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Thread: t42 4L80e 3-4 voodoo

  1. #1
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    Angry t42 4L80e 3-4 voodoo

    I'm hoping for some kind of sanity check here...

    This is a classic truck with an L92 6.2 E38 and T42 4L80e. The truck starts and runs great, the issue is with 3rd gear and 4th.

    This is a complicated issue, I will try to put most of it down along with a log soon.

    Basically everything you physically feel and see on the tach/speedo is that it has 1-2-3 gear just fine, no overdrive. Can't get it to command 4th, won't go into 4th under any circumstances.

    however, if watching a scanner (both EFILIVE, and a very nice Snap-on) it digitally shows it going 1-2-4. The ecm commands 4th, the iss is 75% that of the VSS, and no check engine lights. The computers think they are in 4th gear. But they are not.

    While in this state we are driving down the road at 60mph, all the electronics say we are in 4th, but the truck physically is in 3rd the best we can tell (it feels like 3rd and the speedo, tach, rear end ratio agree we are in 3rd)

    *sometimes* under light deceleration I can pull the shifter down into D and the ISS and VSS line up at basically 1:1, but its not perceivable physically while driving. If we get back on the gas they immediately show 4th gear ratios, or if we put on the brakes the truck will go into 2nd and the iss/vss reflect this.

    Also, when pulling the truck down into 1 with the shifter, all is good. Pull it up to 2nd, all is good and it goes into 2nd. Pull it up to 3 and NOTHING changes as far as physical gear change, rpm, ISS/VSS, nothing EXCEPT the scanners show 3rd gear. You can stay in that state forever (actual second gear but ecm reads 3rd). You can then pull shifter up into 4th and you feel the shift to what I would call 3rd gear, but again, the ecm and vss are now showing 4th gear numbers! Im about to pull my hair out!

    The very basic issue I'm seeing is the ISS/VSS showing 4th, (75% overdrive) but the truck in every other sense being in 3rd. How is that possible? I've always seen the ISS and VSS to be a very cut and dry rotation count. is there some way for this to be skewed???

    some more details...

    -We have quadruple checked the harness against stock gen4 4L80e GM diagrams.
    -The truck is on its 3rd transmission (1st was used with issues, then rebuilt it, then went and got a completely different rebuilt trans from a different reputable shop. Both of the final 2 were doing this exact issue).
    -We've tried an 07 operating system in both an e38 and t42 match. We've tried a 2008 van 6.0 matched e38 & t42, we've even tried mixing and matching the combos of these 2 e38s and t42, we always end up with this same result.
    -We've checked grounds until we are blue in the face.
    -the truck has never been right since its completion a couple months ago, the problem has been there the whole time, it wasn't something that developed after the fact.
    -there are no CELs at all
    -the ONLY thing that is left is to start pulling the harness back from its installed location. There are a couple places where the owner got crazy with zip ties and there are some data wires awfully close to some power wires. In my experience they aren't close enough to be an issue, but that's about all we have left at this point. Seems like if there was some sort of data interference we would be having more trouble than this. Other wise the truck is flawless. If I told you it had a th400 in it and to take it around the block you would come back saying it ran awesome.

    I don't expect anyone to diagnose this over the internet, especially without a data log (yet) I'm really hoping there is someone that has seen this crazy issue before and can help out of experience. If nothing else it feels good to type this out and get it off my chest. It makes no sense. Thanks to anybody who might be able to give advice.

  2. #2
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    Going with the 08 Van E38+T42 combo is what I'd recommend as the LY6 is pretty close to the L92 (0.2L smaller, iron vs alum block, different cam, diff injectors, no AFM on LY6).

    Seems like a weird issue, have you checked the linkages as well? Even things like take the shift cable off the trans and manually put it in 3rd to make sure it's actually in 3rd, then do the same with 4th.
    ~Erik~
    2013 Sonic RS Manual - 1.4L I4T E78, starting the mods
    2008 TrailBlazer SS 3SS AWD Summit White - LS2 E67/T42, bolt ons, suspension, etc.
    2002 Chevy TrailBlazer LT 4X4 Summit White - 4.2L I6 P10, lifted, wheels, etc.

  3. #3
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    Yes, it currently has a 2008 matched set of ecm and tcm from a Van. 6.0 and 4L80e. The ecm has been tuned to work with the 6.2 at this point but a few days ago we put everything back to stock 08 van 6.0, only changing the absolute necessary VATs and whatnot to make it run, just to drive down the road and see if something was different with the shifting. It wasn't.

    We tried another set of ECM and TCM. Set up from a 2007 L92 escalade and a 2007 suburban 4L80e t42 TCM file of some sort. It was kinda a hodgepodge, just trying different things. It had the same results.

    Tonight me and a friend double checked EVERY wire involving the TCM and Trans. Everything. They all go from Point A to Point B just like they should. None of them are shorted to ground, and none of them are shorted to power. We even loaded tested them with a light bulb, not just the ohm meter. Everything is solid.

    I'm beginning to think we got 2 totally different transmissions with the exact same internal solenoid issue. Its gotta be a one in a million chance, but I don't see much else possible anymore.

    The only thing still in the back of my mind if there is a wiring or data problem somewhere AWAY from the TCM/Trans wiring, somewhere out in the ECMs wiring that we just haven't checked yet, and its corrupting data somewhere to the ECM and ultimately the TCM. Were running out of diagnostic options at this point.
    Last edited by LS-SWAPPER; May 20th, 2019 at 01:09 PM.

  4. #4
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    Shifter linkage was a little funky 2 weeks ago but is now straightened out. We can feel PRND321 just as it should. We even have the PNP switch connected and the ecm and scanner sees the chosen gear. We've tried it with the PNP unplugged as well just for the heck of it.

  5. #5
    Joe (Moderator) joecar's Avatar
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    Post the transmission tune, let me take a look if it contains things like upside-down or crossed shift tables.

  6. #6
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    Post log files.

  7. #7
    Joe (Moderator) joecar's Avatar
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    ISS is located after the 4th/overdrive planetary gear set;

    so ISS should show faster than engine RPM;

    3rd gear = direct drive
    4th gear = 4th planetary gearset doing overdrive + direct drive after that

    so ISS should be showing same as VSS in 3rd and 4th gears...

    something's not right.

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    Hmm, ok. I wasn't thinking about the ISS being AFTER the 4th planetary. I assumed it was reading in front of it. Yes something definitely isn't right. When the ECM is thinking its in 4th gear its showing ISS rpms that are 0.75 that of the VSS, I mean EXACTLY 75% of the VSS, couldn't be any more exact. I have not seen a normal condition where ISS and VSS are 1:1 with each other. The only possibly exception is like I mentioned sometimes under slight deceleration I see 1:1 but it soon jumps to a higher or lower ratio with any touch of the gas or brake pedal. In other words the ISS and VSS quickly jump to something other than 1:1 and I never really "feel" that the truck has entered 3rd gear.

    When the ECM is commanding 3rd gear I'm seeing ISS:VSS numbers that represent 2nd gear underdriven ratios.

    Here are the 2 files that we have tried the most, many variations of trial and error to force a change have occured, but these are the base files.

    2008 Express T42 4L80e (24239927).ctz

    2007 Suburban 4L80e T42 (24235341).ctz

  9. #9
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    Jon-sunday.efi

    Here is a log file, pretty sure this was with the 2008 van file. I checked basically every box I could find that might give me some insight to what was going on. I noticed somewhere in the data that it "FAILED" a test, mainly:


    SOLCCC-SOLCP Transmission Shift Solenoid C Short to Power Test Unknown . FAILED FAILED Unknown GM.SOLCP Transmission

    This sounds fairly self explanatory, that a wire is shorted to 12v+. Or perhaps a solenoid itself? I cannot find any evidence of this occurring in the harness. Perhaps the internal trans components are doing it but that puts us back to the impossible odds that 2 completely different transmissions had the exact same issue. Maybe, who knows...

  10. #10
    Joe (Moderator) joecar's Avatar
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    In your log, the pids SOLACC, SOLBCC, SOLCCC, TRIS, MGRATIO, SOL32CC are for Allison, not for 4L60E/4L80E (which have only SOLA and SOLB).

    ( 4L60E has a 3-2 solenoid, but the pid SOL32CC is not for this trans )



    You also want to log GM.GEAR and GM.TCCMODE.

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