Transmission pan drop question. How long for fluid to fully mix?

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Oct 29, 2021
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I have a 2002 Explorer. This week I want to drop the transmission pan, change the filter, clean the pan, new gasket etc. and refill. I understand that you only actually change like 5 qts of fluid when doing this. So after that, I want to drive the car for a bit and then just undo the drain plug and then re-fill one more time. I figure after doing this, that it'll be 75% new.

Anyways, my big question is.... how long does a transmission take to circulate/mix all of it's fluid? Would just driving it around the neighborhood for like 15 minutes do the trick? Or should I drive it for an entire day, then drain the next day? Or, would just running it on the jack stands do the trick?
 
 
I have recently done similar on several cars, and it's my understanding that driving it around for a short period, and using all the gears, will mix it up and dilute the new with the old. 15 minutes will probably be sufficient, but I'd opt for a day or two of driving for a much better mix and to listen/feel for any issues, and check it when it's hot and running, per the manual. I found that I somehow underfilled mine on one of my iterations, so measure carefully but also check and double check.

It's a point of diminishing returns, and there's formulas on the internet that can help you. If it's a 15qt capacity and you extract 1/3, the formula is something like this, and you can see that after #3 you are mostly replacing the new fluid to get some of the old. This is in theory assuming they mix very well.
# New %
1 33
2 56
3 70
4 80
5 87
6 91
7 94
 
Thanks everyone. Hopefully we get a nice day this week and I can "Get 'er done". I'm not having any problems with the trans. Just doing for PM. I'm buying a whole case of Mercon V. So maybe I could squeeze 3 changes out of it? We will see. Only thing that sucks about this trans is it has no dipstick and it's a pump from the bottom fill. Fun. LOL.
 
I have recently done similar on several cars, and it's my understanding that driving it around for a short period, and using all the gears, will mix it up and dilute the new with the old. 15 minutes will probably be sufficient, but I'd opt for a day or two of driving for a much better mix and to listen/feel for any issues, and check it when it's hot and running, per the manual. I found that I somehow underfilled mine on one of my iterations, so measure carefully but also check and double check.

It's a point of diminishing returns, and there's formulas on the internet that can help you. If it's a 15qt capacity and you extract 1/3, the formula is something like this, and you can see that after #3 you are mostly replacing the new fluid to get some of the old. This is in theory assuming they mix very well.
# New %
1 33
2 56
3 70
4 80
5 87
6 91
7 94
Made quick spreadsheet for this, you can enter your total capacity and what comes out in a D&F to run the numbers. Depending on how much you can get out, there is diminishing returns after 2nd or 3rd.
 
I would just make sure you drive it enough to shift through all the gears at least a couple of times.
 
The choice is yours of course, but if it were my car I'd do just a fluid change the first time around then on the last of your series of drain/fills do the filters. The less old contaminated fluid left in the system the longer the filters will stay clean. I would think the fluid would mix within a few minutes, but it wouldn't hurt to run it a few days or even a few thousand miles before doing the second drain fill. I just changed the fluid/filters in my Nissan CVT couple months ago at ~30K miles. In my case I drained about 1/2 of the old fluid so I got about 50% dilution rate. I'm planning on doing the same thing at ~60K miles and if I'm still around at ~90-100K my plans are to do about 3 drain fills in 10K miles to get the majority of the fluid renewed and change the filters on the last of the series.
 
15 minutes of driving is plenty. Just make sure it hits all gears including reverse and neutral for a bit to get fluid through all valve bodies and such.

As said above, there (quickly) comes a point of diminishing returns on doing a drain and fill.
When I got my truck with 168,000 miles, I dropped the pan and did a filter change, and installed a drain plug.
After 500 miles, I drained it again, and did it again another 1,000 miles later. I now do a drain and fill every 10,000 miles just to keep it fresh and it is easy to do with the drain plug.
My Scion and did a drain and fill @ 30,000 miles, then did a pan drop and filter change @ 50,000 miles. Will do a drain and fill every 30,000 miles or so on that. Will be doing the same on my wife's Santa Fe (is actually due for the 30,000 mile drain and fill).
 
UPDATE. All done! Spent most of the day messing with it, but I'm happy now. Here's what I did....

Pan drop with new filter. Fluid looked kind of dark. Refill with 5 qts. of Mercon V. Ran car for 15 minutes on jackstands, alternating from 1,2,3,OD and R.
Shut car off, drain and fill again with 4 qts. (a quart less because pan/filter wasn't dropped) Repeated above run/shift procedure.
Then did a drain/fill again.
Then one more time.
So 5+4+4+4 qts. of fresh fluid. On the last drain it looked pretty dang clean to me and remembered the phrase above "diminishing returns" ... so I called it good.
Took it for a test drive and she is shifting as smooth as could be.

Thanks everyone.
 
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