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Thread: 8L90 codes, tuning or wiring issue?

  1. #1
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    8L90 codes, tuning or wiring issue?

    I just completed a L86 8L90 swap and have numerous codes which should get resolved soon but the transmission has a number of codes that make the transmission unusable. P0706 P0708 P0751 P0762 and U0101. Is this a tuning or wiring issue? I didn?t have a TCM that came with my transmission so I had to buy one from a wrecked truck. I sent the TCM to my tuner when I sent the ECM but thought I would run this by you guys before I send it back to him.

  2. #2
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    Do you have a T87A or T87. The A type needs to be unlocked and I believe that cost is around the high $300's mark. I know the TCM and the PCM don't mate with each other from the factory as I have 2 different units but they are E92 / T87. Did your tuner person do a factory reset/unlock?

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    Quote Originally Posted by JKLTGuy View Post
    Do you have a T87A or T87. The A type needs to be unlocked and I believe that cost is around the high $300's mark. I know the TCM and the PCM don't mate with each other from the factory as I have 2 different units but they are E92 / T87. Did your tuner person do a factory reset/unlock?
    It’s a T87. Not sure if he did a reset or unlock. But he put in the gear ratio and tire size stuff so I’m assuming he did.

  4. #4
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    Still getting U0100 (engine) and U0101 (transmission). I checked for continuity between the ECM, OBDII port and TCM, all connections are good. There are 4 of those white and dark blue data wires going into the TCM connector but only 2 of them are giving me a reading on the multimeter. Power ground and ignition power are good going to the connector. Since it seems to be wired correctly going to the connector, is this a tuning problem? You said that the ECM and TCM don’t mate but from what I can tell from the diagrams they are linked through the OBD port.

  5. #5
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    It sounds like the ECM and TCM aren't communicating with each other (U0100 from the TCM to the ECM, and U0101 from the ECM to the TCM).

    It sounds more like a wiring problem, than a tuning problem, but if the ECM and TCM have incompatible calibrations in them, you'll get the exact symptom you have here. Can you post the calibration information for the ECM and the TCM? I'd be happy to check if there might be a compatibility issue.

  6. #6
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    Unfortunately I don’t currently have a way to read it. I do have the program code on the tag. The program is 15311. My engine and trans didn’t come with an ECM or TCM. I sourced both part numbers from GM Partsdirect and bought used from salvage yards so it’s possible that my T87 has the wrong program.

  7. #7
    Tuning Addict blindsquirrel's Avatar
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    Answering if it's tune-related or wiring-related will require some actual data on both how it is wired and what the tune is. Without anything to look at I guess it could be... aliens? Who knows.

  8. #8
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    I’m going to try to send the TCM back to my tuner to see if he can figure out what’s going on.

  9. #9
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    I spoke with my tuner who said that I need to bring the ECM and TCM into a dealer and flash everything to the same VIN. The closest dealer to me said they can try to program a new vin but it probably won’t work. Their price for flashing is also $450!!! Anyone on here have this capability?

  10. #10
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    Quote Originally Posted by blackdog76 View Post
    I spoke with my tuner who said that I need to bring the ECM and TCM into a dealer and flash everything to the same VIN. The closest dealer to me said they can try to program a new vin but it probably won’t work. Their price for flashing is also $450!!! Anyone on here have this capability?
    Where are you located?

  11. #11
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    Quote Originally Posted by sevinn View Post
    Where are you located?

    San Francisco area

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    I’m willing to ship my ECM TCM to someone who can do it.

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    I shipped the ECM and TCM back with instructions to flash to the same vin at the dealer and upload the TUN for my transmission to the TCM. I got them back, reinstalled them and same exact issue.

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    I’m also getting some additional codes when I push the pedal and put the trans into gear with the engine running: P0700
    P2635
    P0657
    P0706
    P0708
    P0751
    P0762
    P0842
    P0843
    P0872

  15. #15
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    In addition to the U0100 U0101 and P0073

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    I think I found the answer. The codes I am getting are Allison trans codes. From what I can find, the 8L90 codes don’t match what I’m getting. Anyone have a T87 from an 8 speed truck they can sell me? Or point me in the direction of a way to find the correct program code printed on the sticker for an 8 speed T87?

  17. #17
    Tuning Addict blindsquirrel's Avatar
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    I'm sorry dude, but without a HP Tuners tune file to look at I don't think this is going to be the right place to find your answers. There's no reference in HPT to the kind of stuff on any ID tags you're asking about. If you can give us the Service Number of the ECM & TCM hardware, and the Operating System IDs from both, then you might get somewhere.

    You need either a different tuner or someone non-virtual to physically verify your wiring, or maybe both.

  18. #18
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    The TCM was the issue. An Allison TCM doesn’t like to written over apparently.

  19. #19
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    I installed a used TCM from a 2015 that had an 8 speed and no more comm errors.
    Last edited by blackdog76; 03-29-2022 at 10:34 PM.

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    Glad you were able to get past that issue.