84's might not have an EGR computer, in which case the wire goes from the pickup straight to the cluster. That's how mine is. No tach amp, no computer, no OVP.

http://www.peachparts.com/shopforum/diesel-discussion/376694-84-w123-no-tach-amp.html

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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)