Most aircon systems need full pressure to activate the high pressure switch, this sends a signal to the system that refrigerant is present in the system.
If the sensors indicate low pressure the compressor engages but the rest of the system stops working to protect its self from damage as there's probably no oil in the system.
Check the refrigerant is topped up by a reputable aircon service engineer.
If the fan don't run on the condenser rad there will be no airconditioning even if the pressures ok.
Make sure there's a signal to the fan from the resistor pack (common fault).
Last edited by Alfa 156 JTD; 12-03-11 at 17:34.
Reason: Just remembered