The dwell on one coil is anywhere from 100 to 120 degrees (of 360). That is a 28 to 33 percent duty cycle. That means the coil's negative wire will be near ground, roughly, about 30% of the time. If the coil's negative wire is at, or near, battery voltage the rest of the time, then the average voltage on the coil's negative wire will be about 70% of the battery voltage.
If the battery is 13v, then the coil's negative wire will be somewhere near 9v.
At idle, the dwell is a little shorter, so the voltage on the coil will be slightly closer to the battery voltage at idle (but the battery voltage may be lower at idle).
Also, the igniter doesn't quite pull the voltage down to zero on the coil, it's closer to 1v, so the average voltage will be slightly higher.
The range you measured is exactly what would be expected 8 to 10v.
Can't say why the tach isn't working. Are you supplying it with 12v and ground, in addition to the coil-signal wire?
