2004 Suburban 8.1 with the 4L85E transmission. Trying to improve efficiency and responsiveness a bit as the stock transmission calibration does not at all make use of the torque the 8.1 has to offer.

First issue - the stock converter tables lock the torque converter in 4th gear at 45 mph, which is around 1600 rpm. GMT800 4L60E calibrations do this much lower, in the 1100 rpm region, and it seems like for around-town driving, earlier lockup would help. Problem is, no matter how low you command lockup speed, it will not lock in 4th below 45 mph. Is there some other parameter that we don't have access to for the 4L80 that limits this, or have other people had luck commanding slower minimum lockup speeds?

Second issue - TCC lockup during shifts. I've searched all over the place and can't seem to find anything on this. The stock calibration commands lockup in 2nd (45 mph), 3rd (56 mph), and 4th (70 mph) from around 50% throttle all the way up to WOT and this is confirmed by the 4L80E guides that have been posted. The trans shifts 1-2, then the converter locks in 2nd around 4000 rpm, shift into 3rd locked directly (I have NOT confirmed this with a datalog, but this is what the tables say is happening). The WOT 3-4 shift happens faster than I need to go in a Suburban and I really don't care about it. What I do care about is that, when I'm towing, the shift tuning gives be a 1-2 shift, a 2-3 shift, and then locks the converter in 3rd once I'm over 56 mph but still 50-75% throttle so no 3-4 shift. Eventually I go downhill, reach the speed I want, etc, and lift off the throttle, and it shifts from 3rd locked to 4th locked because I'm over the flat 70 mph TCC lock in 4th gear. About 0.5 to 1 second after this very hard shift occurs, the torque converter completely unlocks and then ramps back into lock. As best I can tell, TCC condition is "TCC unlocked due to slip" during datalogs, though I'm not sure if that means the converter is slipping or the trans is slipping because the converter is locked during the shift. Trans torque reduction is stock, but there is no torque reduction for the 3-4 shift in the stock tune.

In the logs, the torque converter shows it is "releasing" during the shift event, but TCC duty is still 98+%. During the hiccup afterwards, the TCC is off, then apply, then locked. There are 3 moderate throttle runs and 1 WOT run in the attached log that show this behavior, using a slightly altered stock tune that allows me to replicate the shift more easily without a trailer on the truck.

Has anyone been able to tune around this? I like the lockup in 3rd for towing, and want to fix the shift so it isn't so brutal and doesn't hurt the trans. I'd love to be able to command a faster TCC release or something, but the ramp rate parameters aren't in this OS like they are for my truck with a 4L60 which is a bummer. The closest thing I see is D2905 TCC pressure apply rate but haven't fiddled with this yet. I'd love to hear others' experiences tuning that table, and wasn't able to find anything when searching.


Any thoughts or pointers on what to look at? Searching has proven basically useless for both of these issues, unfortunately.

2004 Burb 8.1 Original Tune.ctz
20220109.pld