What year/model are you driving? This is for the 1976-1985 W123 diesels, and can be applied to other models as well.
http://www.peachparts.com/shopforum/showthread.php?t=121294&highlight=W123+service
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.