How and why do alternators fail?

A dismantled car alternator on a cardboard sheet

Many alternators outlive the cars they are fitted to. However, in the real world, they can, and do fail for a variety of reasons. Some of these reasons are listed below-

Bearing failure
Alternator bearings usually fail for one of three reasons, these reasons being-

  • Excessive tension on the drive belt, which overloads the bearings
  • Improper installation of bearings during repairs or rebuilding
  • The use of cheap, substandard bearings during repairs or rebuilding

Rectifier failure
Rectifiers in alternators use three sets of diodes to convert the alternating current the alternator generates to direct current. However, diodes fail only rarely.

In practice, rectifiers usually fail only because of poor contact between any diode and the structure of the rectifier. These joints are usually soldered, but excessive vibration and/or poor assembly can sometimes break the joints loose. The only reliable remedy for this condition is the replacement of the rectifier.

Voltage regulator failure
Conventional voltage regulators contain brushes that rub against a slip ring on the rotating rotor. Therefore, most voltage regulator failures occur when the brushes wear out and lose contact with the slip ring.  In many cases, a voltage regulator can be replaced without dismantling the alternator.

NOTE: Smart alternators are controlled by a computer. This means that if the voltage regulator fails, you may have to replace the computer or control module. This procedure often calls for the programming or re-programming of one or more control modules with advanced diagnostic equipment.

Short circuits
Short circuits can occur in the windings of the rotor or the stator. Sometimes, a short circuit can occur in other internal wiring, but this does not happen often on high-quality alternators. However, they are a common cause of failures on cheap aftermarket or rebuilt units.

As a practical matter, it is not possible to predict how long any given alternator will or should last. Nonetheless, new, high-quality versions should last for at least 150,000 km.

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