Hey Gab - On "Miss Purdy" I've found that most gauge issues simply needed the edge connectors and/or the ribbon circuitry cleaned a bit. There are a couple of good Rennlist write-ups on "How To" clean the connector plugs/printed circuit traces for a 1984 instrument panel. Here's one -
https://rennlist.com/forums/928-forum/706384-instrument-cluster-and-central-warning-unit-with-color-wires.html
The tachometer signal originates from the Ignition Control Unit (passengers inner fender well - near radiator hose). Then to the sideways center black edge connector in the photo. Next it heads to the 3 brass terminal nuts (B+/Grd/Tach Input Signal - shown in the center of instrument panel). You might try very gently reaching up behind the instrument panel with a long screwdriver and "SLIGHTLY" moving the black connector plug. Make sure to disconnect the battery for safety and be careful not to pull the edge connector completely off. If you are a lucky duck,
re-seating the connector plug will be enough to reestablish your tach signal. If this doesn't help - the instrument pod may have to come out to clean/tighten the 3 brass nuts a bit.
Good Luck- Michael
