Tach amp and tach needs power to operate. Maybe fuse 3 supplies power to it?

Jumpy tach can be pickup gap too wide (try adjusting it to a narrow gap), or a flaky tach amp.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked