Transmission Question re: manual fluid exchange - how many times?

Is the fluid clean now, does what you have done helped, I would not worry or try to get it all, I just drain it out every oil change or 2. If it still wont shift synthetic wont help.

Your theory is good, but in practice I'm afraid you missed the mark.

Every time you drain and fill, the fluids mix! So, while the initial calculation is right, thereafter the mixing causes less non-synthetic to be replaced.

Math is not my strong point, but it is going to take a while...

I found this in a oil forum by some named "pitzel" which might be interesting. Not sure of the logic, but it does make sense. Here's a shortcut link:

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"Well, that is a somewhat 'slow' way to change the ATF, and ends up wasting a lot of fluid.

For example, my 4T60 transaxle holds 12L of ATF. If I change 2L or 17% of the fluid at a time:

First Change:

17% New/83% Old = 83% old

2nd change:

83%*83% = 69% old

3rd change:

69% * 83% = 57% old

4th change:

57% * 83% = 47% old

5th change:

47% * 83% = 39% old

6th change:

39% * 83% = 33% old

7th change:

33% * 83% = 27% old

8th change:

27% * 83% = 25% old

9th change:

25% * 83% = 19% old

10th change:

19% * 83% = 16% old

11th change:

16% * 83% = 13% old

12th change:

13% * 0.83% = 11% old

nth change:

0.83^n = % old

Doing a pan drop evacuates approx. 8 quarts on the same transaxle, leaving 4 in the torque converter.

First change: 33% old fluid

2nd Change: 11% old fluid. nth change: (1-66%)^n

As you can see, using 16L of fluid with 2 pan drops replaces 89% of the fluid. To do it 2L at a time, would take 24L of fluid to achieve the same concentration of 'new' fluid.

Ultimately, you have to decide whether it is worth wasting a bunch of oil to accomplish a change. I hate to recommend wasting oil, but actually doing it 2L at a time does make significant sense if you don't care about doing a filter change or any of the other benefits a pan drop would provide."

Just focus on how much synthetic has been added on a net basis. If you drain hot, and leave overnight almost another quart will come out. Three or four times, you've replaced most of the fluid. If your numbers are correct, and you're draining 2 quarts out of 8 that would be one fourth. Just focus on how much synthetic has been added, since that's what you want to know. First time, you add 2. Next time, you add 2, but remove one fourth of the previous 2 - equals two fourths or one half quart new synthetic lost. Total remaining, two new, plus one and a half prior quarts equals three and a half. Next time you add two. Plus you lose one fourth of the prior 3 /12,and so on. It's a process of diminishing returns, but after a few times, you've got most of it. On my 4 cylinder, the total capacity is 6.7 qts., and draining over night can remove around 2.8 qts., so you'd be adding new fluid more rapidly.

Thanks! I did a quazi-fluid exchange based on reading the forums over at BOBISTHEOILGUY -

(main page:

- great reading on both sites)

Basically one disconnects the fluid output line running from the tranny to the radiator cooler, connects a line to it so it flows into a container, starts the car and lets it pump out about 2 quarts until the feed slows and bubbles appear. Then reconnect the line, refill with Synthetic ATF, drive around a little, repeat 2 more times. I've read that's a good way of getting the fluid out of the converter.

The question is, where does the fluid come from which is being pumped out of the tranny to the radiator cooler? Is it from the pan or from some circuit off the converter?

Anyway, yesterday I fed it 8 quarts of Mobil1 synthetic ATF with one pan drain and the rest using the procedure above.

I feel pretty good about getting it more or less mostly synthetic by now.

I figure a pan drain of the tranny at every oil change ought to keep things peachy.

You'll never actually get ALL of the old fluid out but to do so is not necessary. It isn't AIDS. A little doesn't contaminate a lot. For many years I've been using a Might Vac hand pump, in between pan drops and filter changes, to pull out 3 or 4 quarts of fluid yearly, then replacing with fresh. It keeps the fluid fresh enough so that I have never had a problem with an automatic transmission in 41 years of car ownership. It may be just me and just anecdotal, but that is my experience. YMMV, of course FWIW DFB

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