The following thread discusses a 470 ohm resistor that I believe was used to get an extra 50 or so RPMs. I believe this may have been applicable to your '89, although the thread may indicate otherwise.
http://www.peachparts.com/shopforum/showthread.php?t=5169&highlight=470+ohm
There's been talk in other threads about this resistor mod to bump rpms. Try the SEARCH function
The OVP is along the firewall, about midway across. Here's what it looks like. This device among other things provides power to the idle controller.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1EQ0PTV2S1KD0Z5JCU&year=1989&make=MB&model=300-SEL-006&category=P&part=Overload+Relay
Same for the conversion back to R-12. Use SEARCH facility. Many discussions. Larry Bible has had much to say on this subject.