Transportation Science

How do I know if my car's alternator is failing?
Answered by Science Channel
  • Science Channel

    Science Channel

  1. The alternator supplies electricity to a car battery using power from the engine; this allows the battery to power the car's electrical components while recharging itself. The car relies on the alternator, because without it the battery is going to live a very short life. An alternator's own lifespan depends on many factors, such as the engine, the conditions the car is used in and how much electrical equipment it runs. Sometimes alternators can die after just 40,000 miles (65,000 kilometers) or well after 100,000 miles (160,000 km). When an alternator is dying, the electrical components may lose power after the car idles for a while. If the battery is frequently dying, or not charging while the car runs, it's probably because of the alternator.

    To check whether it's the alternator or the battery that's malfunctioning, you can use a voltmeter to read the battery output while turning on and off various electrical components such as the headlights and radio. If the voltage on the battery dips lower than 12 volts at any time during testing, and the reading from the alternator terminal is similarly low, it's probably time for a new alternator. It's often best to have cars repaired by professionals, but if you have the know-how and tools, replacing a bad alternator yourself can save you some money. In general, it's just a matter of locating the part, detaching it from the engine, disconnecting its wire connections and replacing it with a new one. Be sure that the engine has cooled before you begin any repair work. And whenever you're working with a car's electrical components, remember to first disconnect the battery leads to avoid shocking yourself.

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