I found the following interesting post on one of the newsgroups....Perhaps blaming it on mites is easier than admitting copper does not play well with aluminum. Our maybe mites really do eat Mercedes-Benz evaporators. Who am I to say?
I've seen and replaced the evaporator core on countless 140 body Mercedes.
It is a problem that Mercedes-Benz is aware of and has released an
"unofficial" explanation for: the Southeastern US has a particular variety
of mites that apparently "eats" and "digests" the bare aluminum. The result
is a hole that develops in the evaporator core and allows the R134A
(refrigerant) to leak. The new parts that are being installed are "mite
resistant" - no kidding. The parts are dipped in plastic and coated with
chemicals. By the way, the 124 body Mercedes has had this problem for
years. I have heard some new Mercedes have actually lost the refrigerant on
the new car lot before it was sold. If your car is out of warranty there is
no alternative for repair payment. If you have an extended warranty, that
company should pick up the expense. If you have any other questions check
us out at www.peachparts.com.
Benzmac