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Originally Posted by Ryland
the biggest killer of A/C is letting them sit for months without running them, that is why some cars the A/C kicks in for just a moment every time you start the car, others it kicks in with the defrost (something you can turn off with alot of cars, but only after it's turned it's self on) sure this drys the air but it also saves the environment by keeping the pump seals from drying out.
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Modern A/Cs don't have this problem. (After all, HVAC parts frequently sit unused for months or even years and work just fine when installed.)
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All A/C systems have some form of low-pressure shutoff switch to protect the compressor from fluid lockup. If your evaporator "freezes up" the compressor needs to shut off to keep from continually icing it which will cause high-pressure liquid to back up into the compressor on the high pressure side by evacuating the low-pressure side through the compressor running when it isn't needed (the super-cold evaporator will cool the air without refrigerant running through it while it thaws).
Locate the low-pressure shutoff switch. Figure out what you have to do to force the compressor off; in my old truck removing the connector leaving the switch circuit open forced the A/C off while jumpering the switch terminals with a wire forced the A/C on. If this is the case splice into the wire (or one of the wires) with a manual toggle switch you can mount under your dash.
The switch will be located on somewhere between where the evaporator tube passes through the firewall and the compressor (not in the run that includes the condensor, the other half of the circuit). It will probably be closer to the firewall rather than near the compressor.
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Actually, the low pressure control is to prevent the compressor from running with excessive compression ratios if the charge is a little low. A properly operating A/C system will not freeze up. A slight loss of charge can cause the coils to freeze up, however.