Take it to an auto parts store and have them perform a bench test on it.
It checks many things, connections, belt, and voltage regulator. I have never checked one with a load test. On a modern car or truck, never ever disconnect the battery from the alternator with the motor running to check the alternator. It can burnup a lot of electronics instantly.
Charge it, replace it,test the alternator,Charge it, replace it,test the alternator,
perform a tube leak test perform a tube leak test perform a tube leak test perform a tube leak test perform a tube leak test perform a tube leak test perform a tube leak test
Developer perform the validation test
Car would either have to be running or alternator removed for bench test
run it and then disconnect the negative wire from the battery if alternator dies its bad.
with a voltmeter
Either drive to your local auto parts store and have them test it in the vehicle or you can remove the alternator and take it to your local auto parts store and they can bench test it for you.
Full fielding is a diagnostic test of the alternator You make the alternator work full out and measure the voltage and amperage output to test it against a published specification.
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Battery light usually means alternator not charging. Most common solutions would be be a bad alternator, bad connection, or loose alternator belt. If the battery and alt test ok than check the connections to the alternator and battery.