Rahulio,
Have you tested the coolant temp sensor?
It is the last sensor on top of the valve cover,back of the engine compartment.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1FX0MB1RS1GB0YO1R3&year=1989&make=MB&model=300-E-002&category=C&part=Water+Temp.+Sensor&appEngines=_any
You will need to buy a multimeter to troubleshoot this.
Only costs about $30.00 for a pretty good one.
To be most usefull for your car you will want to get one that has %duty setting, Ohm's setting, and 20Amps setting, as well as the usual Volts.
Marty