took my car in to be serviced for a/c today and the guy told me that I need a new compressor because the pressure was too high from the compressor on the high side? I noticed that my a/c isn't as cold during the 110 degree weather here, but when I'm driving at 40 mph + it seems ok or even when it cools off a little the a/c seems great. I don't know too much about a/c systems so could this be true? bHe said I only had about 12 ounces of freon when I came in to him, he tried to recover and recharge the system but said the pressure was too high. I think he said he added 4 more ounces before I left which made the car a/c worse so I took it back and he said he would put it back down to 12 ounces since I said the a/c was cooler than when he added the 4 ounces. Can someone please help elaborate on this a/c system thing and how it works?

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1990 190E 3.0L