The 2 wire blue sensor . It will not come on until 105C.
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42AR0R5UPD&year=1991&make=MB&model=300-E-002&category=R&part=A%2FC+ACC+Temp+Sensor&appEngines=_any
If you are exceeding 105C, then your problem is most likely the fuse on top of the relay behind fuse box.

__________________
A Dalton