The AC voltage measured is dependant on the duty cycle of the signal. Bert says 1.5VAC - 4VAC because the duty cycle at idle is less than that at say 2000-3000 RPM. The signal you are trying to measure looks like this (this is probed directly at the primary winding of the Cyl.1 coil on a K75):
As the RPM increases, those spikes get closer together, causing a rise in true RMS volts read by the multimeter. A reading of 0.3VAC indicates to me that the pulses, if any, are coming intermittently. My guess is the HES is not reliably telling the ICU when to generate the primary coil pulses; it points to the HES itself or the wiring. Your later posts indicate a malfunctioning HES. Using an oscilloscope to measure the ICU signals on Pins 9, 12 and 14 would be the best way to confirm your diagnosis.
When testing the HES, have you tried the trick of passing a feeler gauge through the gap? That will activate the hall effect and show you signals on the voltmeter that vary slowly, allowing the LED/meter to respond properly. The procedure is illustrated
in this file.
As for polarity, I believe the HES is polarity sensitive. I believe the magnetic field would not be generated in the correct direction if the supply is reversed.
