I have a problem - long shift times on the 1-2 upshift and 2-3 upshift.
I've been mucking around with base shift pressure tables, shift time tables, TCC apply and release tables, high and low pressure adjust tables to try and get the times down. Not working yet...

Here is the background and the catch. The application is on 0411 converted 98 GMC Savana 2500. I'm running a 2002 6.0L/4L80e 4x4 tune with the 2002 Express van L31/4L80e Engine and Transmission segments swapped into the 6.0L tune - why? I converted the Savana to 4WD and wanted low range shift mode to work. The current L31 is tired with 216K on it. The 4L80e is rebuilt (stock rebuild, no performance parts/mods) with about 9k on it now.

I'm also pretty new to tuning and to automatics.

Here is the catch - behind the 4L80e and in front of the NP205 transfercase I have a USGear underdrive (2 speed transmission, 1:1 Hi and 1.25:1 Low gear ratios).
So to be clear the driveline looks like this: L31/4L80e/USGear Underdrive/NP205

I've been trying to tune the performance shift maps to deal with the underdrive in low and the normal shift maps for running the underdrive in hi (1:1).
The van has 33" tires and a 3.73 gear ratio - with the underdrive in low this is effectively like having a 4.66 differential gear ratio at the axle, which I use for driving around town and climbing steep grades on the highway, the only time I really use the underdrive in Hi (1:1) is cruising on the highway aside from steep grades. The van is quite heavy, about 8500lbs on the scales at the local metal recycler, and lately with the extra tools/equipment I've been dragging round in it, likely 9000lbs. Driving around in low on the underdrive is about the only thing that makes the tired L31 tolerable on the mountain roads in CO at 7500ft. Initially, when I installed the USGear underdrive and NP205 transfer case on the original 4L80e, driving with the underdrive in low with appropriate shift maps, the issue this caused was an incorrect gear ratio and I think a trans slipping code and it defaulted to full line pressure shifts (harsh).

I should also note that the VSS is installed on the NP205 rear output shaft housing/rear output shaft, downstream of the underdrive (instead of the transmission output shaft location which would be upstream of the underdrive) in order to have an accurate speedometer whether the underdrive is in lo, hi or the transfercase is in low or hi. To deal with the full line pressure shifts I disabled the incorrect gear ratio code and the trans slipping code (I realize this is not the beast idea, but the only way I found at the time to deal with the full line pressure shifting with the underdrive in low, maybe someone has another take on how to do this?). This seemed to work except there was a pretty bad lag on the 1-2 WOT upshift and this was still the original 4L80e when the van had about 200K or so on it in 2016. I ended up taking all the torque reduction out on the 1-2 shift as there was a lot of timing being pulled out during what was a pretty long shift (again, wasn't putting it all together here, that the issue was long shift times and that the TR was being implemented for that lengthy shift making the van feel like it was coming to a standstill on the 1-2 upshift). This seemed to solve the issue, or maybe rather mask it... You more experienced guys can probably see where this is going. Then some things happened and the van went into storage until summer 2017 so I didn't really get to work out the tune issues at that time.

In the fall 2017 the van has about 207K after getting it out of storage, I pulled a 4L80e from an 01 Express 3500 at a local salvage yard had it rebuilt at a local shop. I installed that transmission and all seemed well - at the time I wasn't paying much attention to the shift times or mucking with the tune, I was more focused on some pretty bad driveline vibration issues that turned out to be a combination of really bad misfire issues and a single piece rear driveshaft exceeding critical speed. I did do one log at that time and looking back on it now, with the rebuilt 4L80e the shift times were still too long, as they still are currently 9k miles later on the rebuilt 4L80e.

Just to be clear - this is not an issue with shifting the transfer case into low range (1.96:1 on the NP205) and using that low range shift map - that works fine (when the switch on the NP205 low range shift rail works that is), all of the above refers to the USGear underdrive in low (1.25:1) and the transfercase in hi range (1:1) driving at road/highway speed.

So, I'm trying some different things to get my shift times down and hoping I haven't burnt up the rebuilt 4L80e. I have some specific questions:
What can anyone tell me about shift stabilization ratios (Table D4601) and how they affect the shifting?

In my TCC table D2801 I have a parameter D1011 - Shift Stabilization Enable - this is currently enabled, this parameter being visible might be in the tune as a result of a .CAX file I found and implemented? - can't remember right now if that normally shows up in the TCC parameters. So, by having this enabled is it possible that the TC is trying to manipulate the input shaft speed to try and assist the shift based on the ratios in table D4601? - these ratios would essentially be correct with the underdrive in hi (1:1) but not correct with the underdrive in low due to the VSS placement on the transfer case rear output.

The only post I've found so far that mentions shift stabilization is this: https://forum.efilive.com/showthread...=stabilization

I noticed while looking at some recently logged data that my Shift Delay is quite high, so I'm wondering if the shift stabilization is the reason for this and the long shift times. My next test will be a run with the underdrive in hi (so 1:1) to see if I still have high shift times (which by now I may have beat the trans up so bad that this test might not prove anything...). Then I may try disabling the shift stabilization to see what happens there...

Any thoughts?