Alternators are pretty rugged, but can succumb to excessive heat and overwork. They can also be damaged by sudden voltage overloads (as when someone attempts to jump start a dead battery and crosses up the jumper connections or if someone disconnects a battery cable from the battery while the engine is running).

Sometimes alternators can partially fail. In the back of every alternator is a "diode trio" that converts the alternators AC (alternating current) output to DC (direct current). If one or more of these diodes fail, the alternator's amperage output will be reduced. It may continue to produce some current, but not enough to keep the battery fully charged -- especially at idle or low speed.

Most service facilities have test equipment that can identify these kind of problems. So if you suspect a weak alternator, you should have it tested to see if it needs replacing. Most service facilities do not repair or rebuild alternators because it's too time consuming and requires special parts. Most will replace your old unit with a new or remanufactured unit. Your old alternator is usually traded in or exchanged for a credit (so it can be remanufactured and sold to someone else).

Caution: If you're replacing an alternator yourself, always disconnect the battery before unhooking the wiring on the alternator. This step will eliminate the possibility of accidentally shorting out a hot ware and damaging something or starting a fire.

The alternator drive belt should be inspected at this time, and replaced if it is cracked, oil soaked, glazed, badly worn or otherwise damaged. The belt should be adjusted for proper tension following the vehicle manufacturer's guidelines. Too much tension can overload the alternator's bearings and shorten the unit's life (as well as belt life), while too little tension may allow the belt to slip.

In older vehicles, the regulator was a separate component usually mounted somewhere in the engine compartment. If this type of regulator failed, it could be easily replaced in a matter of minutes with a new one. But for the last decade or more, most regulators have been mounted in or on the alternator itself. This was done by the vehicle manufacturers to simplify wiring and assembly. It was also made possible by advances in electronics that allowed the regulator to be reduced in size to a small chip.

Charging systems that have a separate regulator mounted away from the alternator are referred to as "externally regulated" charging systems while those that have the regulator in or on the alternator are called "internally regulated" charging systems. On some vehicles there is no regulator at all! Voltage regulation is controlled by the engine computer.

Unfortunately, internally regulated alternators are packaged as a unit -- which means that if either component fails (alternator or regulator) both must be replaced. This is because internal regulators are not available separately (at least not to the general public or the typical service facility). Electrical shops and remanufacturers who rebuild alternators can get them and can replace the regulator separately if that's all that's wrong with the unit -- but they'll usually charge you the same as if you bought a rebuilt alternator.

The truth is, the high cost of labor today has made it impractical for most service facilities to fool around trying to rebuild or repair components like alternators, starters, carburetors, front-wheel driveshafts, transmissions and even engines. It's faster, easier and usually cheaper to simply replace the old unit with a new or remanufactured one than to try to overhaul or fix it. Besides, most new and remanufactured parts come with a guarantee.