Infamous 8 Speed Transmission Clunk - Dealer/GM Current Position on the Issue?

sgiangregorio · March 4, 2021

Well I'm about a month past the TSB for the fluid change, and I still have problems. Today when pulling out of garage (truck's temp was probably around 60 degrees), I put it into drive and she rev'd up to about 2000 before it dropped into gear and started moving. Then it was fine the rest of the day. And still the occasional hard shift. Time to go back for the full Torque Converter replacement?

bsprtsgrp · April 7, 2021

I had my fluid swapped out at 18K miles on my 2017 back in December and today (April 7th) at 22K miles the clunking and hard shifting is already back again.

lucas287 · April 8, 2021

On 3/4/2021 at 3:58 PM, sgiangregorio said:

Well I'm about a month past the TSB for the fluid change, and I still have problems. Today when pulling out of garage (truck's temp was probably around 60 degrees), I put it into drive and she rev'd up to about 2000 before it dropped into gear and started moving. Then it was fine the rest of the day. And still the occasional hard shift. Time to go back for the full Torque Converter replacement?

If you're under warranty, yeah I'd pitch a fit about that.

lucas287 · April 8, 2021

On 4/7/2021 at 6:40 AM, bsprtsgrp said:

I had my fluid swapped out at 18K miles on my 2017 back in December and today (April 7th) at 22K miles the clunking and hard shifting is already back again.

SO much of the clunking/hard shifting can be attributed to the programming. The TC stays locked through shifts at part throttle and combined with the Driver Demand table commanding high negative torque at low speeds it feels clunky. If you have DOD enabled, that commands a different amount of slip too, which can cause shuddering. I've been able to tune my 8 speed to be a very compliant and happy transmission. It's quite remarkable when you realize how sensitive the programming is. I can make a simple MAF-fueling adjustment and it changes that way it shifts. Torque-based ECM/TCMs are very finicky and I'll GM left a lot to be desired when it comes to driveability.

I say all this, because you might be chasing your tail. The transmission itself is quite robust. The programming just sucks.

bsprtsgrp · April 8, 2021

My AFM is turned off via my Blackbear ECU Flash. Blackbear does not do TCU tuning as the TCU is locked by GM. I'm taking it back in to see if there are any TCU SW updates and a re-learn. My mechanic there told me upfront that the Mobil1 HP Fluid swap was only short term solution

Edited April 8, 2021 by bsprtsgrp
spelling

lucas287 · April 8, 2021

23 minutes ago, bsprtsgrp said:

My AFM is turned off via my Blackbear ECU Flash. Blackbear does not do TCU tuning as the TCU is locked by GM. I'm taking it back in to see if there are any TCU SW updates and a re-learn. My mechanic there told me upfront that the Mobil1 HP Fluid swap was only short term solution

you CAN tune 2017 TCM, it's just costly

Thankfully, my TC was replaced at 34,000 miles back when they still allowed for that. I've fully accepted that one day it might screw up again, by then I hope the aftermarket support has caught up and can get an aftermarket TC. Even then, some very smart people have told me that the TCM algorithms are flawed and it will always be a quirky acting transmission. (even with tuning)

bsprtsgrp · April 8, 2021

HP Tuners?

lucas287 · April 8, 2021

29 minutes ago, bsprtsgrp said:

HP Tuners?

Yup, seem to remember seeing something about EFILive not working on the T87A.

ic3man5 · April 8, 2021

4 hours ago, lucas287 said:

you CAN tune 2017 TCM, it's just costly

Thankfully, my TC was replaced at 34,000 miles back when they still allowed for that. I've fully accepted that one day it might screw up again, by then I hope the aftermarket support has caught up and can get an aftermarket TC. Even then, some very smart people have told me that the TCM algorithms are flawed and it will always be a quirky acting transmission. (even with tuning)

As someone that has spent about 4-5 years with the 6L80 (6-speed) I can confirm the quirky-ness comes from adapts, TCC slip, and driver demand tables mostly. I have adapts turned off and it feels amazing now.

bsprtsgrp · April 8, 2021

I have the M5U Tranny...

Nitrousbird · April 9, 2021

Exactly 1 year and just under 4000 miles since the fluid change and it fixed my shudder issues. Includes a 700 mile tow in there as well, some of which was through mountains. At least in my case, it looks like the fluid was the total solution.

Agriont · April 9, 2021

Three relearn resets. Transmission flush. TC changed. One more relearn reset. Still pushing and clunking. Still under warranty. GM wants to take the transmission apart to look at what is causing it. Should have kept my 2010.