2000 D2, when cold the Tach is jumpy, does it less as it warms up but still will do it. Cant tell if its power surging (truck is heavy and slow lol) but Alternator is charging between 13.2-13.7v. Does the tach get a signal from a Crank sensor?
The tacho reading i based on crank sensor signal but not directly, it's managed by the ECU and IMO if the crank signal is erratic it would affect bad engine running, also there should be a crank signal related fault code logged too... or if you plug in a tester read the rpm with it cos if it's not steady the tacho is ok but the engine speed is not
first of all give some contact spray in connector C0047