An alternator works together with the battery and a voltage regulator to provide power to the gadgets and gizmos in your car, such as the interior and exterior lights, the windshield wipers and the radio. The alternator is powered by the drive belt, and when it begins to break, it may exhibit symptoms similar to a dying car battery - the lights will dim, the radio will cut out and other electronics in the car will stop working as well.
When the alternator begins to fail, it can also ruin the battery if it is not repaired or replaced quickly. The alternator takes energy generated by the pistons and converts it into electricity, feeding it into the battery. A faulty alternator will strain the battery, and all of the draining and recharging can drastically shorten its life.
A clear sign that the alternator is in trouble is a light illuminating on the car's dash, which may read "ALT" or "GEN." The owner's manual will explain all of the different warning symbols on the dash. Another tell of a bad alternator is if the electronics begin to work poorly or stop working all together.
Use your senses to detect problems with the alternator. Listen for low, rumbling and grinding sounds that could be warnings of impending alternator death. This noise may mean that the alternator pulley is not properly aligned with the belt or the bearings or bushings that support the pulley are wearing out. Even if they do not indicate an issue with the alternator, unusual sounds coming from the engine are generally worth investigating anyway.
Do you smell burning rubber, even though you haven't been peeling out of any parking lots lately? That scent could be coming from an engine belt heating up due to an out-of-tune pulley.
If you notice these things, or suspect that the alternator may be on the fritz, you can pop the hood and do a visual inspection of the belt. If it's hanging off the engine block or it's so loose that you could probably just pull the whole thing out on the spot, there is definitely a problem, which may be caused by a faulty alternator.
Tension is important as a belt that is too tight or too loose, even if it may not be dangling from the engine, will be unable to properly run the alternator. You should also inspect the belt for wear and damages such as cracks and tears.