6 Speed Transmission Hack Now an official GM procedure

rav3 · November 5, 2021

This may be old news to some, but I ran across this Service Bulletin "#21-NA-299 - Information on Installation of a New Thermal Bypass Valve (TBV)" where GM now does the "Hack" with a new up-dated kit. I think the P/N is 86774933 but you can read the Bulletin here. I'll paste a copy below in case the link doesn't work. Now GM wants the transmission to operate @ 158 F (70C) instead of the 194 F (90C). I did the un-official change over when I got my truck & transmission runs so much cooler & better.

mikeyk101 · November 5, 2021

I will have the new valve and gasket soon and am going to do this. One thing is that I have found information about Service Bulletin 21-NA-299 but not the actual Service Bulletin. The changeover should be simple enough because I also did the pill flip on mine earlier this year. I should have done it a long time ago. One big difference between the pill flip mod and using this new valve is that full cooling happens all the time with the unofficial mod but won't happen until 158⁰ F with the updated valve. But that is still a tremendous improvement over the original 190⁰ F valve...

jwhjr · November 5, 2021

My parts showed up today and I plan to swap them out this weekend hopefully.

Grumpy Bear · November 5, 2021

Somebody please report back on actual throttled temperature once it's open. Please.

dieselfan1 · November 5, 2021

I wonder if they just opened a huge can of worms.

Now I'd your trans fails you can say it was GMs fault since now they want it to run cooler.

I'm leaving my bone stock truck alone and let it fail.

I have a 150,000 mile warranty with 85,000 left.

Now my lifted truck I flipped it and I'll leave it .

The new setup opens at 158° and I'll bet it never stays that cool.

Now that it's colder my fluid rarely goes above 130°.

I'm with Grumpy let's see some results with this new setup.

newdude · November 5, 2021

The new t-stat is good, but its 5 years too late. They should have had this from when these 2014's came out.

This is a great option however, for those that want to have the thermostat especially for winter operations. Running cooler is good, but running too cool (winter) could potentially lead to moisture concerns in the fluid (think GM 8 speed fluid where the prior formulas were too absorbing of water).

Driving hard in my Sure Cool'd 6L80 on a hot day, I can get it to maybe 170F but no hotter. The new stat should perform somewhat similarly when its opened.

Also, one thing to take note, at some point in 2015 GM stopped flowing the trans through the radiator and the cooler. Late 2015's its trans straight out to the cooler that is part of the condenser. 2014 to early 2015 its trans to the engine radiator, then to the aux cooler, so those trucks with a t-stat delete would still run warmer than a late 2015-2018 and 2019 LD.

Grumpy Bear · November 6, 2021

5 hours ago, newdude said:

Also, one thing to take note, at some point in 2015 GM stopped flowing the trans through the radiator and the cooler. Late 2015's its trans straight out to the cooler that is part of the condenser. 2014 to early 2015 its trans to the engine radiator, then to the aux cooler, so those trucks with a t-stat delete would still run warmer than a late 2015-2018 and 2019 LD.

Pepper was built 3/8/2015. Goes to the radiator only. I've posted her ATF vs Air Temp many times and it does run pretty cool in the winter. I've thought several times about adding an aftermarket thermostat to set a 'floor' on the temperature and an additional air exchanger to clip the top which now, in stalled or stop and go, will get mid 180's a few times a year.

Most days in the summer she sits mid 150's and I find that perfect. My wanting to wait for more feedback is predicated on the fact that the 192 F unit, two of them, throttled over 220 F on a average summer day running under 60 mph on the open road. If that holds true for the 158 F unit I'm still over 200 F. A point I don't care to flirt with. If it will hold 165 F, I'm in.

newdude · November 6, 2021

11 hours ago, Grumpy Bear said:

Pepper was built 3/8/2015. Goes to the radiator only. I've posted her ATF vs Air Temp many times and it does run pretty cool in the winter. I've thought several times about adding an aftermarket thermostat to set a 'floor' on the temperature and an additional air exchanger to clip the top which now, in stalled or stop and go, will get mid 180's a few times a year.

Most days in the summer she sits mid 150's and I find that perfect. My wanting to wait for more feedback is predicated on the fact that the 192 F unit, two of them, throttled over 220 F on a average summer day running under 60 mph on the open road. If that holds true for the 158 F unit I'm still over 200 F. A point I don't care to flirt with. If it will hold 165 F, I'm in.

Yea the V6 don't have the aux cooler in the condenser I should have mentioned that.

mikeyk101 · November 7, 2021

Well I did one last run up to my Wisconsin place with the pill flip mod. Temps were in low 50's and the drive was roughly 55 miles. It was a mix of about 5 miles first on city streets and then about 40 miles of highway then back to city streets. After about a half hour, it got to around 135⁰ F and before I stopped at destination, it was at 147⁰ F.

So now it was time for new thermo valve. I got it on and drove home later. It took probably about 1/2 hour before it reached the 158⁰ and then after getting on the highway, it did initially get up to 183⁰ F but after some stop and go and then accelerating pretty heavy to get up to speed on highway. After a bit,, it did drop down to about 171⁰ F. After about 40 miles I got off highway again. This time I noticed something unusual. It was hard shifting from 1st to 2nd. Then after about a mile, I noticed the trans was slipping a bit. Then came the odor. I could smell trans fluid. I quickly hurried home so I could take a look. Sure enough, there was a nice coating of trans fluid all over the undercarriage from where the valve was all the way back. I checked the trans dipstick and there was no fluid at all! So I got out the socket set to remove it and take a look.

Didn't take long to find the problem. I forgot to make sure that the o-ring was on both cooler lines. One had it and one didn't. Doh! So this will be an easy fix but hopefully no damage was done... I had to make a run back up to Wisconsin place in my wife's car. A quick check in the oil pan I had under trans and I found the missing o-ring. I will tackle it again tomorrow. I picked up 2 gallons of Dexron VI to replace the missing stuff. Not sure how much got lost...

So quick tip, make sure the o-ring is on both trans fluid cooler lines before tightening them down. On a positive note, had this happened and I was running the pill flip mod, I probably would have ended up far worse off. And I knew better too because I saw them the first time when doing the mod. I got careless.

M1ck3y · November 7, 2021

This makes absolutely zero sense. I mean that gm is doing this now...

Grumpy Bear · November 7, 2021

13 hours ago, mikeyk101 said:

Well I did one last run up to my Wisconsin place with the pill flip mod. Temps were in low 50's and the drive was roughly 55 miles. It was a mix of about 5 miles first on city streets and then about 40 miles of highway then back to city streets. After about a half hour, it got to around 135⁰ F and before I stopped at destination, it was at 147⁰ F.

So now it was time for new thermo valve. I got it on and drove home later. It took probably about 1/2 hour before it reached the 158⁰ and then after getting on the highway, it did initially get up to 183⁰ F but after some stop and go and then accelerating pretty heavy to get up to speed on highway. After a bit,, it did drop down to about 171⁰ F. After about 40 miles I got off highway again.

Great writeup. Hope there is no damage. I wonder if the temps would be lower if the oil was going to the cooler instead of all over the bottom of your truck. The missing "O" ring. Was that the top or the bottom line?

One of the things about the OEM thermostats is that they do not have a 'minimum flow bypass' like the aftermarket units. And because of that the trans takes longer to warm up. The cooler is a heater until the trans fluid is higher than the water temperature in trucks that still use the water tank side primary cooler.

Anyway....once you get it repaired and have time to observe its behavior please give us the 411... if you please.

mikeyk101 · November 7, 2021

Update...

I got the missing o-ring back on. It was on the lower cooling line. Got it all buttoned up then started the truck to get trans temp to around 70⁰ F for cold fluid check and it was probably about a quart or so low so I added some. (I had to go with Castrol Transmax Dexron VI full synthetic because that's about all I could get late on a Sat nite.) Then while still parked I ran it through the gears and then kept pressing brake and accelerator to get the temp above 160⁰ F. It didn't take too long and now a check showed it barely reaching bottom of dipstick. What a pain it is to not know how much fluid was lost because it took forever adding, running through the gears, letting the fluid settle out of fill pipe and then trying to read that stupid dipstick. If I was doing a fluid exchange, I could just add the amount I took out and it would be good. I ended up adding a tad over 4 qts and was able to get it to top of "Hot" hash mark.

But it looks like I dodged a bullet. No unusual sounds coming from trans, harsh 1st to 2nd shift gone, and no more slipping. It seems to be acting normally. I drove it to the car wash several miles away to get the undercarriage and back end cleaned off. I did a couple hard accelerations as well as some normal stop and go driving. Before leaving to go the trans temp was around 172⁰ F because of getting it warmed up for fill. It never went above 176⁰ F but stayed mostly in the 174⁰ F range. Outside temps were mid 50⁰'s.

Worst thing so far was the customizing of my right fender last night when trying to get close to the garage. The alley is unfortunately narrow and garage door isn't high enough to pull truck in. That's the main reason I don't do anything here and do all the maintenance up at other place. But last night, I didn't have a choice. There is a chain link fence that is lower than front end so it is a serious blind spot and while trying to maneuver past it to park in front of overhead door, I clipped it. Bumper was fine as it looks like I only caught the cap and top clamp. Unfortunately that will not buff out...

Edited November 7, 2021 by mikeyk101

Grumpy Bear · November 8, 2021

6 hours ago, mikeyk101 said:

Update...

Trouble magnet. Yikes!! That's sad sir. As they say, "No good deed goes unpunished".

Perhaps when you get a few hours on the Interstate you will be able to get steady state down the road numbers and share them? But please, don't take a deer down doing it.

Thanks for all you hard work.

mikeyk101 · November 8, 2021

Just now, Grumpy Bear said:

Trouble magnet. Yikes!! That's sad sir. As they say, "No good deed goes unpunished".

Perhaps when you get a few hours on the Interstate you will be able to get steady state down the road numbers and share them? But please, don't take a deer down doing it.

Thanks for all you hard work.

You just had to add in the deer comment and jinx me... Not nice, LOL! Last deer I tangled with was almost 20 years ago while driving a 99 Tahoe. Got almost an entire new front end.

The photo looks way worse than I think it is. It looks like if I go with OEM wide fender flares (although not a big fan of them) they just might be wide enough to make it disappear. I found them listed at $280 which would be way cheaper than body work.

Although a deer strike would get the entire front end fixed with insurance and no more worries. Just thinking out loud...

Grumpy Bear · November 8, 2021

23 minutes ago, mikeyk101 said:

You just had to add in the deer comment and jinx me... Not nice, LOL! Last deer I tangled with was almost 20 years ago while driving a 99 Tahoe. Got almost an entire new front end.

The photo looks way worse than I think it is. It looks like if I go with OEM wide fender flares (although not a big fan of them) they just might be wide enough to make it disappear. I found them listed at $280 which would be way cheaper than body work.

Although a deer strike would get the entire front end fixed with insurance and no more worries. Just thinking out loud...

My last was hitting a downed deer, on her knees, head on with a Civic. I was blinded by lights from the truck that took her down. Drove right over the top of her. All four wheels off the ground like a ramp jump. Bent the AC lower brace. That was it. The guy behind me caught the blood spray. Never knew he was there. He caught me a few days later at the dinner laughing his butt off.

"Good air"! he laughed and finished with, "Ya own me a car wash".

"Charge it to the dust and let the rain settle it" I giggled.

Took it for an oil change a few days later and asked Jason to look for damage I hadn't seen. Said the deer took the brunt of the hit and charged me for washing the underside in the bay on the lift. Shop smelled all summer. YUCK.