 6L80e, part number 86774933 referenced in the SB. installation look about 10 min and only required a 10mm and 13mm open end. I do my trans fluid and filter every 22,500 as per the service manual and my fluid temp would hover around 185-195 towing or not towing hot outside or cold. As i live in CT the ambient air temp can be 10 degrees or 90. After the installation of the new bypass valve the trans sits at 145 and never got over 150 on a 2 hour 116mile drive. I also think this is much better than the "pill flip" as you retain the bypass ability for colder weather so the trans still gets up to temp quickly. I have not yet towed anything to see if the temp would go back up to the 180-190 range but I figured I would share this info for anyone else who thinks a transmission designed to run at 192 degrees is crazy.

Also for anyone who wants one and doesn't wanna pay if your truck is still under warranty the dealer will install one of these if you complain about anything with the transmission even if the "Customer complaint Cannot be replicated." that being said i got mine for $60 on GMpartsDirect.

mikeyk101 · April 11, 2022

I agree this is a great step forward. Quite a few of us have already done this and think it should have been done right from the start. There are a few discussions on this already including this one:

jwhjr · April 11, 2022

Did this swap a couple of weeks ago. Temp has only gotten to 158 once and when it did it cooled back down when the thermostat opened. I'll keep a close eye once it gets warmer here when I would notice the 190+ temps.

jamief44 · September 20, 2022

Little late to the conversation here, but does this apply to the 2014-2015 5.3 issue where it feels like it kind of slips/slams into gear when just easing around maybe 1st to 2nd gear? It's not very often but annoying. Took it to dealer with about 5k miles when she was still under warranty and of course that was a wasted trip. At 110k now and saw a video about the TSB etc. Anyone have input? Thanks

Edited September 23, 2022 by jamief44
grammar

Tony Adcock · July 30, 2023

Does anyone have the torque specs for the bypass valve bolt?

Grumpy Bear · July 30, 2023

4 hours ago, Tony Adcock said:

Does anyone have the torque specs for the bypass valve bolt?

Go to 4:40

I have never torqued one and done this about 4 times now. Even reused the gasket. No leaks, no fuss.

Edited July 30, 2023 by Grumpy Bear

AmericanPie · August 29, 2023

I'm hoping that this Service Bulletin covers my 2020 Sierra 2500HD L8T/6L90. I bought this vehicle with 92K on it and 96K now, but because it was originally purchased under a fleet contract, powertrain is covered up to 100K from day one. I know this, because GM just covered a new complete L8T (right off the assembly line), as the original had a catastrophic failure on number one rod at 94K, but that's a story for another day.

Anyway, my 6L90 has an intermittent hard 2-1 downshift, and while towing a 34' travel trailer recently the trans temp went up to 204°F. Things may clear up with fresh fluid, a new thermostat bypass valve, and some driving.

I've since done a flush (using the trans pump, and no machine). I have yet to take it for a trip, but I'm hoping the temps drop a bit.

BTW, with 96K on it, the fluid was spent, but the trans pan was very clean and just a bit of the normal fine particulate accumulated on the magnet.

One other thing...if anyone is going to flush their 6L90 in a 2020 GMC, don't count on a YouTube video with correct info on how to. The setups I saw steered me wrong and I had to move the flush exit port to the lower left hand (drivers side), trans cooling line coming out of the radiator.

Rusty Shackleford · December 3, 2023

Well, I'm on my third transmission due to GM's junk torque converter. Both replaced in warranty, 1st one made it 97k miles, second one gave up at 187k miles. When they installed the current one they did the tsb and it dropped my operating temp to the prescribed 145ish range, but in Tx summer traffic or heavy towing the temp would still creep up into the 180's and take forever to cool off once I got moving again. Often taking an hour or more to drop back into the 160's. I average about 50k miles a year, my truck is my 2nd home.

Back in June I installed this kit ( https://www.amazon.com/WHATEVER-TAKES-TRANSMISSION-PARTS-STL010/dp/B0BCL9M23T/ref=sr_1_2?crid=FSXXUFT1XO12&keywords=superior+products+transmission+bypass&qid=1701636535&s=automotive&sprefix=superior+products+transmission+bypas%2Cautomotive%2C159&sr=1-2

It retains a bypass function and eliminates temp regulation. Took about 30-45min's. I also installed a Hayden cooler under the cab (w/fan) that runs constantly.

My trans temp in the summer now still operates in the 140's (seems like that's just where it wants to be), and creeps up into the 160's under loads and in traffic, but cools back down quickly when I traffic breaks loose and I get some air moving. I haven't seen it get over 190 since, not even towing the trailer and tractor 100 miles in August.

I went from burnt fluid every 25-30k miles to fluid that still looks and smells new at 50k.

It baffles me that anyone at gm would think running an auto transmission at 180+ degrees constantly (and regularly well over 200) was a good idea. On top of that, let's merge the trans cooler with the ac condenser coil so it conducts heat into the transmission when the ac is running, then hide all of that behind a stupid set of louvers/shutters that can, and do, close and restrict airflow. Add in the previously mentioned junk converter and it's way beyond "if it fails". All to gain a few tenths of a mpg. There ought to be a class action suit on these things.

I'm going to pull the trans and replace the JMBX converter with something that doesn't eat itself over the Xmas holiday, might have the trans gone thru but will at least have a tuner change some things up on it afterward to help it. I'm currently at 244k miles so will be doing everything I can to make this one live longer.

I'm in the infancy stages of planning a 6.2 swap around 300k, if it lives that long. I have no complaints about the L83, it has been dependable so far in original form, but I sooo love the LS3 in my 5th gen Camaro.