2016 Yukon XL Transmission Failure

2016 Yukon XL, 5.3L, 4x4, 6 speed transmission, 120k miles and dealer “rebuilt” the transmission at 50k miles. Transmission started doing weird things like RPMs jumping all over the place, then stopped shifting, then made a noise kind of hard to describe, like loud whining. I haven’t had it looked at yet but I’m guessing a full rebuild could be in order. I’ve called a few shops I’ve dealt with before and was quoted $3500 by one (not a dedicated transmission shop), ~$5000 by a transmission shop, and the third shop suggested a fully stock, manufactured transmission. (I’m in houston, tx.) So, 2 questions:
What’s the better option between a remanned unit or rebuilding?
Second, if rebuilding is the route to go, does anyone have a “build sheet” of the parts that should be upgraded?
PS: don’t know if this is relevant or not, but a couple years ago I had the transmission shop bypass the thermostat (or something like that… it’s been a while and I don’t remember details) because although normal operating temperature was 196 or so, it was reaching into the 255 range when driving up mountains in Colorado. Immediately afterwards it started acting up but then quickly returned to normal, so not sure if it was relearning something or what. Anyway, even with that modification, it didn’t seem to change the normal operating temp or the very high temps when in the mountains.
And as I’m typing this, a third question came to mind… what about upgrading to the 8 or 10 speed transmission as an option?

I would go the GM Certa transmission route if I were you. 3 yr / 100k nationwide warranty. And, based off the use you seem to do with it, keep it serviced on the SEVERE schedule which is about every 40k or so. Keep the fluid thermostat bypassed for sure and maybe look into a external cooler as an additional level of protection.

When climbing anything, engine rpms are your friend with a lower gear that's happy - lugging everything just makes everything hot. I've done the same stuff in CO with my 2008 Denali - 6 speed. I can attest that things get hot when climbing. I usually never shut my truck off when doing this - an idling truck keeps all the fluids moving and cooling.

B-train said:

I would go the GM Certa transmission route if I were you. 3 yr / 100k nationwide warranty. And, based off the use you seem to do with it, keep it serviced on the SEVERE schedule which is about every 40k or so. Keep the fluid thermostat bypassed for sure and maybe look into a external cooler as an additional level of protection.

When climbing anything, engine rpms are your friend with a lower gear that's happy - lugging everything just makes everything hot. I've done the same stuff in CO with my 2008 Denali - 6 speed. I can attest that things get hot when climbing. I usually never shut my truck off when doing this - an idling truck keeps all the fluids moving and cooling.

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Gotcha, thanks. It has the tow package so it has a cooler from the factory. Are you talking about an additional cooler?

It gets real hot when going slow on trails, as in 4Lo driving. But I've also been on a paved road in 55 degree temps but with steep grades and it still got into the 250's.

The entire tranny is controlled by the electronic TECHM module inside the tranny oil pan. If that goes bad nothing works properly. I would drop the pan and inspect for metal in the pan showing parts are failing inside. If the pan is pretty clean, some dust in there is normal, you might just need a new Techm. I installed a new one in my Escalade a couple years ago. A company in Florida sold me one programmed for my car, complete programmed Techm for less than $500 and it was about a 3 hour install. Worked perfect. I live in California and they shipped it to me super fast and shipped a return box for me to ship the old unit back to them. If needed I can find the name of the company for you. Their phone help line answered on the 2nd ring and walked me right through an install issue I was having. They were great.

Edit: I should have mentioned when the Escalade tranny failed everyone told me the tranny was shot so I had it completely rebuilt but used the original TECHM in it. It had the exact same issues after rebuild that it had before. The Techm was the total reason for failure. As I recall the tranny would shift from 1-3 but go into neutral and leave you stranded rather than shift up to 4th in the 6L80 when mine failed. It would work normally if your throttle was only 15-20% applied, more than that and it broke each time.

EDIT: I just looked the shop up, it is Street Smart Transmissions in Florida, family operation, treated me very well. https://streetsmarttransmission.com/remanufactured-valve-bodies/

I personally would recommend the rebuild route. Warranty work is nice but a reman or replacement from OEM is going to reset your clock at 0 and start over again. I prefer the "over build it to last forever" route. My trans build part list is attached. Probably overkill for you cause this is mostly "stage 2" build out for 800HP so you can save some money but also include a billet torque converter from a reputable brand and a trans cooler like a TrueCool 40k.

The build cost me $5400 including the tune, plus the TC ($1300) which I installed years ago. As long as you are using Sonnex or Raybestos parts, you should be good.

8 speed is a big no-no.....