The 85 tach has different connections on the tachometer itself, as it gets a signal from an intermediate computer.

The 85 still has the pickup for the hall effect sensor, so that a mechanic can run a separate tach from there.

Youll need a true tach amplifier (might as well buy new, most older ones can be flaky) and a tach from an older w123.

I did this the opposite way - to get a working tach on an 85 300D that had an older engine swapped in and had all messed up electrical parts. Worked perfect.

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