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Thread: 4L80E/4L85E 8.1 TCC Minimum Speed and Factory Lockup-Shift Tuning

  1. #1
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    Default 4L80E/4L85E 8.1 TCC Minimum Speed and Factory Lockup-Shift Tuning

    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

  2. #2
    Joe (Moderator) joecar's Avatar
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    To avoid killing the TCC friction surface, observe these rules:
    a. TCC must not be locked during shifts (TCC slips).
    b. TCC must not be locked during significant engine torque or load or WOT (TCC SLIPS).
    c. TCC should be applied only in 3rd (if manual lever is on 3/D) or in 4th (if manual lever is on OD).
    Last edited by joecar; January 11th, 2022 at 02:04 PM.

  3. #3
    Joe (Moderator) joecar's Avatar
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    Also, locking TCC at very low RPM in high gear will lug engine (engine does not make enough torque at that low RPM. 1100 RPM is very low).

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    Quote Originally Posted by joecar View Post
    To avoid killing the TCC friction surface, observe these rules:
    a. TCC must not be locked during shifts (TCC slips).
    b. TCC must not be locked during significant engine torque or load or WOT (TCC SLIPS).
    c. TCC should be applied only in 3rd (if manual lever is on 3/D) or in 4th (if manual lever is on OD).
    So all of your points are what prompted me to post here in the first place.
    a. I would tell me I am crazy too, if I didn't see it for myself. With the stock, untouched tune straight from GM, a WOT acceleration shifts from first to second, locks the converter around 3500 rpm in second, and then eventually shifts into 3rd with the converter still locked. I have the datalogs to prove it on the other computer, can share if interested. But no joke, that is how GM set it up. I even verified on youtube with all those silly 0-60 videos people post: every 8.1 suburban out there you can see locking the converter in second at WOT.

    b. stock tables will kick down before unlocking the converter in both 3rd and 4th when given heavy throttle. You're right, if it didn't kick down it would probably slip, but the stock tune is pretty aggressive about staying locked. I'm not interested in making it any more aggressive at high load than GM did.

    c. Stock tune drives lockup in 2nd at 45 mph over 93% throttle, and 3rd at 55 mph over around 50% throttle so towing often gets you to part of the map where it locks up in 3rd before you hit the 3-4 shift point.

    TCC slip is in all my logs now but I haven't done a WOT test since I added it. I'll have to see what it is doing during those shifts.

    Quote Originally Posted by joecar View Post
    Also, locking TCC at very low RPM in high gear will lug engine (engine does not make enough torque at that low RPM. 1100 RPM is very low).
    Very true - not wanting to go that low on this truck. The main motivation is I can watch instant economy (and injector pw) go from 13 mpg to 16-17 mpg as soon as the converter locks between 45 and 50 mph. Most of my around-town driving is 40-45 mph, so holding lockup down to 1400-1500 rpm instead of 1600-1700 would make for a meaningful improvement.


    I've since tried a few more cals, with limited luck. Still trying to answer this question: is there some reason why I can't get the torque converter to lock below 45 mph? This seems to be the minimum regardless of gear, i.e. if I have the shifter in 3rd and the converter table is set to lock 3rd at 30 mph, it still doesn't lock until 45.

    In an effort to get rid of GM's 3-4 shift with the torque converter locked, I'm trying to move the 4th TCC apply speeds up higher. But either that isn't working, or there is an error of 15+ mph between when the TCC apply is commanded and when it actually occurs. I'll keep moving it out until I see it make a difference, but haven't found that point yet.

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    I've tried to reply and it says the post has to be approved by a moderator. Is it just going into some black hole in the internet?

    EDIT: any time I write more than a sentence, I get the moderator approval message. This is frustrating.
    Last edited by gearheadE30; January 17th, 2022 at 08:53 AM.

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    Well that worked. Odd. I'll try this again:

    I would have agreed with your comments if the factory tune didn't do exactly what you're saying shouldn't be done. I've not seen any other GM vehicles that do this; my 4L60E stuff definitely didn't.

    The factory tune, straight from GM, locks the torque converter at WOT during the 2-3 and 3-4 shift. It's attached to my first post, and I've confirmed it in datalogs looking at TCC apply pressure, and looking at all those acceleration videos people post on Youtube show other 8.1 4L85 trucks doing the same thing.

    Second gear only locks over ~93% throttle.

    The factory tune, straight from GM, locks the converter for high-load operation in 3rd and 4th, and it is very easy to end up in the area of the map where it will lock up in 3rd before shifting into 4th if you're towing in the mountains.

    I'm not trying to lug the engine down that far, but most of my around town driving is ~40 mph. The converter doesn't lock until 45 at the earliest and over 50 most of the time, and it seems that changing the tables doesn't make any difference at all. I can clearly see the fuel rate drop significantly when it does go in at 45, so it seems like there is something to be gained here. That is around 1600 rpm on this truck. I'm just trying to get it to go in at 1400-1500 at very light throttle.

    It also seems like, no matter what I do to move the 4th gear TCC engagement out of the way so it doesn't do the 3-4 shift locked up, it doesn't make a difference. Currently, if it shifts into 4th over ~60 mph, it locks the converter at the same time which is both not smooth and pretty abusive to the transmission. Is it normal for the shift command speeds to be off of VSS by 10+ mph? That's the kind of error I'm seeing now. I guess I just need to go very extreme and then work my way backwards, but this doesn't make sense to me and I haven't found anything else online of people having this problem.

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    Let's see if I can do a short post: stock cal shifts locks in second at WOT, shifts 2-3 in lockup, and shifts 3-4 in lockup when you lift. 3-4 WOT doesn't matter to me, it is a suburban. Stock cal will not lock in second at part throttle, but will in 3rd at high load, and then will shift 3-4 locked. I'm actually trying to prevent this from happening and have been unsuccessful, but the actual shift speed is 10-15 mph off from the tables, if not more. Need to try some extreme values to see what works, but what would cause this to be so consistently far off?

    Also, still no lockup at lower speeds. Goal: lock in 4th around 40 mph, which should be a bit over 1400 rpm.

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    What size tires do you have?
    Also it still has 3.73 gears?
    Last edited by nonnieselman; January 24th, 2022 at 02:26 PM.

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    Quote Originally Posted by nonnieselman View Post
    What size tires do you have?
    Also it still has 3.73 gears?
    Yep, 3.73 gears and stock size tires on stock rims - I believe 245/75r16.

    I haven't touched the speedo or VSS parameters. I have not checked them against each other, but I think the speedo was 2-3 mph high at 70.

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    Sounds like something in the background of the tune that you cant see is holding you back.

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