I have read numerous posts relating to high reading tacho, my 79 is the same. I purchased a new PCB, fitted it but made no difference, although it jumped round quite a bit more than the original so refitted the original. I have a new filter in there now, still doing the same. I thought I might check the tach with just the signal from the distributor and an external power and ground, however doing that, the needle zeroes fine, but the signal won't drive the gauge. Using the power and ground out of the dash and then it works fine again.
I'm a bit lost as to what to try next. Why can I not get the tach to run when it's physically removed from the cluster by using a separate power and ground signal?
I finally had the entire cluster assembly out of the dash today to clean all the connections to no avail. Could it be that the new PCB I purchased has the same fault as the original board? It had always worked fin until recently, not at 60mph it says I'm at about 6000rpm in fourth lol.
I also put a dwell/rpm meter on my distributor tach output and it showed the correct rpm, same as what I got using a light pickup on a crank pulley.
I checked the distributor and cluster points with a fluke, both reading similar frequency at idle.
I also tried giving the tach a square wave signal plus power and ground but got no reading, however the signal may be too low.