When Good Alternators Go Bad (and Why)

Every car has a battery to crank the engine, and power the fuel pump and ignition at startup, but then the alternator starts spinning and providing the power. Because of this, you can often jump start a car with a failing battery, then drive it to the auto parts store to get a new one. With the motor running, and the alternator putting out power, the battery doesn’t have much to do.

There are only a few ways an alternator can go bad, but they tend to happen suddenly without a lot of warning. Here are the common symptoms:

• Undercharging – This is the most common problem, and can often be caught in the early stages. If you notice your headlights get dim with the engine at idle and get bright when you rev it, chances are your alternator is not putting out enough power.

• Overcharging – This isn’t a problem with the alternator itself, but rather a problem with the voltage regulator (often built into it). An overcharging alternator can kill a battery and boil the fluid out of it.

• No Charge at All – This can appear just like an undercharge condition, but revving the motor does nothing. Get home or to a service station quick, as a modern car will only run a short time before the battery is dead.

• Mechanical Issues – This category of failure often just involves the bearings wearing out and squealing, or in extreme cases, seizing up. Besides not charging the battery, this can also throw the serpentine drive belt, causing overheating.

Like all rotating machines, the bearings in an alternator have a finite life and will eventually go bad. You can minimize the chances of this happening prematurely by keeping the drive belt and tensioner in good shape. Nothing will kill the bearings quicker than a too tight belt, but a belt that is too loose isn’t good either. If your car has a spring-loaded tensioner, you should at least inspect it at 50,000 miles when you change the belt. If you have an older car with a manual tensioner, be sure to tighten it to the specification found in your Haynes Manual.