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Thread: 2018 Silverado A6 3-2 Downshift Modification

  1. #1
    Potential Tuner
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    Nov 2019
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    2018 Silverado A6 3-2 Downshift Modification

    Hi All, long time reader, first time posting. I have had an ongoing issue with my 18 Silverado A6 where I have what seems to be an incorrectly timed 3-2 downshift when the truck is at normal operating temps (185 deg F and higher). When coming to a closed throttle stop (foot off gas, slightly braking) I get a 3-2 driveline clunk. What I feel is that the 35R clutch is releasing too soon and the CB26 isn?t coming on quickly enough to prevent a loss of torque on the driveline. After reading up on a lot of posts, I haven?t seen any info on increasing the 3-2 downshift speed or reducing the 3-2 pressure. Does anyone have any suggestions on how I could tackle this and what the possible repercussions would be for the changes? I can post the stock tune later if needed. Thanks in advance!!

  2. #2
    Potential Tuner
    Join Date
    Jan 2020
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    1
    My first step would be to check if you have the latest TCM calibration if you are running a stock tune. If you have access to a scanner, you can access the calibration ID and enter your VIN at this link https://tis2web.service.gm.com/tis2web

    Compare your part numbers you retrieved with the scanner with the ones they provide, or head over to the dealer or an aftermarket shop equipped with a J2534 passthru and get it flashed. Alot of shift concerns addressed with programming updates on these units.

    From a mechanical perspective, the 1234/35r housing is known to crack at the welds, at higher temps any leaks there will be exaggerated. Eventually slippage will occur in reverse, 3rd and 5th. Hope this is of some help.
    Last edited by TransTech; 01-09-2020 at 09:16 PM. Reason: clarification

  3. #3
    Potential Tuner
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    Nov 2019
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    Thanks for the reply. I will check them once I get an MDI from work. One other thing I noticed is that the downshifting issue is happening when I run the defrost. I drove it for about 3 hours with just the vent heat on and no issues, shifted like normal. When I turn the defrost on it gets firmer on the downshift and I hear the 3-2 clunking on the downshift. Any ideas on why that would happen?

  4. #4
    Advanced Tuner IARLLC's Avatar
    Join Date
    Aug 2016
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    708
    With the defrost comes the AC compressor and a few more closed-throttle RPM to go with it. That could give a little bump during the 3-2 downshift.

    The 8 speed gets a closed-throttle 3-1 clunk as it comes to a stop. Running through the relearn process helps a little. Maybe once a month, making sure to let them decel from highway speed with no throttle or brake down to as low a speed as it will coast to helps the TCM know how much pressure/volume to predict for a closed-throttle downshift. Reducing or turning off the TCC for that situation and/or adjusting closed-throttle downshift speeds gets rid of it completely on the 8 speed.