There are two sets of diodes in a classic alternator.
- Rectifier diodes to make DC current from the AS of the alternator.
- Zener diodes to limit the voltage.
Zener diodes start conducting electricity at a specific voltage, for alternators 13,5 of 14V are most common. Above this voltage they short the field of the alternator and the voltage drops.
It is a very simple but effective way of protection against over voltage.
To test them you need something more sophisticated than a multimeter.
With an oscilloscope you van see malfunctions like voltage drops (Rectifier diodes) and spikes (Zener diodes). A bad Zener can easily destroy you batteries and fry your electronics.
Modern alternators are more complex, the voltage is limited by external electronics allowing for changing the charging voltage, AGM batteries can be charged with a higher voltage but are very sensitive to high temperatures and overcharging.
It is easy to see if you have a classic alternator (One big wire for charging and a small one for the charging light) or a modern one (more wires).
Other common problems are:
- A short in one of the coils
- Worn brushes
- Worn cable or connector in the charging wire.
To test an alternator you will need a testing bench where the alternator can be tested at full capacity. Alternators and batteries can generate a lot of current, so unless you want to risk setting your car on fire of fry your electronics do not try to test it in your car.
So if your only tool is a multimeter I suggest to take your alternator to a specialist with the proper tools.
Rob