I'm reading a car electronics textbook that claims that an alternator can maintain correct system voltage (say, 14 volts) when under load, yet not be outputting enough amperage. I can't understand how this is possible. He claims that's why it's important to check the current output even if the voltage is good. Wouldn't it be necessarily putting out enough amps in order to maintain 14 volts?
He is correct.
I tested alternators and sometimes one would have a phase down, so while it would show 14v, it would not output sufficient current.
This is why the correct testing procedures should be followed so that the “real” issue can be found.
Even following the posts on here, one can see that many have had a charging issue, jumped in and replaced the alternator as the voltage was low.. Then found they still have a problem and further testing shows a different issue... Sadly they paid for an alternator that was not needed.
So, do the correct tests.
Edit, you mention a textbook but don’t give any detail. However have a look at this link and it shows you a regulator circuit, older now, newer ones are even more complicated:
I'm not sure about the veracity of your textbook. There are some fundamental electrical laws that govern the current and voltage (and resistance).
Specifically Ohm's law: voltage= resistance * amperage.
If the voltage readings are accurate, in order to create a 14 volt potential across the battery terminals, there must be a certain amount of current proportional to the resistance of the circuit.
The alternator doesn't "allocate" current based on a whim. The current is a function of the difference in potential between battery voltage and alternator output voltage. The current is a result proportional to that voltage delta and the resistance of the charging circuit. The "hunger" of the battery ultimately determines the current, up to the capacity of the alternator. If the alternator cannot keep up with the charging demand, it would not be able to generate a high voltage potential, unless there is some sort of high resistance in the charging circuit - which would not be a fault of the alternator itself.
Voltage regulation, either internal or external to the alternator, determines the alternator output in current to the battery up to the limit of the alternator capacity. If the regulator is working properly, it will not yield a voltage that puts a current demand on the alternator it cannot handle.