OVP is a relay...
...behind battery under black plastic rain cover.
It protects the cold start circut among other things
also suspect the water temp sensor...
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1FX0MB1RS1V30YBUYU&year=1988&make=MB&model=300-SEL-006&category=C&part=Water+Temp.+Sensor
...which is the primary imput to the cold start circut.
If the system is runniing fine when it is warmed up you might want to start with these components, lots have been written about them, use the search function:
http://www.peachparts.com/shopforum/search.php