That is your voltage regulator.
Order a new one, they are cheap.

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1983 Mercedes Benz 300SD
Part: Voltage Regulator
F4010-39368 Voltage Regulator Bosch IN STOCK $33.26

F4010-108169 Voltage Regulator Germany IN STOCK $25.25

http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1OI103FLR1OX03KERU&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1983&product=F4010-30110&application=000272329

__________________
ASE Master Mechanic
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Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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