a/c needs your help

[mr benz]

i own a 190 e 2.3 1987. Recently it began to blow warm air, out of the vents, sometimes even hot air.
My question is the compressor comes on but during the peak hours on the hottest days the compressor doesnt surge. therefore, it feels like im sitting in a sauna. No cold air whatsoever.
Now at the dealer i was told that icould be the pita valve, since there is no mono valve on a 190 e 2.3 1987. correct?
Or it could be an evapaorator sensor. Im not fimiliar with either part but i was wondering what u guys think?
Onother opin said it could be that i need more freon. But i jus added three cans last week, which blew extremely cold while i was adding. but blew hot air the next day when it was hot.
Another was that it could be a pressure switch.
And still another was it could possibly be a leak.

At this time im kinda confused.
Any help at all will be appreciated, since i dont want to spend money wildly.

thanks

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