You dry charge the system first and that will give the system enough refrigerant for switch cut-in for compressor activation.

That sw should be a pressure sw , not a thermistor [ temp sw]

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU24Y0TB6T4&year=1992&make=MB&model=400-SE-001&category=R&part=A%2FC+Pressure+Switch