Okay on my 1990 CRX-Si (D16a)My tach ticks erratically (sometimes at 0-1 while acctually at 3-4)(usually at highway speed) and does so randomly, then kicks back to normal.) What all does the tach run off of? Electronic of the Crank Speed sensor? If so what all should I look at, I've never delt with tachs before because none of my other cars had one. Could this be electrical, or is there a manual linkage somewhere in there (like the speedometer runs off the lil gear in the tranny)

Appreciate any help thanks all.