8l90 slip/false neutral help

[melition]

I have a 2005 ctsv with an 8l90 swapped in using a tcm 2650 with 3.73 rear gears. For the most part the car has been doing great and has been issue free, but i do have a reoccurring issue I can't solve.

I can be cruising the car normally and will give it 30-70% throttle and it will kick down a few gears like it should. If i have to let off the throttle soon after the downshift finishes sometimes the car will fall into a neutral condition and free rev. This can sometimes be predictable or sometimes it isn't and when i go to accelerate it will rev up and act like it is slipping a clutch into gear and then grab. It will really slip and know its not good for this style of transmission. I am on the fence on whether this is a mechanical issue or an issue with the tcm 2650 and the swap part.

In the log the condition occurs at 2 minutes 10 seconds in, you can see where i give it throttle and rpm increases but speed does not. Its also says trans slip rpm is 2400 and its showing the tcm is a little confused on what gear it needs to jump to.

If anyone has dealt with this i could really use some advice before i swap completely over to gen 5 setup

[TheMechanic]

Some issues with the valve body.
Might want to check with TransGo Robert

[TransGo Robert]

Some issues with the valve body.
Might want to check with TransGo Robert

It is a command issue, not an actual transmission issue, but why it is doing that I have no idea. The computer for some reason decides to fires the solenoids in the wrong sequence when it feels like it is neutralizing is not even a combination that is valid for any of the 8 forward ratios. I wish I could help more but all I can really contribute here is to confirm this is not an internal transmission issue.

[melition]

alright well that's good to know then. Its possible the torque table i have setup for the transmission isn't correct because as far as the t87 tune goes to me everything has seemed fine so far. I wonder if the tcm is having a glitch occur because of the tire/gear setup i'm using. I cant seem to find anyone else having this issue occur on a stock setup, but i also haven't seen someone do a 3.73 gear on an 8 speed and then modify the shift scheduling so severely.

Ill start by tuning my pcm for the ls6 and getting the ve as close as possible, then i can log engine torque and surely get a close estimate for the tcm 2650. Once i get the torque sorted out ill report back on here to at least let you guys know.

TransGo Robert, have you had this situation occur on other 8l90s in their normal e92/t87 formats?

[TransGo Robert]

TransGo Robert, have you had this situation occur on other 8l90s in their normal e92/t87 formats?

No, I have never seen that before.

[melition]

ok i appreciate the help. Ill do what i can before i pull the engine for a gen 5 setup!

edit: i changed torque converter and shift scheduling back to a stock settings, then adjusted them for 26.7 tire and 3.55 (car has 3.73) gears. After doing that the car has a lot less confusion going on and i couldn't get the symptoms to reoccur but only time will tell. I did a fast learn and it aborted early, but did finish the purging process so i'm not too sure if that helped any. Seems to me like a shift scheduling issue but ill keep on messing with it.

[melition]

No, I have never seen that before.

Are the 8l90s internally the same between the camaro ss and the silverado, sierras, etc..? I know tailhousing is different but can't find any other info.

I ask because the transmission is from a 2016 silverado and tcm I reflashed with sps to a 2016 camaro ss os (tapshift differences)

[TransGo Robert]

Are the 8l90s internally the same between the camaro ss and the silverado, sierras, etc..? I know tailhousing is different but can't find any other info.

I ask because the transmission is from a 2016 silverado and tcm I reflashed with sps to a 2016 camaro ss os (tapshift differences)

Yes those two are basically the same internally.

[GHuggins]

Don't know if this is your problem or not, but the 8 speeds required solenoid programming or calibrating as each was unique on them in how they performed. That procedure is only able to be performed using OE software and even then if the tranny or vin are from one that was totaled or something like that then GM killed the ability to do it on their side. This is why the 8 speeds suck SO MUCH in swaps to get right as pressure requirements for shifts will be all over the place. I haven't had one go into neutral, but also haven't had anyone try to put one with a gen 4 engine before. Don't know if there's something torque reporting related that can cause problems or not. I just recently saw where killing misfire test caused a 10 speed to go into neutral or downshift like you're experiencing if it helps any.

[TransGo Robert]

Don't know if this is your problem or not, but the 8 speeds required solenoid programming or calibrating as each was unique on them in how they performed. That procedure is only able to be performed using OE software and even then if the tranny or vin are from one that was totaled or something like that then GM killed the ability to do it on their side. This is why the 8 speeds suck SO MUCH in swaps to get right as pressure requirements for shifts will be all over the place. I haven't had one go into neutral, but also haven't had anyone try to put one with a gen 4 engine before. Don't know if there's something torque reporting related that can cause problems or not. I just recently saw where killing misfire test caused a 10 speed to go into neutral or downshift like you're experiencing if it helps any.

Entering the solenoids flow rate into the computer is only so that it can adapt faster, it is a form of quick learn if you want, nothing else. If you don't do it, it is no big deal, the only downside is that it takes a bit longer to adapt. Pretty much all modern transmissions are like that for domestic vehicles, it is simply much easier and cheaper to make solenoids with loose tolerance, then flow test them, and write on the solenoid its specific flow rate range. Ford divides them in 5 range, GM depending on the model of transmission can be broken up in as much as then 10. Then you simply tell the computer what that flow rate is so it does not have to figure it out while driving the vehicle around, it only needs to fine tune the shifts. On the newer units using pintle type solenoids like the 10L and 10R that both Ford and GM designed together it is not a flow rate, it is a PI Curve but in the end it is the thing, you enter that in the computer when replacing a transmission, complete valve body or individual solenoid only as a form of quicklearn, it will adapt on its own eventually if you don't do it.

[melition]

When i did flash the t87 i entered in the correct tun data and was able to get it to complete the flash, In the tech tool it does show valid under solenoid program data as well.
Ive double checked just about as much as i can, even going as far as modeling my torque table as closely to a gen 5 table as i can but with my torque values. The car does great typically and ive even made some 1/4 mile passes with it, But this issue is a concern for me because you can hear the clutches slip occasionally if it decides to engage 7th at like 6k rpm and i just so happen to be doing half throttle. It occurs with a certain type of throttle fluctuation so im not ruling out my torque table or shift scheduling just yet. The one major thing i do notice is both throttle and accelerator pedal percent is linear from 0-100%, whereas gen 3-5 etc throttle percent pid is usually 10-100% (10% when not pressing throttle) and accelerator position pid will be at 0%.

Ive got some ideas on how to go about fixing it but at this point im concerned it could be the t87 not getting certain data to keep the shifts happy and the lack of torque control from the etc on the gen 3 pcm during shifts/downshifts. that being said, i haven't had it happen since i last flashed it. I am about to start speed density tuning and get the torque values close to perfect.

[TransGo Robert]

When i did flash the t87 i entered in the correct tun data and was able to get it to complete the flash, In the tech tool it does show valid under solenoid program data as well.
Ive double checked just about as much as i can, even going as far as modeling my torque table as closely to a gen 5 table as i can but with my torque values. The car does great typically and ive even made some 1/4 mile passes with it, But this issue is a concern for me because you can hear the clutches slip occasionally if it decides to engage 7th at like 6k rpm and i just so happen to be doing half throttle. It occurs with a certain type of throttle fluctuation so im not ruling out my torque table or shift scheduling just yet. The one major thing i do notice is both throttle and accelerator pedal percent is linear from 0-100%, whereas gen 3-5 etc throttle percent pid is usually 10-100% (10% when not pressing throttle) and accelerator position pid will be at 0%.

Ive got some ideas on how to go about fixing it but at this point im concerned it could be the t87 not getting certain data to keep the shifts happy and the lack of torque control from the etc on the gen 3 pcm during shifts/downshifts. that being said, i haven't had it happen since i last flashed it. I am about to start speed density tuning and get the torque values close to perfect.

Good to hear it has not happened since the last tweaks!

[melition]

Good to hear it has not happened since the last tweaks!

well, i spoke too soon. The original issue i hasn't occurred again but this time i was cruising at a steady state with no fluctuations and suddenly i dropped completely into neutral and just free revved. It happened about 40 mph, slowed down hit the brakes and suddenly about 15 mph i feel it jump into a lower gear like it was downshifting.

My wife said this happened to her cruising down the highway at 75 steady state driving and suddenly neutral and honestly i figured she bumped the shifter on accident. But She said it jumped into gear suddenly coming to a stop also. the Fluid level is definitely full and fluid color was still real clean and red. checked for codes after this happened and show no codes reported.

The cars been trouble free for a while and suddenly with these random neutral issues i'm not sure what to think.

[TransGo Robert]

well, i spoke too soon. The original issue i hasn't occurred again but this time i was cruising at a steady state with no fluctuations and suddenly i dropped completely into neutral and just free revved. It happened about 40 mph, slowed down hit the brakes and suddenly about 15 mph i feel it jump into a lower gear like it was downshifting.

My wife said this happened to her cruising down the highway at 75 steady state driving and suddenly neutral and honestly i figured she bumped the shifter on accident. But She said it jumped into gear suddenly coming to a stop also. the Fluid level is definitely full and fluid color was still real clean and red. checked for codes after this happened and show no codes reported.

The cars been trouble free for a while and suddenly with these random neutral issues i'm not sure what to think.

Bummer. Well that can't be diagnosed by the seat of the pants, we need a log of it happening to confirm that it is still a bad computer command and not a transmission issue

[melition]

Bummer. Well that's can't be diagnosed by the seat of the pants, we need a log of it happening to confirm that it is still a bad computer command and not a transmission issue

Thats what I figured honestly. It's another issue that I can't seem to find info on with a regular gen 5 setup too.

I won't be getting a log of it anytime soon as it's only happened twice and months apart. I think I'll be looking for an l86 soon lol

Thanks for the help so far!

[melition]

Bummer. Well that can't be diagnosed by the seat of the pants, we need a log of it happening to confirm that it is still a bad computer command and not a transmission issue

Well the issue happened again today in the same way. Cruising 70mph fell into neutral condition and had to slow to a stop on the highway. Went into gear around 15 mph and started downshifting.

I checked fluid level prior to this today and it was good, fluid cherry red but had a good amount of material floating in it and Pretty glittery. After the issue occurred transmission acted 100% normal even at wot. Should I pull the pan off or attempt to flush it? I'm just generally confused now. I did a pan drop a month ago and it wasnt terrible but did have the same material in fluid. Ill try to drive around and get it captured on a log too.

[TransGo Robert]

Well the issue happened again today in the same way. Cruising 70mph fell into neutral condition and had to slow to a stop on the highway. Went into gear around 15 mph and started downshifting.

I checked fluid level prior to this today and it was good, fluid cherry red but had a good amount of material floating in it, Pretty glittery. After the issue occurred transmission acted 100% normal even at wot. Should I pull the pan off or attempt to flush it? I'm just generally confused now

If you can see material floating in the fluid definitely drop the pan. Too bad you did not have you scanner hooked up at the time, without seing the log to know exactly what happened it is a bit more crystal ball diagnostic then anything else unfortunately.

[melition]

Ill have to see if it happens again ill log it anytime i drive it. at this point im a little unsure of what to think on it but with the way this swap has went i think its time to move on from the tcm 2650. I setup My torque tables very closely to a gen 5 and made sure it modeled my engines output on vcm scanner as well. even went as far as moving the figures into vcm editor to model it into a graph and smooth it.

I did buy the transmission used but after driving it for months issue free and it only having 30k miles, i have a hard time believing the transmission was failing prior with the way the fluid and pan looked.

[TransGo Robert]

Ill have to see if it happens again ill log it anytime i drive it. at this point im a little unsure of what to think on it but with the way this swap has went i think its time to move on from the tcm 2650. I setup My torque tables very closely to a gen 5 and made sure it modeled my engines output on vcm scanner as well. even went as far as moving the figures into vcm editor to model it into a graph and smooth it.

I did buy the transmission used but after driving it for months issue free and it only having 30k miles, i have a hard time believing the transmission was failing prior with the way the fluid and pan looked.

The crazy command most likely took a tole on the unit. I have never seen something like that and still have a bit of a hard time figuring out what gear it was even trying to go in with the solenoid firing order it did when it felt like neutral. It is not a combination that is supposed to ever happen so it is not obvious. In any enent, it does not sound good for sure. As for the type of controller for it and all that stuff, unfortunately for me that's all still Chinese. I know how it all works inside and what normal commands are supposed to look like in the logs, but outside of that I know just enough to be dangerous.

[melition]

The crazy command most likely took a tole on the unit. I have never seen something like that and still have a bit of a hard time figuring out what gear it was even trying to go in with the solenoid firing order it did when it felt like neutral. It is not a combination that is supposed to ever happen so it is not obvious. In any enent, it does not sound good for sure. As for the type of controller for it and all that stuff, unfortunately for me that's all still Chinese. I know how it all works inside and what normal commands are supposed to look like in the logs, but outside of that I know just enough to be dangerous.

Well none the less I do appreciate all the input. Unfortunately I'm one of the few who has done it so there isn't much info. I did look at solenoid command charts for the gears and I see what you mean, its almost like it's a sort of limp mode. Hard to say with no codes or diag options though.

It was a good proof of concept for sure and I'll definitely upload a log if I can capture it. If I'm going to refresh the trans I'll definitely be swapping to a newer setup to run it properly though.