Originally Posted by KrisZ
Originally Posted by T-Stick
Should be: That is the same as saying new fluid on degraded components IS a high risk.
And I'm not disputing that. There is obviously a risk when changing the ATF on a neglected transmission and there is more then enough evidence of this. What I'm disputing is the mechanism that the contaminators in the fluid help keeping the clutches engaged. There is absolutely no scientific basis for this, nor any studies that I have seen. This talk seem to have started with people posting few youtube videos on the subject and it now is taken almost as a fact.
Like I mentioned earlier, in order for particles to help with clutch engagement, they would have to permanently embed themselves into the clutch material in such a way that it would make that surface rougher, which I proved by posting a study that opposite is happening. Since clutch pack engagement is a mechanical coupling, not a fluid coupling, contaminants floating around have no effect on the friction when the clutches are fully engaged, because these contaminants have no anchor point. It's like having sand between two sliding surfaces. Go ahead and grab two pieces of 2x4 and start rubbing them together, now throw some fine sand on them and repeat. You will find sand creating an air gap and rolling around, thus reducing friction, because the sand particles have no anchor point.
Or let me put it this way, why spend money on sand paper right? Just get some paper, throw in some sand on it and viola! Except it doesn't work this way.
What I'm trying to explain is that a traditional, modern automatic transmission is a very complex hydraulic system and ATF is at its heart. Pretty much every automatic transmission since mid to late 90s is controlled by electromagnetic solenoids, ECU and has some sort of adaptive logic. Clutch packs are engaged/disengaged by hydraulic action alone, not like traditional manual clutches where constant spring pressure applies the clamping load. Controlling clutches hydraulically has advantages, the pressure can be adjusted on the fly, making slow and smooth shifts when driven normally, or fast and hard when accelerating at WOT. This logic also adapts to changing conditions, like aging ATF or slower acting control valves.
Now, when a severely worn out, burnt, sheared out and full of contaminants ATF is changed with a brand new one, all of these calibrations are wrong, right away. Then you have the effect of contaminants migrating because the new ATF can now hold more and also creates a cleaning action. So these contaminants can be removed from one place and deposited somewhere else, where before everything was sort of settled down.
Since hydraulic pressure and its control is so vital to the clutch pack operation, my position is that the new fluid creates an "upset" in the system through various mechanisms, not just one, that the pressure cannot be regulated properly and therefore the transmission starts slipping shortly afterwards.
Edit:
To add to the above, I believe that resetting the AT ECU might provide some advantage when a neglected transmission is serviced. But It's only my speculation and at best it would probably only delay the inevitable.
This is a very well reasoned post and worthy of consideration. You also are noting some of the same points I was making. The only point of further consideration for me would be seeing more than one test concerning whether contaminants always cause more slippage. If so, then all new fluid at one time is very bad because it will clean up and circulate more of them and thus hasten the demise. I see the bottom line essentially the same - the drain refill is the preferred method for trying to salvage a neglected transmission because you can't introduce too much new fluid and cleaning at once and you need to remove those contaminants through a series of drains. I have never had a problem to date with using the drain and refill on an older transmission and I typically would keep 6 months to a year between drains and refills. There are a lot of examples of transmissions working OK until they are flushed. Essentially if you flush an old transmission you should do a series of flushes to try to remove contaminants before they kill the transmission.