Alternators keep the car battery charged, and run all the electronics onboard the vehicle. A propperly operating alternator puts out between 13.5 - 14.5 volts. Anything more or less is a malfunction, and requires immediate attention. If an alternator quits charging, the battery / alt light will come on, and eventually the engine will stop, leaving the driver stranded.

To test an alternator, a special tool is required, and not only a volt meter. Replacing an alternator is different in all vehicles. As of for some vehicles, the alternator is ontop and infront of the engine with easy access to replace, while other vehicles are in a hard to reach place, and require several hours of labor, and possibly having to drop the frame from a car just to access the alternator.