Could be a bad compressor like Crowz posted or you could be low on refrigerant. In order to diagnose AC problems, you need a good set of manifold gauges. What were the low side reading, the high side reading, and the ambient air temp? Aux fan kicks on when the high side reading exceeds a certain psi. When the Aux fan is running it cools the refrigerant in the condensor which in turn lower the high side reading. Once the high side reading reaches a lower psi the fan kicks off.

__________________
2006 Infiniti G35
2001 BMW 3.0I E53 X5 Build date 08/2000 SOLD
Lotus Europa 1970 Destroyed by fire
Lotus Europa 1970 S2 Renault Powered
Lotus Type 52 1970 Twincam Webers Powered
PORSCHE 911 Targa 1982 The Garage Queen
Audi Avant donated to Kars for Kids
BMW 525IT Sold
Audi 4000CS Quattro Sold
Jensen Healey Lotus Powered Sold
Opel 1900 Sold
Triumph Spitfire 1971 Sold
Triumph Spitfire 1968 Sold
Plymouth "Cuda" 340 Six pack SOLD