Follow the path of the refrigerant from the compressor. The compressor compresses ( gee!) the refrigerant gas, and that increases the heat content. Then the hot, high pressure gas goes to the condensor where it--guess what--condenses. Actually it loses heat to the cooling air and becomes a liquid. Then its off to the passenger compartment, flowing thru some controls and such along the way. Once it gets to the passenger comaptment it goes to the evaporator. Wanna guess what happens there? The liquid is passed thru a controlled orifice and allowed to evaporate, or return to being a gas. As it looses pressure it also looses heat--you could say it becomes cold. As it leaves the passenger compartment to return to the compressor it is at its lowest state of energy. Somewhere on this side of the system is the port, the low pressure port, for adding freon.
I would add a word of caution here. Refrigerant systems are somewhat dangerous. Adding freon is not a difficult task, but there are real, physical dangers. If you do not know where to put freon into the system, you could be exposing yourself to a potentially dangerous situation. Protect your exposed skin with face shield, gloves, etc.
If you insist on adding freon yourself. Do the smart thing and study a little first. The eyes that you DON'T freeze will thank you.
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