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Allison OEM Discuss repairing & maintaining an OEM Allison automatic transmission.


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Old 05-28-2019, 10:36 AM   #1 (permalink)
Ted White
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Transmission Clunk 2019 Model Year

I'm taking my truck to the dealer tomorrow, but wondered whether anyone else has experienced this issue and resolved the problem. I did not have this issue on my 2016.

From time to time, when braking on approach to a traffic lineup at the lights, there is a clunk that can be felt and heard, but NOT THROUGH THE BRAKE PEDAL.

When it first started happening, I dismissed it as anti-lock braking clicking in, but then I noticed that there is not any vibration or clunk feeling through the brake pedal. It's more like the transmission making a hard downshift.

It does tend to happen during harder level braking, but as mentioned above, I don't feel that it is giving me brake system symptoms.

Over to you.

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Old 05-28-2019, 03:16 PM   #2 (permalink)
redwngr
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My 18 does that sometimes.

I figure it's related to the torque converter not unlocking soon enough.

It seems the L5P trucks have a different torque lockup strategy that has it engaged in lower gears.

It may be better for towing or for fuel economy (got no info one way or the other) but it certainly seems to have a detrimental impact of driveability.


Maybe your buddy can shed some light on this?

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Old 05-28-2019, 03:23 PM   #3 (permalink)
Ted White
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Thank You redwngr for letting me know that you have also experienced the "clunk". My buddy in GM can't offer any insight because he is strictly involved in fuel programming.

Perhaps there's another owner out there who has discussed it with a dealer, but in any case, I wanted to get it on the record so that there's no argument if it turns out to be something nasty later in the game.
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Old 05-28-2019, 08:34 PM   #4 (permalink)
redwngr
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Sometimes people working on different areas of the same program 'hear stuff' that isn't directly related to their area of responsibility.

Besides, seems TCM programming and ECM programming ARE fairly closely related. Especially as far as driveability goes.
We do know that the trans and engine controllers 'talk' to each other. A lot.
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Old 05-29-2019, 01:06 AM   #5 (permalink)
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Believe it or not but according to one of the Jacksonville GMTC instructors, in the mid to late 80s GM actually brought one of their brightest training center instructors to Warren to "Teach" the engine and transmission engineers how their products worked together, Guy from Oregon I believe.
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Old 05-29-2019, 07:48 AM   #6 (permalink)
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Thread relocated over to "Allison OEM"

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Old 05-29-2019, 10:02 AM   #7 (permalink)
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2017 & 2018 had Service Updates for what your describing traveling slow and stop quick. You can feel it in your seat.
There’s lots of info on this on other sites for a service bulliton for the 2019

Last edited by Rvn4me; 05-29-2019 at 10:07 AM.
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Old 05-29-2019, 07:14 PM   #8 (permalink)
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Well, if a service bulletin exists, my local dealership knows nothing about it. The tech took the truck for a drive and returned saying it's completely normal and very common.

I inadvertently started this thread under engine, should have known better. I'll do some searching for that service bulletin.
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Old 06-11-2019, 12:17 PM   #9 (permalink)
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Is it an actual clunk? Depending on the level of my fuel I will feel a "shift" from the fuel sloshing forward.

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Old 06-11-2019, 04:49 PM   #10 (permalink)
Ted White
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It's DEFINITELY a mechanical clunk, not fuel slosh.

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