Volkswagen Repair: A/C Air Flow Problems, thermal expansion valve, vacuum hoses

Question
I have a 2001 Passat GLX AWD wagon with 2.8 engine; problem is with the Climatronic system - when in automatic mode irregardless of temp setting, the A/C will run for a short time then starts this cycling process whereupon the air stops flowing through the dash vents yet the fan is still running.  The only way I can can get cold air to flow is to lower the temp all the way down to LO position.  Am not sure if this is a temp sensor or vacuum line problem.  Thanks in advance for assistance w/ this.

Answer
Hi Don;
There is a "manifold" of vacuum hoses that opens, and closes the "doors" to redirect air flow.  When the "LO" temp control is reached the system automatically engages "Recirculate."  Since the system in your car flows air to the main vents in "Recirculate" only, there is either a hose that is disconnected from the manifold, which is highly unlikely, but certainly possible, OR the system is slightly low on refrigerant.
I honestly think it's due to low refrigerant.  When the system recirculates, it's easier on itself, because it is re-cooling air that has already been cooled, and it "knows" this because it has been programmed to recognize the difference between outside air, and inside air.  The "low pressure switch" allows the system to continue to run because the "thermal expansion valve," "TXV" is closed, or as closed as it can get, and cooling takes place more efficiently, and the refrigerant "backs up" slightly at the TXV and maintains pressure at the low pressure switch (this switch is actually a combination high and low pressure switch), which keeps the compressor running for a longer period of time.  When the system is in another setting, or mode the pressure at the combination switch drops quickly because the TXV is slightly more open, and allows the remaining refrigerant in the system to pass quickly through which triggers a low pressure signal to the control module which then cycles the compressor rapidly, and opens the defroster doors, in an effort to keep the windshield clear for safer driving.
Since the 2008 model Volkswagens are beginning their release at the end of this month, your car, for all intents and purposes is now 7 years old, and refrigerant will escape over that period of time.
Not being able to place a manifold, and gauge set on your car personally to see what the actual pressures are, that is the best diagnosis I can give "at a distance."  Hope this helps, and your A/C runs again soon.  Good luck, Don.