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:
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.
If you notice an acidic smell or your battery is leaking fluid, it may be because the alternator is overcharging. You can easily check by measuring the voltage at the battery with the car running, which should not go above 15 volts even with the engine revving. First, check that all the wires to the alternator and regulator are connected properly and not broken. If your car uses a voltage regulator, either separate or built into the alternator, it is inexpensive and easy to change. If you have a car where the voltage is regulated by the ECU, you may need to talk to an expert technician.
An alternator that won’t put out at least 13.8 volts is in need of replacement or at least a rebuild. But, if the low output is combined with squealing noises, it may just be a case of a slipping serpentine belt. If the belt is good, low output can be caused by the insulation breaking down on the internal alternator windings, or it may just be worn brushes; either way, the unit needs to come off and come apart.
Finally, if there is no charge at all, there are several possibilities. It is possible that the drive belt has broken or come off the pulley, or the tensioner went bad completely. Alternators put out alternating current, and the battery needs direct current to charge, so there is a voltage rectifier that converts it, and it can suddenly go bad, too. A faulty voltage regulator can also stop an alternator from charging at all. Otherwise, a no charge condition can mean an internal break in the windings, bad brushes, or burned out slip rings. If all the external wiring seems good, it is likely an issue inside the alternator.
The single best thing you can do to keep your alternator working well is to keep it clean and dry. Dust and dirt can get into the alternator, and over time cause shorts or wear on the brushes. The wire windings of the alternator only have a thin insulation on them and won’t produce a magnetic field if it fails, so avoid getting solvent on them, like carb cleaner or gasoline. Heat and oil can also shorten the life of an alternator, so fix under hood leaks as soon as possible.