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Thread: 4L80E 3-4 Large Shift Time Error/TCC Cycling After Shift

  1. #1
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    Default 4L80E 3-4 Large Shift Time Error/TCC Cycling After Shift

    2004 Suburban 2500 with the 8.1 and the 4L85E. Fundamentally the same as the 4L80E.

    Ever since I got it, every time the transmission shifts from 3rd to 4th and the converter was either previously locked in 3rd or the tables say it should immediately lock in 4th, the shift will happen, the converter will lock, unlock, and then lock again and be fine. The transmission and torque converter are mechanically fine and not slipping. No temperature issues or any other odd behavior. It does this with the stock tune. The converter appears to be a stock replacement rather than an original, and I am not sure if the stall speed is stock or not.

    When this occurs, everything is good in 3rd gear, the 3-4 shift occurs (and feels nice and quick), and as soon as this happens the TCC slip shows ~-200 to -300 in the datalogs (I believe it still is using 3rd gear for the maths). TCC state then goes to "off due to slip" and the converter unlocks. As soon as it unlocks, TCC slip goes positive again and the converter re-locks. The shift error time roughly corresponds with the ECM thinking the 3-4 shift is "complete" when the converter unlocks and the slip number goes positive again. The 3-4 shift has a very large time error (sometimes as large as 5-6 seconds and always more than how long the shift actually takes).

    I have seen one other person report this exact problem, but they never posted a solution and it was nearly 5 years ago now.

    Any ideas? If the converter in this truck is higher stall than stock, can that cause the PCM to not "see" the shift? How does the PCM know that the shift is complete? Is there anything that EFILive has control over (desired shift time, shift stabilization ratios?) that would actually impact this?

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    Did some more logging. The gear ratio PID shows ratio falling from 1:1 (3rd) to 8.75ish:1 before eventually falling to the correct 0.75:1. This happens even if I disable lockup in 3rd gear. The 3-4 shift doesn't feel terrible, but it is much softer than it should be. Shift time is right around 2 seconds. The transmission does not slip, get hot, or have any other bad habits. At this point it seems like it is probably a hardware issue that I can't tune out, but so far I've not read of anything that would impact only the 3-4 shift and not really have any other signs. Any suggestions would be appreciated.

  3. #3
    Joe (Moderator) joecar's Avatar
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    It appears various "things" are going wrong.

    Does your VSS indicate the true speed (i.e. are tire size and axle ratio correctly accounted for)...?

    Was the PCM previously running a 4L80E...?

    Post your .ctz file and a few log files.

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    Needed to get some more logs and finally got that done. VSS is correct, all the other sensors are working properly in the trans (turbine speed, output speed, temperature). PCM is the original for this truck and has always been running a 4L85E. I can see that the turbine speed is what is causing the ratio reporting now; it isn't settling properly after the shift takes place.

    At this point, I'm 99% sure I have a hardware problem, and given the lack of good information on the internet, I doubt I'll be able to diagnose it without taking it apart.

    Trouble spot:
    Click image for larger version. 

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    Here's the log that goes with it. Several shifts through the gears, generally 3-4 is at low load. Shift times are occasionally acceptable if I happen to be rolling off the throttle during the shift, which makes sense. Note the regular timing dips - this log also showed me my AC system needs attention.

    20220513 trans log 2.pld

  6. #6
    Joe (Moderator) joecar's Avatar
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    Instead of TURBINE, look at TIS.

    Take note that TIS is read from the Forward Clutch housing which is driven by the Overdrive clutch, i.e. is input speed after overdrive, i.e. for gears 1, 2, 3 TIS is turbine speed, and for gear 4 it is turbine speed overdriven.
    Last edited by joecar; 3 Weeks Ago at 09:23 AM.

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    @joecar thanks for the tip, I did switch to using TIS which shows that TIS and output speed seem to be what the gear ratio parameter is calculated off of, and TIS shows the engine speed instability following a shift.

    I did end up replacing the valve body because I don't really have time to pull it apart, make and test individual changes, etc. I'm not sure what was wrong with the old one, but several things changed. What I am seeing of the adapt shifts in the log is the pressures are trending back towards 0 psi (previously it was pulling a lot of pressure/negative adapt). It also doesn't feel like I'm getting rear ended every time it shifts in tow haul mode, which makes some sense since the calibration says that pressure learning is disabled in tow-haul. The 3-4 shift is dramatically improved but not 100% resolved. Instead of a 1.88s shift fault, I'm generally getting a 0.3 to 0.8 second shift.

    It appears that, even though there are valid shift times in the target time table for the 3-4 shift, and the adapt range is set to +16 psi across the board for all gears and -0 psi for the 3-4 shift, that the 3-4 shift is not adaptable. For every 3-4 shift I have logged so far, the "adaptable shift" state parameter drops to "no" as soon as the 3-4 shift takes place. It is yes for all other shifts under relatively steady throttle and once the engine and transmission are at operating temperature. I'd love for adaptation to work for 3-4, but I don't see any other parameters that would enable it.

    I also have not found a way to reset the trans adaptations. V8 doesn't have any control that I can see to do the reset for the LS1B ECM, and if I go into the old V7, I can connect and activate DVT and there is an icon at the top that says it shows the transmission adaptation table for each shift. They are all populated with 0 psi, and clicking the "reset adaptation" button just gives an error that "this feature will be added to a later version" which is a bummer. My other scan tools don't have the functionality to view and reset this either, and leaving the battery disconnected overnight seemed to also make no difference based on the datalogs. So for now, I'm just driving carefully and waiting for the algorithm to catch up.

  8. #8
    Joe (Moderator) joecar's Avatar
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    Post your tune .ctz file, let me take a look around in it.

    Do you have the latest V7 software installed...?

    Also log TFT, this may give us some insight (if fluid getting too hot then TC is having problems (or trans clutches/bands may be slipping)).

    When you removed the VB did you notice anything unusual...? Was fluid good color/smell and was fluid level correct...?

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    I'll post it in a bit, on vacation with no computer.

    I do have the latest softwares installed for both V7 and V8, and updated the flashscan while I was troubleshooting so all set there.

    TFT is great, even towing heavy in the mountains it has never been over 200F and usually sits around 150F to 175F. I still have the stock small trans cooler core.

    The fluid was great, no debris in the pan, level was good before draining. The only quirks I found in the VB are a few check ball seats in the separator plate were quite worn and the shift accumulator pistons were pretty darn sticky. I didn't go too deep in tearing it down though. The manual valve bore seemed okay, but I didn't check the other shift spools. Likewise on the solenoids and EPC valve. There wasn't really any material in the filter or in that little screen in the valve body either.

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