http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1D00YU6RY1KL0SZM8T&year=1991&make=MB&model=300-TE4M-001&category=P&part=Water+Temperature+Sender

If it is hooked up, take an ohm reading to ground