If there is a pressure switch, it will be on the hard line or the cooler/dryer. To recharge properly you will want to vacuum the system first then add the correct amount of refrigerant. There may be a sticker somewhere in the engine bay that says the gas type and quantity. Hopefully it’s r134a and not the old r12 as that’s hard to find. You really need a professional to do it properly. Probably going to cost you $300 - $400.