HELP

I have a 2000 Focus TDDI LX. when I switch on the AIR CON, the little green light comes on but nothing happens. I have found the relevant relay in the fuse box and it does not appear to be energised. If I remove the relay and short out the relevant terminals, I can hear the compressor solenoid engage but still the air from the vents is not cold. Does anybody know if there is an interlock which prevents the compressor engaging if there is low gas pressure?

Any suggestions??