It usually doesn't take much vacuum to actuate the diaphragm and close the heater valve. These cars have a well know history of vacuum leaks both in the engine compartment and the HVAC system, so it is unlikely that your heater control valve is bad. But that can easily be checked by connecting an engine vacuum line with plenty of vacuum to the valve temporarily and see it it actuates. If you have a leak in the HVAC system which it probably is, we can give you some hints where to look.

What ambient temperatures are you running the A/C in? The reason I ask is because it was -8F here last night. A/C cycling is a function of temperature and state of charge. Adding R134a is relatively simple nowadays and cheap. You can buy a can of R134a and a gauge for ~$20 USD at most all Wal-marts.

DEWFPO

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1998 S90 077,400 and 1995 964 154,100