On that year chassis, there were several changes. But if you have the 4 prong, it is correct. ..here are the different ones for that year, according to serial#.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1W70PZW4N2080GHWOX&year=1991&make=MB&model=190-E-005&category=C&part=Water+Temp.+Sensor&appEngines=_any

The four pin should read the same ohms diagonally across from each other. The sesnor is actually 2 sensors in one casing and they are thermistor.
If one set of diag pins reads different that the other , the sensor is bad.
Do a search on 'OVP' relay........