1-2, 2-3, and 3-4 shifts occurring to late after rear gear change on 8L90

[cmitchell17]

I put a 3.73 differential from a manual SS into my 2017 Camaro SS. I just started to use the Bluecat tool and it's amazing how perfect it was in predicting the correct lead times and shift speeds to properly shift my 8L90 at WOT so many thanks to Bluecat.

However, for the lower gears, mostly it seems like 1st and 2nd gear have the worst problem, will not shift when I command them to? I have it set down to 4 mph for the 1-2 shift and it ignores it and won't shift until 9 mph? Anyone ever seen something like this? Is there any parameters I could be missing? It appears there is a hidden table or scaler than prevents a shift below a certain speed?

I also wonder if I may need to adjust driver demand, upon thinking about it I wouldn't think it would be possible that anything torque table related could be the cause but then again the DD tables reference vehicle speed and throttle position?

Is there anywhere else besides the tire diameter and pulses and diff ratio in the speedometer section as well as the shift points in the trans section? I don't see anything in the ECM about how it knows what gear ratio is in the car, I guess it dosen't care, but then again if drive demand torque is in wheel torque, then technically I should automatically multiply all my driver demand tables by 1+(2.77/3.73)?

[cmitchell17]

My only thought is it has something to do with PAS (performance algorithm shifting), which I think I have seen somewhere in some other 8L90 operating systems but not mine. I guess it could just be another table we are missing that says don't allow a upshift until this speed.

Is there anyway I can trick it to make it think its going faster than it is in only those gears?

Also, I tried shifting it manually but it still basically shifts at the same RPM and speed, normally accelerating I can try to manually shift it and it just won't respond and you can see the gear indicator number flash on the dash. I need to experiment with it more to see if I can barely crawl along and see if it will let me shift it manually.

I guess another option I have is maybe try to program in the shift speeds to make it skip a gear.

[cmitchell17]

I found this thread, but its for Fords, but I think NV ratio might have something to do with my problem.

https://forum.hptuners.com/showthrea...g-Tip-N-V-Base

Although the big thing is I think the NV ratio in the GM ECMs is only used for driver demand torque stuff and nothing to do with TCM and shift commands. However, the late shifts still might be related to NV ratio, logging the throttle position it seems like its giving it normal throttle, but it may artificially giving it more throttle than it needs at lower speeds (much higher axel torque at lower speeds so the gear ratio increase (~.25) is greatly amplified at lower speeds and is kind of negligible at higher speeds).

The Ford thread gives a formula to calculate the NV ratio which mine ends up being around: 45 = (Rear Gear Ratio*Tire Revs per Mile)/60

I am going to input this into the NV ratio in the ECM and let the ECM "calculate the VSS for the DD tables by the NV ratio.

[TheMechanic]

BlueCat. That will fix it.

[cmitchell17]

Yes, I did use bluecat and it got my WOT shift MPH's dead on, like it shifts exactly at the RPMs I want it to.

However, if I watch in the scan tool the accelerator and throttle position (I believe the shift point mph's are referenced from the throttle angle and not accelerator angle) and the VSS speed (both vehicle speed, undriven and driven speed) I can see that the low throttle shifts in gears 1, 2, and 3, and maybe 4 are not what I command them in the tables.

I am starting to think that the grade breaking feature is what's causing this. I think that this is done by the chassis control module? (CCM) which on the Camaro I am not sure if the CCM is seperate from the FSCM? I have access to the FSCM/CCM and I know we have the tables for the NPP exhaust valve control, I also know in the speedometer section were you input the final drive ratio, there is a seperate one for the FSCM. My stock tune actual had it set wrong, the TCM and ECM both had 2.77 which is the actual stock ratio, the FSCM had like 3.08, so at first I tied making it 3.73+(3.08-2.77) and I also tried putting 3.73 in it. It had no effects. I still think the FSCM or CCM or ESCM or EBCM or whatever doesn't know the correct gear ratio.

The only way I see maybe fixing it would be to flash a CCM or ESCM or whatever it is to be to flash it with a calibration from a manual car with the higher gear ratio I have.

[TheMechanic]

Did you use the new gear calculator in the Gear Tire Wizard? Had an issue with a Corvette a little bit ago with a similar problem.

[cmitchell17]

Yeah I tried that too using stock shift points also. I think its got to be the CCM or something taking over and trying to apply grade breaking or something because it thinks it still has 2.77 gears in it. There is also a lot of other things that can effect the shift points of the 8L90, like performance algorithm shifting and grade breaking, and we don't have access to control any of that so who knows what could be effecting it.

My only thought would be that maybe I can manually set a code that would force the TCM to ignore weird shifting requests to it from the CCM and other modules? I am sure this would break all kinds of other things that I want but maybe prove the point.

The only other thing I know to do is to switch to the 3.27 differential that comes in the manual V6 and 2.0, and hope that the 3.27 gear is close enough to the stock gear ratio to not cause problems.

[cmitchell17]

Wait a second, do these tables need to be changed from stock due to the new lower rear gear ratio?
Capture.JPG

I noticed a 2016 Camaro manual SS which is where I got my differential from, has the pulses from the VSS sensor as 17 instead of 36?

[cmitchell17]

So I tried messing with the VSS pulses, I set it to 16 (like the stock manual SS tune), and .42*68=28. It did actually make it shift at what seemed like a reasonable and correct time, but the shift was all messed up I guess from the VSS signal being incorrect. It clunked and shifted really hard into 2nd and 3rd.

[TheMechanic]

Whenever I use the gear change wizard it seems to change numerous speedo and ABS items. So I would not doubt it. Maybe see if in the repository there is another camaro that has a different gearing that may lead you to the answer.

[cmitchell17]

I've looked through all the tune files I can find the manual does have a different number, but I think that's because the speed sensor for the manual transmissions is different.

I wonder if I could put the original tire revolutions back to stock for the TCM and just change the ECM revolutions, that way I would have a correctly reading speedometer, but then maybe the trans would shift right? I'm sure it would probably break like 50 other functions and cause all kinds of random throttle closings and timing pulls though.

[cmitchell17]

Ok so I changed the trans final drive ratio TCM [5004] back to the stock setting of 2.77. It seems to have fixed the problem with the part throttle shifts for lower gears hanging, but now it broke WOT shift and especially broke WOT downshifts. On WOT downshifts most of the time it won't shift down to the lowest gear and if it does it engages traction control light and cuts to like half throttle.

[cmitchell17]

So I wonder now if the fix might have to be changing the axel ratios back to stock (or at least just the TCM axel ratio back to stock), then compensate by changing the TCM [5006] tire revolutions per mile scalar? Basically I should just be able to multiple the tire revolutions by (2.77/3.73)=.74?

I feel like someone has already have had to of figured this out by now? I just can't find any examples of earlier posts of people searching how to do gear ratio changes, even with a 6L80 which I think should be able the same as the 8L90. I know my Camaro has 4 wheel speed sensors on each wheel to work the 4 wheel ABS, but I also still think there is still a transmission output shaft speed sensor? I believe there is also a intermediate one.

[cmitchell17]

So looking back at what the HP Tuners "Gear/Tire Wizard" does, it makes no sense. I tried taking a stock tune and putting in the same stock tire diameter but put 3.73 gears from the stock 2.77:

Here is the stock tune:
Capture.JPG

Here is the modified stock tune for same stock tire size just 3.73 gears instead of the stock 2.77:
Capture2.JPG

So in theory, the only thing you should have to change in the speedometer section is the actual gear ratio tables, which the wizard does change for you (and by the way the FSCM having the 3.07 random gear ratio makes no sense either), but it also changes the VSS sensor "calibration" and it changes it to some random value thats pretty close?

The only think I can think of, is this is just some rounding error in the wizard?

[TheMechanic]

Looking in the wrong area. This is your same page of a 2008 C6 I fixed for a similar issue. Screenshot 2021-01-20 141753.jpg
Here is the wizard that you find in the upper menu under "Edit". Screenshot 2021-01-20 141822.jpg

[cmitchell17]

Sorry, I'm not sure what you're trying to say? The 2 screenshots above is from the stock tune settings from my car and the other one is the same stock tune, but I used the tire/gear ratio wizard (like in your second screenshot) to change the gear ratio from 2.77 to 3.73. My point was to illustrate that the gear/tire wizard changes slightly changes the "sensor calibration" scalars, when it seems like they shouldn't need messing with for just a gear ratio change and the same diameter tire.

I think I am going back to the driver demand tables being the problem. I think in someway or the other the lower gear I have installed is messing the wheel torque references up or something like that. It makes a lot of sense that the hanging gears happen in the lower gears at slower speeds where wheel torque is a lot higher.

I was looking at the manual transmission Camaro file, it uses engine torque instead of wheel torque, but you can still tell it has a different shape than the auto Camaro driver demand table. I just don't know a good and proper way to calculate the manual Camaro driver demand table engine torque to wheel torque. I may try pasting it to Excel and multiplying the values by the 8L90 gear ratios and rear gear.

[SSDION]

i HAVE A SIMILAR PROBLEM BECAUSE i HAVE CHNAGE REAREND ON A C7Z06. i WENT FROM 2.41 RATIO TO 3.42 AND SHIFTING 1-2 IS BAD. IT GO TO REV LIMITER, CUT THROTTLE,REDUCE TIMMING SLOW DOWN AND SHIFT.

[village_idiot]

Ssdion on your deal try lowering your wot mph significantly and see if it shifts early. If so, raise the wot mph until you get to the shift rpm you?re looking for

[SSDION]

I did try to lower shift MPH 2 mph and 4 mph and 6 mph and 8 mph and 10 mph with no goo results!