77 ohms says the mag pickup is ok. You bought a new tach amp so assume that is OK. I'd try cleaning all contacts at the tach amp and also crimp the female contacts so they fit tighter on the pins. Also try a known good tach.

If you connect a DVM set on AC volts on the lowest (milli volt) scale across tach amp pins 2 (gnd) and pin 3 (output of tach amp to the tach) and run the engine, you should read a small voltage there that is proportional to engine (rises with) RPM. If you get that, the tach amp and pickup is working and the problem is wiring to the tach or the tach in the cluster itself. Check fuses for the cluster and cluster grounds before trying another cluster tach.

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