Don't do it. New freon is much less efficient in heat transfer and the SL's
system is already marginal. It short, you won't get enough cold air to do
yourself any good in hot weather. A good, tight R12 system is far superior
in cold air output.
Be sure the mechanic replaces the freon pressure switches. I suspect many
people who didn't get adequate cooling after conversion might be trying to
run R-134 within R-12 vapor pressures--not a very efficient way to go. Also
check all compressor seals and the low side suction hose for leaks. However
if your system isn't leaking, then I'd just add R-12 until it becomes
necessary to convert.
The "New Freon" is not freon at all but a refrigerant called 134A. It is
sold at all Wall Mart stores! Converting requires the replacing of the
receiver/dryer, standard procedure. Any good service station can do the
conversion. Find one who injects fluorescein into the system. It will reveal
the exact source of a leak when exposed to UV light. To convert is not
voodoo, but removing the contaminants from the system, installing the 134A,
changing the screw-on caps for both ends with ones that indicate the new
refrigerant. It will cost less if you source the new receiver/dryer from
Rusty at 800-741-5252. He discounts all MB parts and is the official parts
supplier for mbz.org. My conversion costs under $100. P.
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