I thought I'd post up on this thread on some issues I've been having with my AC in an attempt to keep the AC conversation in one place.
I drive the same route to work every day. Weather is about the only changes the car undergoes between the air conditioning working and it not working. I have linked the air conditioning problems to what seems to be the outside ambient temperature.
During highway driving the air conditioning is cold and works well. Occasionally there is a quick and short lived "warm up" in the air coming out of the vents. I think this may be an attempt for the automatic AC controller attempting to maintain the set temperature. This behavior was pretty consistent for temperatures below 90 degrees Farenheit. Just to be sure that it was controller related and not refrigerant related I bought a can of r134a fitted with a pressure gauge and 'topped off' the system. After closing the valve and reading the pressure gauge it was evident I got a little too much refrigerant in the system.
The system, running while filling the refrigerant, started to cycle the compressor on and off 1 second at a time. I then bled some of the pressure off a bit until the compressor ran in normal cycle times.
The air conditioning system seemed to work normally throughout temperatures up to 90 degrees. Things changed, however, when the temperature outside crept up over 90 degrees. During the same highway driving the air conditioning would "drop out" after running well for 10-15 minutes. The compressor would start the rapid cycling at the same time the air conditioning went into this mode. Air temperatures out of the vents become warm and humid in this mode. Occasionally the AC button can be cycled and I may get lucky for a short time with some cold air. Eventually it goes back to warm and humid.
This past week we experienced above normal temps near 100 degrees. I again purged some of the refrigerant to lower the pressure a bit more. I didn't notice a big difference in the behavior of the system. I can still get cold air for around 10-15 minutes of the drive and then the car goes to the quick cycle and warm humid air out the vents.
This may be a case of still having too much refrigerant in the system but I also am thinking a possible faulty pressure switch. I consider the switch because at idle the air isn't as cold as when the car is at operating rpm.
I apologize for the long winded post but I wanted to throw out some of my experiences that may help others chase their symptoms to a solution. I don't really have a lot to offer on the system since I'm not a HVAC guy. I can say that I don't have a leak in the system although this thread has seemed to prove this as the catch all answer.