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Thread: 8L90e Need Help Diagnosing

  1. #1
    Potential Tuner
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    Oct 2018
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    8L90e Need Help Diagnosing

    Need some help/advice on an 8L90 issue. I've installed an L86 8L90 combo from a 2017 vehicle into a square body Chevrolet pickup. Engine runs fine, but having an issue with the transmission. It seems to take off in some gear higher than 1st gear and doesn't ever shift. I can command a different gear using the scanner and it doesn't change anything. I'm able to read vehicle speed, transmission input/output shaft speed, gear selector position, etc using the scanner and everything seems to be working on those channels on the scanner. I can unplug the TCM and it goes immediately into limp mode, I know that doesn't mean anything, I just wanted to do that test to see what would happen.

    I've not unlocked or licensed the 8L90 TCM, so it's configured as stock. I am aware of the process of sending these TCM's in to unlock, etc but as I understand that is only required if your wanting to modify the stock tune on the TCM which I am not

    is there any other tests or tricks to try to diagnose what's going on here? I'm hoping it's not a mechanical failure but anything is possible here.

    Thanks for the help guys

  2. #2
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    Finally discovered I had missed an ignition power wire at the TCM when I did the harness work. Strange thing is that it gave me no trouble codes for anything related to the trans and it was still reading input/output shaft speed, etc.
    Got the ignition power wire hooked up and she shifts in all 8 gears perfectly.

  3. #3
    Senior Tuner Ben Charles's Avatar
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    Nice update

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  4. #4
    Advanced Tuner lt1z350's Avatar
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    We have had a few late GM trucks now in the shop with the trans and according to GM and my diag software adapts are stored in the bcm. On your swap did you some how include the bcm into this mix? I have seen on an earlier GM 6t70 a failing bcm not let the trans shift correctly. So being curious as I do repair many of these stock vehicles what’s the go around or is it incorporated into the swap? Nice build!!
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  5. #5
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    I am having the same issue, do you remember where you put the ignition wire on the TCM(which pinout)? This would be a huge help for me if you could pass me some insight.