Lot's can happen with a 40 year old tach. Components (especially capacitors) can degrade, solder joints fail (bad tach amp soldering job from the factory), potted component gets hot and burn out (power input resistor).

From your description, it could be too large a gap between the round tooth on the harmonic balancer and the magnetic pickup. Try making that gap smaller (make sure they don't collide), which will increase the output voltage of the pickup a little and see if that fixes it.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked