Thread: The Reason That Your Transmission May Act Up After a Flush

Yes, there is a reason, and a very simple solution. Our cars have what is called "Adaptive Learn", which means that the the car learns how you drive, and adjusts fuel trims, transmission line pressure, shift points, etc., based on how heavy your right foot is. The transmission learns to shift according to your driving style with the old fluid, and when you flush it out and replace it all with brand new, It cant tell the difference. It still has all the shift points, shift times, line pressure, etc. stored in memory, but they are all based on readings using the old fluid. So before you condemn your tranny, try disconnecting the battery for a few minutes to reset the PCM's adaptive learn. I have seen many tranny's "fixed" after a flush by doing this. Of course it won't "fix" all of them, but if your tranny is acting up after a flush, it is worth a shot.

I'm not sure what year Adaptive learn was introduced, but I want to say 1997. I'm sure trannyman can fill in where I have missed.

In relation to the tranny, it only adjust shift pressures. Shift points are coded into the PCM and only change with tuning (or performance shift, I think).

That's odd...
When I change those settings in the PCM they stay at what I change them too...
Also, I've never seen a table or a PID that supports this theory...
Wouldn't this make it hard to switch from driving around town to driving on the toll road?
Or, switching from you driving to your wife, husband, kid, etc?

I'm not saying you're wrong, but I am disagreeing with this post.

It's fact. GM's SI manual describes it. It's called TAP (transmission adaptive pressure). All it does is learn what shift pressures accomplish the desired shift time. If you're a heavier throttle driver, heavier pressure is required to accomplish the shift within the PCM's programmed limits.

It learns very quickly. If you hot dog it for a while, then go back to gentle driving, the first few shifts will feel a little more harsh. It doesn't remember what throttle percent you used, all it knows is that on the last 1->2 shift it had to run XX pressure. Once you drive soft and the more crisp shift happens, it realizes that it can tone the pressure down a bit. Next shift it will learn a little more, and by the third things are back to being gentle.

The only other "learning" the PCM does is fuel trims, which doesn't vary by person.

I'd like to read more on that, just to be informed...

If it's that fast to learn though...
Wouldn't it correct itself before you noticed the difference from the transmission flush?

Originally Posted by FastKatt

I'd like to read more on that, just to be informed...

Check this article out: http://www.sonnax.com/tech-articles/TASC-TIP-05-05.pdf
And the follow up: http://www.sonnax.com/tech-articles/TASC-TIP-06-05.pdf

Sonnax has a bunch of good tech articles.

Typically doing a fluid change/service/flush will create a softer smoother shift feel as the fluid breaks down over time and you dont notice it because the pcm learns and this is what can throw a wrench into things. Unless the trans is shot or weak it really isnt a concern and the pcm will relearn pretty quickly and in town stop and go driving at light a part throttle conditions is best. The 97s are the worst as they are slower pcms and take longer to learn, the 98+ pcms learn much quicker and generally make the changes in as short as a few shift and key cycles.

Thank you all for your additions and corrections.