Electronic tach

1 wire is the signal wire (often blue, or a striped red, or a green wire from the tach) and it attaches to the ignition coil terminal that the breaker points tie to (small wire to distributor).

1 wire ties to the ground (you likely have a negative ground, so it is probably a black wire (-) from the tach you want here)

1 wire goes to the battery (probably red (+))- ideally off of a lug on the ignition switch so the tach is not always powered when the engine is off- otherwise that same wire can tie to the ignition coil on the terminal that goes to the ignition switch.

I am not familiar with what Dat-com is trying to measure if they are trying to measure engine speed off of an accsesory that may have several choices for driven pulley diameter, several choves for drive pulley diameter, and the number of pulses per revolution of a given alternator or generator.
If the tach is measuring the waveform off of the charging system (not always accurate but can be done, it is only going to have 1 signal/source voltage wire and a ground (may have one lead for light bulb too)

If the coil is in right, there is probably a bad diode (most common) or other component in the tach.