26-NA-049 and Transmission ‘fix’. BIG news IMO

[dequardo]

In a separate post under the main discussion forum I noted my dealer had applied 26-NA-049 ostensibly to fix the ‘Service Brake Assist’ error message I was constantly seeing.

WELL, I have no idea whether it did that BUT I now see that my RPMs at 70mph are now about 1800 vs 2200 for the entire 15 months we!ve had the vehicle. I’ve verified the rpm differences now in both directions on the freeway to eliminate any wind effect.

I think this is big and something everyone should be aware and informed about. I’d certainly ask your dealer about it stat. This can’t help but improve the poor MPGs many of us report.

Would love to read any details that people may have on this bulletin/update.

Mike

 

[GaryM]

In a separate post under the main discussion forum I noted my dealer had applied 26-NA-049 ostensibly to fix the ‘Service Brake Assist’ error message I was constantly seeing.

WELL, I have no idea whether it did that BUT I now see that my RPMs at 70mph are now about 1800 vs 2200 for the entire 15 months we!ve had the vehicle. I’ve verified the rpm differences now in both directions on the freeway to eliminate any wind effect.

I think this is big and something everyone should be aware and informed about. I’d certainly ask your dealer about it stat. This can’t help but improve the poor MPGs many of us report.

Would love to read any details that people may have on this bulletin/update.

Mike

Thanks for the update. I have one of the first 2025 RS to come off the line and 8th gear has been an issue from day one. Reported it many times but the old "working as designed" comment was the result. I "learned" how to achieve 8th gear through trial and error however it would never shift to 8th on it's own without some "technique" to get it there, ala, less than 62 mph, no throttle. I forwarded your comments to my service rep. Maybe it will get them thinking about this as a way to get me off their back about it.

 

[dequardo]

After driving it yesterday I still think this needs work. It was back to 2200rpms at 70mph. I tried shifting to L8 manually but no change. Then suddenly it dropped back to 1800. Not sure what to think now.

 

[GaryM]

After driving it yesterday I still think this needs work. It was back to 2200rpms at 70mph. I tried shifting to L8 manually but no change. Then suddenly it dropped back to 1800. Not sure what to think now.

Hmmmm. I have a friend who worked at GM test track. His take on this whole thing was the original download to the TCM was corrupted by a disconnect before the download completed. Given the nature of the defect it became pretty obvious, at least to him, the issue wasn't mechanical. The fact that 8th gear can be achieved and held w/o issue points to a programming error or exclusion in the parameters coding for 8th gear acceptance. We're going to reprogram the TCM Wednesday and I'll make sure this update you reported is included. We also found the transmission reacts to ATF when it's colder than 150 deg F. Previous models (6 spd) used a dual role ATF cooler to heat it with coolant during warm up. Starting in 2025 they went to a cool only system allowing the transmission fluid to warm on it's own. That's a slow process especially when it's near zero like it is around here. Mine was sticking in 6th gear, ignoring 7th and 8th all together. I changed the ATF the factory uses to a product Mobil 1 makes. It's still a GM approved Dexron VI, just better cold weather properties. That helped quite a bit. Still didn't get me 8th gear, but all the other shifting performance has improved. I still have to use a technique to get 8th gear. Once there it performs very well.

 

[dequardo]

Good stuff Gary. Reprogramming 26-NA 049?

 

[Sewtrapped]

Hmmmm. I have a friend who worked at GM test track. His take on this whole thing was the original download to the TCM was corrupted by a disconnect before the download completed. Given the nature of the defect it became pretty obvious, at least to him, the issue wasn't mechanical. The fact that 8th gear can be achieved and held w/o issue points to a programming error or exclusion in the parameters coding for 8th gear acceptance. We're going to reprogram the TCM Wednesday and I'll make sure this update you reported is included. We also found the transmission reacts to ATF when it's colder than 150 deg F. Previous models (6 spd) used a dual role ATF cooler to heat it with coolant during warm up. Starting in 2025 they went to a cool only system allowing the transmission fluid to warm on it's own. That's a slow process especially when it's near zero like it is around here. Mine was sticking in 6th gear, ignoring 7th and 8th all together. I changed the ATF the factory uses to a product Mobil 1 makes. It's still a GM approved Dexron VI, just better cold weather properties. That helped quite a bit. Still didn't get me 8th gear, but all the other shifting performance has improved. I still have to use a technique to get 8th gear. Once there it performs very well.

2026 LT (no CVT). Stupid question: how can I see what gear I'm in without using the manual shifting paddles?

 

[GaryM]

2026 LT (no CVT). Stupid question: how can I see what gear I'm in without using the manual shifting paddles?

As far as I know that's the only way unless you just remember what speed/rpm represents the gear. I use 70 as a guide. 2800 in 6th, 2200 in 7th and 1800 in 8th. The paddle shift will indicate a gear but if all the parameters in the coding aren't met it won't actually shift into that gear. Even though a paddle shift is there the coding in the TCM still dictates whether it's allowed to shift to that gear.

 

[dequardo]

To be clear. In my case at least, there is/was no difference in L7 or L8. 2200 rpms at 70 in each. When it’s working correctly it’s 1800

 

[GaryM]

To be clear. In my case at least, there is/was no difference in L7 or L8. 2200 rpms at 70 in each. When it’s working correctly it’s 1800

Exactly. The paddle may show that 8th is selected but unless all the criteria is met it will stay in 7th gear, which is 2200 RPM. You can achieve 8th gear w/o the paddle shift by adhering to this criteria, < 62 mph, no throttle. It will shift every time. However, if you give it a bit too much throttle to get up to road speed it will down shift to 7th where it will stay. If it's shifting right, at 70 mph engine RPM will be 1800. The criteria is quite complex but in short it contains speed, throttle position, acceleration rate, ATF temp, loading % (combination of acceleration and engine load factor). The TCM has programming that uses all that plus fill time in the shift cylinders to determine if it's "safe" to move to the next gear. That's where the ATF viscosity comes into play. The factory fill is GM Dexron VI. Good stuff but in cold weather it's thick, which slows down the fill rate in the shift cylinders. That could cause it not to shift to the next gear, or in their terms. reject the shift. In some cases it may allow the shift but because the shift cylinder fills slightly slower you get a "shudder" in that gear. I had it in second gear until I changed the ATF to Mobil 1 Devron VI which has a better cold weather pour point. Shudder is gone.

 

[Sewtrapped]

Whew! TMI! I guess I'll ignore all this and just drive and hope it works as designed. I'm old and this is prob my last car. After only 6 months of ownership, I achieved only 1500 miles. Best of luck to you all.