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Just caught this today in the GM Recall Center.

GM Program #:
N182200470

Date Issued:
May 06, 2019

Program Title:
Transmission Clutch Drag Causing Overheat Warning

Program Description:
Certain 2018 model year Buick Enclave and LaCrosse vehicles, and 2018 – 2019 model year GMC Terrain vehicles, may have a condition that occurs when the driver places the vehicle in neutral immediately following an auto start from an auto stop condition. The transmission can become stuck in a command that will cause damage to the transmission clutch plates.

Repair Description:
Reprogram Transmission Control Module with SPS.
 

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Why would someone throw the transmission into NEUTRAL immediately after an Auto-Start event? is this a way to disable Stop/Start?

Whatever .... this sounds like a simple software update to close a potentially damaging logic loophole.
 

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No, not to disable stop/start, to have the general buy you a new tranny.
 

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Bear with me for a moment. You brake it goes to AUTO STOP, you ease off the brake it AUTO STARTS but doesn't move. You brake again but this time it doesn't AUTO STOP and you place it in 'N'.

As long as the car isn't moving what difference would it be going from 'P' to 'N'? or 'D' to 'N'?
 

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I'm sure there are people who do this. There are people who with older cars would put it in neutral at a stop thinking they were saving there transmission. So obviously someone must be doing it and seems there is error in the programming of the computer when this happens. I hope I can wait to have it done since it should be less than a hour to do.
 

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Bear with me for a moment. You brake it goes to AUTO STOP, you ease off the brake it AUTO STARTS but doesn't move. You brake again but this time it doesn't AUTO STOP and you place it in 'N'.

As long as the car isn't moving what difference would it be going from 'P' to 'N'? or 'D' to 'N'?
The difference is: (controlling) software.

I've seen software where someone tried to implement a software State Machine, but then didn't code it out completely (leaving loopholes) such that the "machine" would sometimes end up in 'no-mans land' - with no way of getting out - short of a power down of all related subsystems to 're-sync' them.

And then any time you create multiple paths to the same destination, you have to make sure one or more of those paths aren't inadvertently bypassing something they shouldn't be. A wireless key FOB signal needs to be introduced in the software at the right point so that it not only seamlessly follows the same path of the key, but also transparently considers additional interlocks that the key doesn't have to. If you get some part of this wrong, you can introduce a bug in one of the other, or both.

In this case, the engine is being re-started via a secondary path: STOP/START. But now the driver has thrown the transmission into NEUTRAL. There must be some logic loophole in there whereby the clutch plates are not releasing (??) because the software is mistaken as to which state the transmission is actually in? Just guessing here.

This is not that surprising, really. Most software is coded and tested to a Functional Spec, and little else. The better systems go further with "defensive coding" and more exhaustive testing, but that takes more time and money. Plus - you've got Quality Control people who really don't provide any "Quality" at all. They're mostly only catching the most obvious stuff: "Does this Test document test every requirement in this Functional Spec?" Well, yeah, it does ... but what if something out of the ordinary happens? Did you test for any of *that*? "No. We don't have time for that!".

That's called "User Test". LOL!
 

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When the engine stops so does the trans. front pump so trans. loses internal pressure and clutches disengage. Just like shifting into neutral. I see accelerated trans. wear from all that start/stopping. Unless there is an accumulator keeping internal line pressures up engaging trans. clutches I don't see these tranny's lasting 200k miles. IMO another case for spending the money for a start/stop deactivator.
 

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When the engine stops so does the trans. front pump so trans. loses internal pressure and clutches disengage. Just like shifting into neutral. I see accelerated trans. wear from all that start/stopping. Unless there is an accumulator keeping internal line pressures up engaging trans. clutches I don't see these tranny's lasting 200k miles. IMO another case for spending the money for a start/stop deactivator.
I don't know about other transmissions, but the 9-speed in the '18 2.0T 'Nox has an accumulator as part of the package with start/stop. It's advertised as providing the ability for "immediate" movement of vehicle after the "start" feature activates.
 
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