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

[gearheadE30]

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.

24003
24004

[joecar]

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).

[joecar]

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).

[gearheadE30]

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.


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.

[gearheadE30]

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.

[gearheadE30]

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.

[gearheadE30]

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.

[nonnieselman]

What size tires do you have?
Also it still has 3.73 gears?

[gearheadE30]

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.

[nonnieselman]

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

[nonnieselman]

24025

Just for the heck of it, give this a try and see if the TCC acts different.

I searched all my tunes and i don't see that OS. I know on the larger trucks with the 8.1L there are several tables that we cant get to.
On a friends Kodiak with a 8.1L the Throttle wouldn't go over 50% over a certain speed. Ended up finding the original guy that did the calibrations on those trucks for GM and sure enough after changing some stuff to the 2500HD Truck tunes it made all the difference.

[joecar]

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.For some reason it happens automatically, I'm not sure why it happens.

[joecar]

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.



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.
Various of GM's tunes are not done sanely, they have other objectives in mind (or maybe they gave it to the interns).

[gearheadE30]

@joecar - no worries, I'll just be brief in my messages or multi post longer stuff. Now that I know it will let me do short posts at least I can work around it now.

@nonnieselman - I think you'd get a giggle out of my current tune - I ended up at very similar changes to the shift patterns that you have. 3-4 upshift in particular, the lines are almost on top of each other.

Is there something different in the engine OS section? It's telling me the checksum changed, but not giving me a visible parameter for it. I'm in the habit of checking tunes having bricked an ECM with someone else's tune on a different car in the past - nothing personal just being cautious before I go test it :thumb_yello:

[gearheadE30]

@joecar - no worries, I'll just be brief in my messages or multi post longer stuff. Now that I know it will let me do short posts at least I can work around it now.

[gearheadE30]

For some reason it happens automatically, I'm not sure why it happens.

Strange. Okay, I'll deal with it then.

[gearheadE30]

24025

Just for the heck of it, give this a try and see if the TCC acts different.

I searched all my tunes and i don't see that OS. I know on the larger trucks with the 8.1L there are several tables that we cant get to.
On a friends Kodiak with a 8.1L the Throttle wouldn't go over 50% over a certain speed. Ended up finding the original guy that did the calibrations on those trucks for GM and sure enough after changing some stuff to the 2500HD Truck tunes it made all the difference.

I think you'd get a giggle out of my current tune - I ended up at very similar changes to the shift patterns that you have. 3-4 upshift in particular, the lines are almost on top of each other. I also didn't find anything else with this OS, but there is so little information about the 8.1 around. That's pretty interesting about the Kodiak - if I had a .bin I'm sure I could go crazy trying to find stuff in there, but I'm not sure I care enough to expend that kind of energy.

Is there something different in the engine OS section? It's telling me the checksum changed, but not giving me a visible parameter for it. I'm in the habit of checking tunes having bricked an ECM with someone else's tune on a different car in the past - nothing personal just being cautious before I go test it :thumb_yello:

[nonnieselman]

I understand completely.
I only changed shift points and TCC settings. nothing in the engine side of the tune.
Ill look for the Kodiak tune in a little while.

To play it safe just do a compare and copy the TCC and Shift parameters over. Id be interested if anything helped or if there is anything in the background that is limiting you..
Similar to the 4l80e Minimum TPS TCC unlock on the LS1B pcms. You have to make a CAX file or use HP Tuners to change it.

[gearheadE30]

I understand completely.
I only changed shift points and TCC settings. nothing in the engine side of the tune.
Ill look for the Kodiak tune in a little while.

To play it safe just do a compare and copy the TCC and Shift parameters over. Id be interested if anything helped or if there is anything in the background that is limiting you..
Similar to the 4l80e Minimum TPS TCC unlock on the LS1B pcms. You have to make a CAX file or use HP Tuners to change it.

the minimum TPS thing definitely is definitely true here too, even with those TCC tables. I'm not sure the .CAX that Lextech was using would work or not, and even if it did he doesn't seem to be too active these days. I've not found a copy of that file to try out, but it sure would be nice to be able to stay locked at 0% throttle. I don't care enough to go out and get HPT just for that though.

There is also some other processing going on for it to decide if it wants to lock. Sometimes it will go in at 45 mph. Sometimes it will wait forever at 55 mph and then lock. Not getting any misfires or anything, it just seems like it is waiting for some other condition to be met.

I did get a chance to try it out, but only briefly. Biggest difference is the VSS and speedo are way off compared to GPS. The stock settings are dead nuts on VSS and 2 mph off at 55 for speedo. However, if I ignore the speedo and just look at GPS, yes it does make the converter lock up at a lower speed in 4th. No lockup in the other gears, so it is responding to changes. So I suppose one option would be to skew the VSS high and then change the speedo output signal to make that accurate again if I really want to pull lockup speed down. Not sure what side effects that would have.

[nonnieselman]

Ok good atleast that answers one question.

So there is a difference in the Speedometer versus what the PCM calculates in the background.. I found out the hard way. Just cause the speedo shows "45mph" it might be calculating "42mph" in the background..
I caught a ton of issues with a truck last year that had a 4l80e / 38s /"4.10" gears ( he told me it had 4.56 but it didnt) it took me quite awhile to figure out what was going on. Once i got the tune fudged enough everything started working but the speedometer needed adjusting to match the VSS in the scan tool.

Lex is around. I email him every now and again and he gets back to me pretty quick to be a busy working man.
Never hurts to ask him. Do you have his email?

[gearheadE30]

Yeah, speedo a few percent high vs. VSS is how they have always been set up on the few GM vehicles I've played with. I think as long as the speedometer/odometer/cruise/instant economy work properly (i.e. they use the speedo offset number and not the VSS number), using that workaround doesn't bother me too much. I probably won't have time to mess with that for a bit but will report back on what I find.

I think I actually do have his email - I got the 0411 wiring harness file for one of my GMT400s from him so will try that route. You are right, never hurts to ask.

I know we're in the trans tuning section, but looking at other 8.1 files I did find it interesting that the early 8.1s with the higher ratings (pre-'04) have a higher VE table across the board by around 3% for most of the map and around 8% around WOT. That's a huge drop in fueling for the late 8.1s considering there isn't any documented VE-impacting hardware change that I can find in those years. EGR has its own modifier tables for when it is active.