There is a tag on the A/C system somewhere that states the refrigerant used. Please do not use anything that will interrupt the dynamics of the ozone layer. There are drop ins you can get at auto supply houses if you need to change out R 12.
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=== Update (03/24/2016) ===
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Also note the weight of the charge. Most drop in replacements are usually 3/4 of the R-12 charge. Add slow with a thermometer at air vent till temp of air is 20 degrees F. cooler then the ambient air. Do not let frost line to ice up back to compressor. This is after you have troubleshot low pressure switch. Then before you do any adding of refrigerant, ask yourself how did refrigerant get out of system. Check your gauges to see if suction side is pulling a vacuum. Is clutch on compressor engaged? Is there power to low pressure switch? Every time you add refrigerant there is oil mixed with it and after so many times the refrigerant leaks out but not the oil and you could be oil logged. Is cap tube plugged? And remember with most if not all drop ins, you add as liquid not vapor so don't slug the compressor with liquid. And most important keep the number of a air-con tech handy for when you blow it up. Good luck if you don't have good skills:)
There is a tag on the A/C system somewhere that states the refrigerant used. Please do not use anything that will interrupt the dynamics of the ozone layer. There are drop ins you can get at auto supply houses if you need to change out R 12.