I would suggest pulling another gauge from a diesel W126. I don't think the ones from the 85 and older diesel W126s will work, since the newer W126 diesels have little lights that give ABS warnings and low windshield washer fluid level warnings.

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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.