90 20V Quattro 1989 model Aircon

[whitehill]

Hi helpful crew.
I aircond has stop working.
I have diagnosed this far
Push the aircond button and the magnetic clutch does not pull in although I can hear a relay somewhere under the dash pulling in.
I pulled of the wire to the magnetic clutch and put 12 volts on it and it pulls in but and seems to run OK when I start the car but it will not cool - perhaps that it needs re-gassing.
Is there a sensor somewhere that detects low pressure in the compressor? 

And where do I find it?


Does the unit have to be modified to re-gas with ozone friendly refrigerant?

Thanks in anticipation
Rick