Why would my transmission fluid only be "partially clean" instead of "completely clean" after a fluid change?

Dear Car Talk

Dear Car Talk | Nov 01, 1996

Dear Tom and Ray:

I recently took my 1992 Toyota Camry with 67,000 miles to the dealer for an
oil change. I requested that all fluids be checked. When the service was
completed, the service advisor informed me that the transmission fluid was
at its proper level and was "fairly" clean. I maintain the car religiously,
and had a complete transmission service at 60,000 miles, so I can't
understand why the fluid was only "fairly" clean. Why not "completely"

The service advisor stated that only about half the fluid can be drained
during transmission service, and that if I really wanted it totally clean,
I should change the fluid two or three more times to get all the fluid out.
It has been my understanding that changing the transmission fluid every
30,000 miles is adequate. Is it? -- Paul

TOM: Yes, it is. The dealer is right that you can't get all of the
transmission fluid out simply by removing the drain plug or by dropping the
pan. When you drain it, there's still fluid in the torque converter and
some of the passages that doesn't come out.

RAY: And if you really wanted to change "all" of the fluid in there, you
would have to drain it several times.

TOM: But it's really not necessary. Changing most of the fluid every 30,000
miles is good enough. In fact, some manufacturers now tell you not to
bother changing the transmission fluid at all. "If the engine's still
running at 100,000 miles," they say, "go ahead and change it then."

RAY: I think the real problem here was semantic, Paul. He told you your
fluid was "fairly clean." He should have just told you that your fluid was
just "clean" -- which it was, in the overall scheme of things. When it
comes to transmission fluid, dirty, burnt-smelling, brown gunk with chunks
of metal in it is a problem, but "clean," "fairly clean," "pretty clean,"
"medium clean," "Mr. Clean," is all as it should be.

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