As a part of getting regular maintenance on my 2 Legacy's, I decided to do fluid change myself. The question is, rather than doing a full tranny flush once a year or so, can I empty and replace what's in the pan every 2 or 3 oil changes instead?

lots of people chance the oil frequently in a shorter time period to clean out whats impossible to get with just a single oil change. but if you want to remove the pan thats fine, but real messy and theres a magnet in there you can clean as well

I don't want to remove the pan, just drain the AT fluid and refill.

Once you get at least 90% new fluid in circulation, then the occassional drain and fill is feasible. Maybe someone can post the calcs again of how many drain, fill, operate, drain, fill you'd need to get to at least 90% new fluid. You want to get to 90% new fluid fairly quickly. Many oil blenders say ideally on the change you want at least 90% of the same brand and type fluid in there.

 

Then after that, try to stay with the same brand and type fluid for your periodic maintenance drain&fills. Mobil1 syn atf works nicely, as does redline, amsoil, and others.

i usually do 3 drains and refills in very short succession. just run it enough to circulate and do it again. do this at mileage intervals the owners manual sugggests. i would not do it multiple times a year myself. the system and fluids are designed with the intent of being replaced at large intervals (compared to engine oil changes), i would rather stick to those recommendations than try mixing one change in every now and again.

 

sounds like you're trying to save some work...do two things at once. not a bad idea. but if you follow the recommended ATF change intervals, it's not very often.

If your Legacy is the same as my daughter's former '93 I would recommend dropping the pan and changing the filter. Like theflystyle said there is a magnet that is in the bottom of the pan. Probably looks like a fuss ball if you have any miles at all on the cars. This also gives you a chance to clean the inside of the pan which will be coated with a pretty nasty layer of fluid. When dropping the filter out you should get another half quart or more. My daughter's Legacy showed in the maintence records a change at 60K and flush at 97k. I changed fluid and filter at 130K and the pan and magnet were pretty nasty. Then I did just a drain at 30k intervals. A coworker bought the car after my daughter hit a bear with it and it has around 215k last I looked and transmission is running fine.

i disagree with replacing the internal filters. really you're right in a way, but my real problem is that it's not a filter, if it was i would say replace it as well. but it's just a mesh screen. your oil sump in your engine oil pan and your fuel pump sock have one as well, but noone ever replaces those and without a doubt the most likely one to need it is the fuel pump since anything it collects can't go anywhere.

 

that being said, i've pulled apart a number of auto trans and used to replace the "filters" as well. mostly on 20 year old XT6's with 150,000+ miles on them. i soon realized they don't get dirty, because their job is to prevent stuff from getting sucked up, not collecting it like a typical fliters. yes, there's a difference. proper drain intervals will drain any items the mesh screen prevents from getting sucked out. so in essence rather than collecting it really just makes sure particulate stuff stays in the pan, which a fluid drain removes. it gains you nothing and requires new gaskets, lots of time dealing with trans pan bolts. the bolts can be problematic, the gaskets sometimes don't seal well, pan bolts holes can get ruined...etc.

 

the newer style external filters starting around 1998 shoudl definitely be replaced. ACTUAL filters finally and easy to do. but those screens just aren't worth the time and risk....under normal circumstances.

Grossgary I believe you are correct. Sometimes my memory slips by a little. That is probably why I never dropped the pan again after the first change. I guess the intent of my reply was to say that if the transmission has been serviced regularly there would be no need for extra fluid changes as the transmissions are not prone to failure if treated and maintaind properly.

good thing you brought it up, probably good info for him to have..particularly if one of his legacy's is newer (with external filter) and the other is older (without). he'd surely be scratching his head about that.

[...]Maybe someone can post the calcs again of how many drain, fill, operate, drain, fill you'd need to get to at least 90% new fluid.[...]

Sure . I did the calculations a while back; here's what I previously posted:

 

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Based on the full capacity of the 4EAT and typical drainage from the pan by just removing the plug (with the car level), I roughly calculated the percentage of old ATF replaced with new by multiple drain/fills.

The following is in the form #-of-changes -- %-fresh-ATF:

 

1 -- 40%

2 -- 64%

3 -- 78%

4 -- 87%

5 -- 92%

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Naturally, if you wait significant time (months or more) between the drain/fills, the "fresh" ATF won't all be as fresh as if you do the changes in rapid succession.

I haven't seen anything in the screen type filters either. Which makes sense since yes I believe it really a strainer on the pump suction. Unless you already have a phase II AT with the external filter on the outlet to the cooler, adding a filter in the cooling circuit is not a bad idea. Subaru had those retrofit kits for the '90- something's because of junk/shreddings/etc; these can plug up the cooler and lines and stuff.