The alternator diode works as a switch to open and close the circuit when the alternator is running or off.
When the alternator is spinning and producing voltage the alternator diode switch is supposed to close to allow the charging volts to go to the battery to supply power to the engine and battery.
Also the alternator diode and voltage regulator helps to convert AC current to DC that your vehicle requires.
When an alternator diode goes bad the most common symptom is a dead car battery after the car sits overnight or your engine runs a bit rough at idle.
If your cars battery goes dead overnight and you have not left anything on in the car then the alternator diode could be bad.
Also sometimes your headlights will flicker or your battery light will come on in the cars dash.
Low charging volts can also be a symptom of a bad alternator diode.
An alternator can still be bad even if the alternator tests good.
A vehicles alternator can still have an output charge of 14 volts but still the alternator may be bad and have a short in it.
I've had an alternator that tested good and put out the proper charging voltage and it was charging the battery but it also had a short in the diode since the diode went bad.
The bad alternator diode was causing a short in the electrical system and causing my alternators gauge to drop down.
The engine would start fine, then start running rough especially at idle and I tested the alternator with a Voltmeter and was getting the 14.4 Volts in normally puts out.
But the alternator was not supplying enough electricity to keep the engine running properly.
I've also had a bad alternator cause the battery to go dead overnight.