Nissan Repair: air conditioning, nissan altima, compressor cycles

Question
Hi Von.  I have a 2000 Nissan Altima. My a/c is switching from back & forth from cold air to warm air.  I took it to the shop and they replaced:scrader valve, o-rings, dryer, triple pressure sensor, and charged the freon.  After $650 in "repairs", the same problem still persists.  Any ideas what it could be??

Thank you for for your help!!

Answer
CC,

I assume that means the car is blowing cold air followed by ambient air and back again.  If you have the computer control it could be the blend door moving back and forth?  If not, I would be looking at a system that is over charged and that could cause the evaporator to freeze over and then thaw.  That would cause the condition you discuss.  Another cause would be the overpressure causing the compressor to turn off to keep the system from blowing a hose.  You can check the latter by opening the hood and seeing if the compressor cycles when the car blows warm air.  Email me back with what you find.