> I've always wondered why manufacturers don't put drain plugs in
> transmission pans.
I've heard several justifications for that; the one that seems most likely
to me is that changing transmission fluid is a rare enough occurrence that
it isn't worth the extra cost and complexity of a drain plug plus an
external oil filter (so the filter can be changed without dropping the pan).
But it's also a Good Thing IMO to wipe out the bottom of the pan, to both
remove dirt and to see any evidence of impending failure.
I guess I've been lucky, all of my vehicles with automatic transmissions
have gone essentially their entire service life with only routine
maintenance to the AT. Except for the motorhome (Torqueflite 727 behind a
440-2) that had a tendency to overheat the ATF until I added a cooler. The
4L60 in my current hauler has gone some 260k with only a (partial) fluid and
filter change every 100,000.
My opinion, unless you've let it go too long, changing just part of the
fluid and the filter is good enough. The oil itself does not wear out, and
by replacing part of it you have both adequately removed a substantial
portion of the dirt, and replenished the additives that do wear out.
-- Randall
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