Okay, apparently the voltage regulating zener should be 12V.
When 12V is applied to the black(0V) and brown(12V) lines, I measure a 12V across white and black.
When the fan starts spinning, I record 6VAC across white and black.
SO when the fan doesn't spin,
12VDC is not able to pass through the capacitor, so NPN does not allow CE current to flow.
When fan spins, AC passes through the capacitor to allow transistor to conduct.
I have no idea what is going on inside the pulse generator
EDIT:
Wait, if I rotate the fan motor just a notch when stationary, it presents either 12VDC or 0VDC across white and black.
Blueskull mentioned this earlier:
1 of the 3 pins of the motor is a sense wire, it should be connected in the commutator somewhere in the motor. As the shaft rotates, it generates pulses. When it stops, it will stuck at either Vcc or 0V. That's why there is a capacitor between it and the NPN, it only allows pulse (AC) to pass, so when the motor stops, it won't turn on the NPN, so the LET won't lit.
EDIT:
The plot thickens. Got my hands on 2 more identical units of this fan hiding deep in my storeroom, one with the problem of fluctuating LED voltages and one without. I swapped the motor detector between units, but the problem persisted in the faulty unit and did not appear in the okay unit. (see video below)
https://youtu.be/WrqvayIwkhkThe faulty unit has a slower sounding motor. That's all I could discern, both regulate 230VAC to 12V in the motor detector and both motors are identical in the way the pulse generator behaves.
Could a slow or maybe irregular RPM be causing this fluctuation?
If the DC voltage is regulated at 12V, how am I observing spikes of voltage up to 20VDC?
Could such occasional short spikes be the reason why the LED dies all the time?
Ben