If you have the 722.1 and older transmission in your diesel, you'll always start off in 2nd gear unless you hit the kickdown switch. If you have the 722.3 and newer transmission in your diesel, you start off in 1st gear, all the time. The last year for the 2nd gear start in the diesels was 1980.

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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.