The common signs of a bad automotive alternator include a weak battery, dimmed headlights, an odd or burning smell and a whining or grinding noise coming from the front of the vehicle. Alternators are electromechanical devices that power the electrical requirements of the vehicle and charge automotive batteries while the engine is in operation.
Alternator failures can be classified as electrical or mechanical in nature. Electronic components, such as the voltage regulator and rectifier, over or under supply the required voltage output to the mechanical parts of the alternator, and in some cases cause electrical failures. An electrical breakdown usually produces an odd burning smell and prompts the charge lamp on the vehicle's dashboard to warn drivers. Other common electrical failures include burnt copper windings, loose terminal connections, broken slip rings and depleted carbon brushes.
Since alternators are powered by automotive engines via fan belts connected to crankshafts, rotor bearing failures may arise from under- or over-tensioned fan belts. The rotor bearings can overheat and eventually fail. This type of mechanical failure commonly produces a whining or grinding sound.
Other common mechanical failures include broken mounting studs and broken casings. Mounting studs are susceptible to slight movement due to vibrations, and out of position mountings cause low power output and put stress on rotor bearings that lead to potential damage.