Anybody could have answered this question because the non-working/erratic tach is a common issue.
If it is not working at all first verify you have a tach signal present at the cluster connectors.
The tach signal is in AC voltage so be sure youyr digital voltmeter is set on the correct scale and it will read approx. 2.40 AC volts.
If the signal is present remove the cluster and reflow the solder joints on the circuit board.
Unfortunately if the tach still does not work there is no way to test the drive module or tach itself other than replacement.
My daytona did the same thing and to fix it I just gave the dash a good slap above the cluster and it started to work again, this happens like maybe a few times every 2-3 months, so far this slap trick has worked every time
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