Adams' Apple said: It is most likely a problem with the circuit board, but before condemning it, check to make sure the tach wire is plugged into the distributor cap properly. A loose connection there can certainly kill the tach. A broken wire coming off of the coil could also be a possibility. You could remove the top coil cover and have a look-see to verify that.
Sometimes the connection for the board to the gauge panel can be loose/corroded, or otherwise faulty. You might try unplugging it, and plugging ti back it to see if it helps. If none of the above corrects the issue, then I'd have to go with the board taking a doo-doo.
Joel, would a loss of signal from the tach make it read less than zero? What I'm wondering is if I still have 12V shouldn't the tach at least read zero if there is no signal or am I missing something? I'll have to get my meter in there and check for voltage of course but is there a way of checking for signal with a meter?
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Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ