This information is from www.AirCondition.com http://www.aircondition.com/tech/questions/56/Should-I-Retrofit-to-R134a%3F "Retrofitting, or converting to R-134a does not always require great expense. Sometimes, service port adapters and synthetic refrigerant oil are the only extra expense. Not all cars are created equal though. Some vehicles will require additional high side pressure switches, and other vehicles may require condenser replacement - or - condenser fan upgrade. Since R-134a is less efficient, sometimes these changes are necessary. This largely depends on how the vehicle was originally designed. A system that performed marginally with R-12 will probably require the most retrofit associated cost. Also, if the system didnt work at all with R-12, it certainly wont work with R-134a."
"I'm not the AC expert here but I think there are only 2 basic reasons that the compressor would cycle (shut off and on) . One is if you are low on freon which would trigger the pressure switch located near the dryer and the other would be if the thermostatic expansion valve is freezing up. You may have a bad or plugged valve " The compressor shuts off anytime the thermostat tells it that the car is cool enough. Actually, of course, it's not measuring the cars temp but the temp inside the plenum. The thermostat tells the compressor to shut off because the air in the plenum is sufficiently cold. The system on my 328 was recently checked/serviced and it is functioning correctly with a correct charge. So there are two obvious "solutions." One is to move more air so the plenum will not be cold enough to shut off the compressor but a lot more cold ir will be delivered to the compartment; the other is to adjust/modify the temp sensor so it will not shut off the compressor. On similar old style add-on units in US cars circa the early 60's (and the Ferrari's is an "add on" unit, despite how it appears), if the temp was turned to max cool, the compressor would never shut off. IMHO, that's what the car needs. Actually, I'm surprised that the temp control doesn't do that and I wonder if it's actually supposed to but is not functioning correctly...I'll check the owner's manual more closely. That being said, if the compressor is left on it is possible for the evaporator to freeze up and this was not uncommon. But it is easier to deal with that that because at least you can manually ensure maximum capability.
