Alternator noise

I have found that when an alternator starts to whine, it has lost one of its six diodes. This causes a hole in the normally smooth output waveform. There are six diodes each responsible for conducting one half of each of the three phases. You can read about diodes call rectifier diode as resposible for this.

The regulator will compensate for the loss of output so it appears to be working normally except for the whine.

 

I wouldnt hold that as 100%...
Unless there are a ton of bad alt in new and old cars....Sometimes i pick up an alt from a car or another truck and times i dont....So there is more to it than a simple diode gone