Very likely low refrigerant regardless of observed low and high-side pressures.
I know nothing about your system - but a plugged orifice tube - or a plugged screen on your expansion valve can do it. So can too much air - or an overfill of system oil.
Often an AC system that blows cool at first and then gets warm has too much air in the system. Or . . . not enough refrigerant.
I've got one here that sort of defies reason - at least it defies my reason.
98 Dodge Grand Caravan. Just put a new compressor in it, evacuated for an hour at 29 inches, and then put in the exact OEM charge - i.e. 34 ounces. When done - it blew cold air when first started and then gradually got warmer the longer the engine ran. Did it every time I started it.
Checked all pressures against ambient temps and all checked perfect. I then went on to further checks - with a thermocoupler on the discharge line. When you compare line temps to low and high side pressures it is supposed to tell you if charge is too low or high. Again . . . all checked perfect.
So - I then brought it to an AC shop and the guy told me my orifice tube is plugged. Well . . . my system does NOT have an orifice tube - so I decided I did not trust the "expert." My system uses an expansion valve.
Went home - added two more 12 ounce cans of refrigerant - and it took off and now works pretty good. Drove 100 miles yesterday - 86 degrees outside and the AC blew 50 degrees F for the whole trip. Maybe not perfect - but good enough for now. I checked my Ford F250 and it blew 42 degrees F out the ducts - but it IS a different system and also uses R12 and not 134A.
If someone knows why my system works when 24 ounces overfull - I'd like to hear some ideas. But for now - I'm leaving well enough alone. And by the way, the low and high side pressures barely changed with the added refrigerant.
I did wonder if some of my 10 year old cans of refrigerant were maybe, not full? So I weighed one full and empty. 15 ounces full and 3 ounces empty - so I guess that's not the problem.
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