advanced auto parts stores can check it for you
alternator is bad
water pump or seal gone bad
Typically, a bad or "going bad" alternator.
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The alternator went bad. You need a new one
You have a bad alternator.
Turn car on remove positive battery terminal if car turns off alternator is bad
When your battery does not get charge.
alternator on a 2000 Pontiac runs off a serpentine belt if its loose then chances are ur serpentine belt is worn or stretched or your tensioner pulley is bad or the tensioner itself..alternator bolts stable does not move like the old setup.
Typically you know your alternator is out when you have to jump start your car numerous times. A way you can test it is by turning on your car and while it's running discount the + terminal. If the car turns off then your alternator is bad. If the car remains in then alternator is good and possible bad battery. Hope this helps
If it is makeing a noise or is not putting out at least 13.5 volts it is bad. Take the car or the alternator to an Auto Parts store and have them load test the alternator to be sure.