With points, rotating the engine will change the voltage as the coil is switched on/off. But with electronic ignition, it depends on how the unit works. The factory service manual for it will provide test procedures, example it's the Bosch/Bluebird EI unit


Quote:
1. Tacho wire disconnected from coil will read one thing
2. Tacho wire connected to coil will read the same as the coil terminal, which is various depending on how the module works. With engine off, perhaps Ground (briefly) followed by 12V via the coil winding (which will not repeat). With engine cranking, something like 9V average as it switches on/off