What is the best method for changing transmission oil?

Dear Car Talk

Dear Car Talk | Feb 01, 2000

Dear Tom and Ray:

What is the best method to use in changing a car's automatic transmission oil? One method is to remove the transmission pan, drain out as much oil as you can, then
change the filter. I understand that method leaves a lot of the oil still in the transmission. The other method, used by Jiffy Lube service stations, is to suck the oil out of
the system without removing the pan. Supposedly, this method removes nearly 100 percent of the old oil. Which method is better? -- Charlie

RAY: These new "purge" machines do get almost all of the transmission fluid out, Charlie. So they are better. Specifically, they get the fluid out of the torque converter
(a separate part of the transmission), which the traditional fluid change can't touch.

TOM: The only disadvantage is that they don't change your transmission pan gasket or your filter.

RAY: During a traditional transmission service, the gasket and filter will get replaced. So if your gasket is leaking at all (which they often do), then you might as well get
the traditional fluid change. Otherwise, you'll be paying for the same job twice.

TOM: But if the gasket is good, then the purge method is great.

RAY: And if your filter is in decent shape (i.e. if you've serviced the transmission on any kind of a regular basis), then you shouldn't need to change the filter. So again,
the purge method is best.

TOM: On the other hand, if the fluid hasn't been changed since Dodge Darts roamed the earth, chances are the filter is full of crud, too. In which case, you probably
want to do both: a purge and a traditional filter change. That'll cost you more, but that's what you get for ignoring your car for all these years.

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