What Are the Causes of Alternator Failure?


Automotive alternating generator systems provide electrical power to the primary ignition system and the secondary accessory systems through a negative grounded battery. The alternator is an improvement on previous generator systems and is designed to put out up to 15 amps of power under a full load. Several components of an alternator can wear out and fail, usually requiring replacement of the entire unit as most alternators are not user serviceable. The average backyard mechanic can replace and alternator in about 1 hour.

Test the Output

  • When an alternator wears out, it does so in stages. An alternator produces power by rotating three plates across a winding of copper wires. These plates can fail one at a time and reduce, but not stop, the power output. As more plates fail, the power drops to levels that will eventually cause the battery to die and the ignition system to fail. Testing the output regularly with a voltage tester while the car is running can give indications that these plates, or other components, are wearing out. Most automotive parts stores will test alternators and batteries for free, as they can then sell parts to the owner. If the voltage drops below 12 volts, or the amperage falls below 11 amps when under a load, then the alternator may be failing.

Check the Battery

  • Many times, the alternator will fail from being "stressed" by a dead or dying battery. The duty cycle of the average alternator will charge a battery while the car is running, and accessories like an air conditioner or lighting system will not give the alternator a break. Charging at maximum capacity for days or even weeks can wear out an alternator quickly; the unit tries to recharge a battery that acts like a massive load instead of an energy bank, using up the alternator's lifespan in a short amount of time. The battery will have a charge of about 12 volts at rest, and could exceed 14 volts while charging. This could be reduced by accessories, but the voltage should not fall below 12 volts under normal usage (except when starting the vehicle).

Listening for Sounds

  • Inside the alternator is a rotor that spins on a set of ball bearings. As these bearing become worn, they can "catch" as they develop flat spots. This causes a horrible racket and can become a sustained grinding noise. The sound can be so loud that it drowns out the sound of the engine running or even the car's stereo system. The only remedy for the typical owner is to replace the alternator. Alternator rebuilding companies will replace these bearings, but they are not user serviceable.

Losing Ground

  • The primary mount of the alternator serves as the negative ground and usually the mount will be wired to the vehicle's frame with a large cable. This cable is held onto the mount with one bolt, and it is known to become loose or disconnected. When the alternator loses its ground, it can become damaged as it surges and could cause catastrophic damage to the voltage regulator within it. Most alternators manufactured since the 1980s have internal voltage regulators, which can vary in design. If the electronics are of a cheap make, the voltage regulator can overcharge the battery or even destroy the electrical system of the car from extreme power surges. Tighten the ground wire or braided strap to the alternator mount, and check it often.

Belt Slippage

  • A worn or damaged drive belt can slip on the alternator's pulley wheel, causing the unit to put out a reduced voltage. This is often accompanied by a loud squealing sound, but it can be faint and hardly noticeable. A slipping belt can make the alternator undercharge the system, which will eventually kill the battery and damage the alternator.


  • Photo Credit www.rallydesign.co.uk
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