Federal '84's don't have that EGR computer. At least, mine doesn't and never had one either.
I think 85 federals do have it, which would make the federal '84's an oddball configuration as far as the tach goes.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)