The alternator functions as a generator that powers the electrical system and all of the electronics in your vehicle when the engine is running. It also serves to recharge the battery. The alternator is mounted on your engine and is driven by a serpentine belt that turns its drive pulley. Whenever your engine is running and the crankshaft is spinning, the serpentine belt (moved by the crankshaft pulley) runs the alternator.
A built-in voltage regulator and rectifier ensure that the electric current remains constant and correct to operate your vehicle’s various systems. Although an auto electrical system runs on DC power, and an alternator generates AC power, the rectifier diodes within the alternator convert AC to DC. Without the alternator, your battery would quickly wear down from the current draw of all your vehicle’s electrical needs.
If your alternator fails when you are driving, a dashboard warning light (usually shaped like a battery or with the letters “ALT”) will come on. Not long after the warning light appears, the electric components of your vehicle will begin to lose power - the headlights, interior lights, audio system, and the like. The battery will try to keep them working, but it simply does not have enough reserve power to run everything for long. Eventually, your vehicle will stall because the engine needs the alternator to operate. Jumpstarting your vehicle will not correct the problem.
Other signs of a bad alternator that happen if the internal pulley shaft bearing fails include grinding or whining noises that rise and fall with changes in engine speed. You might also notice advance signs of a bad alternator if your engine turns over slowly when starting, as if the battery is weak. A weak battery can be a sign of a bad alternator. A technician can also test the alternator to determine whether or not it is putting out the proper voltage.