A friend owns a large transmission shop (28 service bays in one third of the
shop, bench work area in another third, plus a converter department and
parts storage, locker room, etc. in the last section). He has a couple of
transmission flushing machines, but they never use them. As Eric Murray
said, it stirs up settled material and can often cause failures which might
otherwise have never occurred, or would at least have taken a couple more
years to happen.
One of the worst practices in flushing, as seen at many quick-lube shops and
at least some car dealers, is to just flush the trans and never remove the
pan and change the filter. Really, the pan should be cleaned and the filter
changed before flushing, and at the very least the pan should be cleaned
again and the filter checked after flushing, with another filter replacement
again if there's any indication of more debris having come down.
The best thing is still just to service a transmission regularly, going no
longer than the manufacturer recommends for "severe service".
Karl
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