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Thread: Long Shift Times

  1. #1
    Junior Member
    Join Date
    Feb 2016

    Default Long Shift Times

    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:

    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?

  2. #2
    Joe (Moderator) joecar's Avatar
    Join Date
    Apr 2003


    I'm still reading thru your post...

    if the VSS is after the GVOD then it is reading the wrong speeds.

  3. #3
    Junior Member
    Join Date
    Feb 2016


    Quote Originally Posted by joecar View Post
    I'm still reading thru your post...

    if the VSS is after the GVOD then it is reading the wrong speeds.
    Joe - Thanks so much for reading this and any suggestions you have. To be clear this is not a Gear Vendors unit nor is it an overdrive, but same idea (I think GV unit would sit behind the transfer case and only be usable in 2WD for a 4WD vehicle because it only affects the rear driveshaft speed, the USGear unit I have sits in front of the transfer case and can be used in 2 or 4WD) - a picture of the unit bolted to the transfer case:

    Click image for larger version. 

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    Picture of the van:
    Click image for larger version. 

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    On the late 90's 4WD GM/Chevy trucks my understanding is that the 4L80e's rear output speed sensor was eliminated and the VSS at the transfer case is used instead for this purpose, I was trying to mimic that configuration here. If I put the VSS at the rear position of the 4L80e transmission, which I can do because my 4L80E is the 2WD version that has been modified using the 4WD extension housing and I chopped the 2WD output shaft short so the USGear UD will bolt up correctly - so the sensor at the rear of the transmission is still present but currently unused, then my speedometer would not read correctly when operating the underdrive in low (1.25:1) or the transfer case in low range (1.96:1) or double low (1.25x1.96 = 2.45:1), agree?

    I'm not seeing another input on the 0411 pinout to drive the speedometer separately on Lextech's Vortec Blackbox to 0411 conversion spreadsheet. I'd prefer to solve this in the transmission calibration, if possible. I do have some other ideas on how to make it tick if this isn't possible, although my experiment has likely become rather expensive at this point, so that's my fault.

    Last edited by Savana25004x4; February 6th, 2019 at 01:22 PM.

  4. #4
    Joe (Moderator) joecar's Avatar
    Join Date
    Apr 2003


    Do this:

    - check level and condition (color, smell, feel) of trans fluid.
    - check pressure at line tap (left side of case), is it pegged...?
    - check for any current or pending DTC's.

    Log these:
    - RPM
    - VSS
    - TP
    - TFMPRS
    - TRQENG
    - GEAR
    - MAF
    - MAP
    - LONGFT1/2
    - TFT
    - IAT
    - ECT

    and post the log here.

  5. #5
    Junior Member
    Join Date
    Feb 2016


    Hi Joe - thanks for getting back with me. Here is the update on what I've been testing out. I thought more about what you said regarding the position of the VSS. Yesterday I moved the VSS feed to the PCM from the VSS at the transfer case (after the underdrive) to the transmission output shaft (before the underdrive). I modified the tune file I've been working on to reflect a 4.66 gear ratio (3.73 (axle)*1.25(underdrive)) and then logged data. With the previous tunes I was trying to keep the VSS after the underdrive but was seeing shift times typically greater than 1 second. So with the VSS feed moved the shift times improved. I haven't finished processing the data in excel yet, there were a couple instances where something was going wrong on the 2-3 upshift, so had I been logging spark advance as you suggest above I think those events would be easy to find - it felt like it pulled timing. I'll see if I can use the data I have to find what happened. Here is the log file and tune:


    With this tune it seems to be shifting as intended with the underdrive in low (let me know if anything in the current tune file v25 looks off to you). If I shift the underdrive to hi (1:1) I suspect the only issue will be the speedometer being incorrect, since the PCM doesn't know the underdrive is there.

    Here a previous log and tune showing the long shift times with the VSS located after the underdrive (at the transfer case):


    Both tune files have the same base pressure shift maps, low pressure adjust tables and high pressure adjust tables. The performance shift maps are basically identical except for some small changes to WOT values and TCC apply and release tables, the Normal shift maps are not relevant as both logs were taken with the performance map running. Both logs had the underdrive engaged (1.25:1) which is what the performance map shift speeds are adjusted for. The main difference here is that the v25 tune had the VSS placed at the back of the transmission (before the underdrive) and on the v24 tune had the VSS placed at the back of the transfer case (after the underdrive). I'd like to understand why or how the transmission is operating such that would cause this difference in shift times. My guess right now is that this has to do with shift stabilization ratios being accurate to what was really happening last night vs. inaccurate on the v24 tune and VSS placement at the back of the transfer case.

    I was planning to install a line pressure gauge on the trans this weekend and borrowed some shop time at my brother's place to do so. I ordered the wrong gauge so I'm re-ordering that. Probably won't get to installing it until next weekend. I'll be working on my Comanche this weekend since I have some shop space to work with...

    It is due for a trans fluid/filter change, I intended to do this when I install the pressure gauge. Once I have that done I'll adjust my PID list to match this list you gave above and I do need to check for codes to see if anything comes up that might explain the 2-3 upshift issue from last night.

    I'm coming to the conclusion that leaving the VSS that the PCM sees at the back of the transmission so that it will shift correctly and not have long shift times and seeing if I can drive a completely independent of the PCM speedometer off the VSS at the transfer case might be how this needs to be done so I can have an accurate speedometer and be able to shift between hi and lo on the underdrive without damage to the transmission. Any other thoughts?

    Another question I have is I used 2002 6.0L 4L80e 4x4 base shift pressure maps upped by a factor of 1.2 and a little more in some areas rather than the 2002 express van base shift pressure maps while I was trying to reduce the long shift times. Those are the current base pressure shift maps in the above tune files. What I also noticed while doing that was that the force motor current map for the 2002 6.0L 4L80e 4x4 truck are different than the 01 and 02 Express van maps - I used the Express van FMC map since my transmission came out of an 01 Express (the 02 and 01 Express van FMC maps are the same). Any thoughts on why these differ or if one might be advantageous? Is is just simply the L31 vs. 6.0L engine difference?

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