Yes. It sounds like that switch is not opening as it should. Just to be sure, check for continuity through the switch with the car cold. If it has continuity, replace the switch.

If the switch looks like this, you can change it with no refrigerant loss:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23H0N2A6324F16YR7A&year=1979&make=MB&model=240-D-002&category=R&part=A%2FC+Temp.+Switch