8L90 Questions (OS updates, TCM location, Manual shift programming, etc.)

Where is the TCM located on the 8L90 trucks? Is it in the tranny or external?

Can I swap a 2015 GMC 8L90 TCM with a 2016 GMC 8L90 TCM to try out the 2016 8L90 OS on the 2015? Will it plug right in and run or will there be VIN mismatch issues?

Another option - Can I update 2015 TCM to 2016 OS? Do I just need to buy credits for both 2015 and 2016 vehicles? Any VIN issues or anything that would not make the 2016 TCM OS run properly on the 2015 TCM?

On the trucks the manual shift mode simply limits the max gear to the selected gear (i.e. if you select M6, it will shift around as it pleases in gears 1-6 but not 7-8). Is there a way to program the software on the truck to basically lock the selected gear so the truck doesn't shift until you push the "+" or "-" button?
One article I read on the 8L90 calls this Driver Shift Control - "Driver Shift Control allows the driver to shift the transmission like a clutchless manual gearbox. Electronic safeguards prevent over-revving if the wrong gear position is selected." (http://gmauthority.com/blog/gm/gm-transmissions/m5u/)

Thanks in advance

If the option is available, in the software, have you checked to see if TUTD is enabled or disabled?

Sounds like a similar issue with 12-15 Camaro 2LS trim models, "sport mode" didn't allow full control of the shifting with the paddle shifters, it was just as you specify here, just limited the max gear... The work around was to enable the TUTD in the tune, zero out a couple tables, and bam it worked like you would want it to... I would thinking you should be able to do something similar with your tune, but the a8 tcm tables are limited. Just looking at my 2016 a8 camaro, I would imagine you might be able to pull it off.

Might help if you upload a copy of your tune.

In the Auto shift properties tab, Might try enabling TUTD, Serial, and then max out the TUTD upshift values in each column in the auto shift speed tab (if TUTD shows up)

Thanks for the response. See attached tune.

I think the tables are limited, but I can't make sense out of the ones that I have available to me. :confused: To start, I am assuming there should be (at least) 4 tables to handle the shifter being in full auto or manual and then whether or not the Tow/Haul button is pushed. I was hoping maybe I could play with the Tow/Haul tables or simply the manual shift tables to try some things out so that I could revert back to the auto table (shifter back in "D") if I did something incorrectly. But, looking at what is available I can't conclusively understand which tables apply specifically to manual mode and/or Tow/Haul...

Can you elaborate on some of the switches and tables you would suggest modifying to get TUTD to hold gears rather than simply limit max gear?

Any explanation and help is appreciated.

Thanks

I would guess that either pattern A or pattern B is related to the tow haul mode? Not sure which, just a guess. Judging from the shift speeds, I would guess pattern A?

It looks like the TUTD tables I mentioned are already populated correctly (at least similar to my 2016 camaro)

Looking at your tune, Transmission > Auto Shift Properties > General

TUTD is disabled. Try enabling it? In theory it should then be active when the gear selector is in Drive and you use the + - button to change gears. I would give that a shot.

On the 12-15 camaro, you would ENABLE TUTD, and set the active range to D4 (not sure why, it just worked) in the Auto autoshift properties > General tab, then in the Auto shift speed tab zero out the SAFTEY SHIFTS tables for D1, D2, D3, and 5th and 6th in the D2 and D3 rows. Then you would zero out the Tap Up / Tap Down table in the FULL THROTTLE SHIFT SPEED column. Then it would work... On a 12-15 2LS v6 camaro...

That's about all I can suggest. hope that makes sense and works. I don't have a truck to experiment on .



ALSO, looking at your tune, it says you have an A6??? You said you have an A8 above? Very strange.

It says A6 because I uploaded the wrong file! My apologies. I deleted and uploaded the correct one above.

Is the TUTD normally supposed to work with the gear selector in "D"? I was thinking it would have to be in manual mode "M"? That's the same place I use the buttons now (they just unfortunately only limit the max gear not hold the gear).

I can see where to enable the TUTD, but lose you where you talk about setting the active range to D4 and zeroing out the safety shift tables. I don't believe those exist in the Sierra tune?

I will likely have to do some experimenting simply to find out which tables are which unless someone can shed some light on the default table vs. Tow/Haul mode, etc. It would be nice if they were labeled a bit more clear or at least gave some reference in the description to what they might be.

It appears the TCM is external on the 8 speeds based on post 5 and 8 here:
https://ls1tech.com/forums/automatic...l#post19042726

Anybody have any insight as to whether I could take a 2016 TCM and simply swap it to my 2015? I'm concerned about potential VIN mismatch issues with other controllers on the serial bus...???

The 8L90 TCM is located on the firewall with the trucks, Real easy to change. The issue with switching them around like that, is the calibration stored in the TCM is mated to the valve body characteristics. It would likely need a full learn to make the trans happy. Can't say for sure if a VIN mismatch would cause issues as well.

Thanks for that info and insight on the TCM. Can we force a tranny learn with HPTuners? Or, is a Tech2 or similar (dealership only) type device needed?

As far as I know, a GM MDI would be needed to initialize the valve body to the TCM.

There would be a VIN mismatch as well. Dang near every module is married to the VIN of the vehicle now.

Sorry to dig up an old thread, but has anyone seen if TUTD will work if enabled, in drive, with the +/- button on the selector?

yes does work very well, when enabled. changes the D & M to both M and swaps the modes around.

I should have mentioned I was asking for my truck just to make sure the cars weren?t different.

What I was looking to do is tap up or down a gear temporarily while in drive, have it change the gear, them eventually time out and go back to drive.

Could anyone advise what tables and what types of changes to speed up the time it take from
Pressing the “+” or “-” button and the transmission starting its shift? On my 18 8 speed, (TCM unlocked), there is a delay between me tapping the button and the transmission starting the change.