All my cars, '79 300D, '95 C280 and '89 300SE have had their TC drained at the time of trans fluid/filter service.

You have to realize that the car needs to occupy the service rack for up to 1 hour until the TC drains.

That may be why some shops decide not to drain the TC. "get 'em in, move 'em out"

I've paid $150 for this 'full' service at Peter Schmid transmissions.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD