AutoTips   Alternator Page

Alternator on car   11,589 bytesAlternator cut away view   altrvu1.jpg   10,925 bytes    
Alternator mounted on engine and cut away view.

What it does:
Supply current to charge the battery and run the electrical components of a vehicle.

Typical Failures: Low or no output. Failures can be due to:

  • The internal regulator (common on many models), faulty diodes, or worn brushes.  
  • A worn belt can slip and cause low output on the alternator. Low alternator output can cause the battery not to fully charge, especially when driving with lights, and other accessories, such as air conditioning.
  • Bad connections between the alternator and battery, corroded battery terminals, poor ground connections.
  • Quick Troubleshooting if you suspect the alternator has low output.

  • Inspect the belt with the engine off for cracks or glazing. Check for a snug fit and adjust as specified in the service manual. Some cars may have a tag under the hood. Many cars have spring loaded pulleys, and no manual adjustment of tension is required.
  • If the belt checks ok, test the battery voltage with the engine at idle, and accessories off. It should be around 13.8 to 15 volts. Also check the battery terminal connections. If the voltage is in the proper range, turn on the headlights, to add some load. The voltage should not drop below about 12.8 to 13 volts with the engine at idle. If it does, you may have a problem with the alternator. Many auto parts stores have an alternator tester that can do a more complete test.
  • When you go to buy a replacement:
    Alternators are rated for a current output capacity (such as 50 amps or 90 amps ) at a specific RPM , and physical size (normally by car model and year). If you are fortunate enough to have a factory service manual, you may be able to find a table of specifications that indicates the proper rating for your vehicle. Sometimes you may find a rating on the alternator. In most cases, the auto parts store will have a listing for your car.

    Pulley: Because the same alternator is used on many models, the pulley on the alternator at the store listed for your car may not match the one on the current alternator. Pulleys are not always easy to remove, however many of the parts stores have the proper tools and can swap pulleys for you if you take your old alternator in.

    How it works: A current through the rotor supplied by the battery generates a magnetic field as the alternator spins. The magnetic field cuts through coils of wire and generates a current. The regulator monitors the battery voltage, and varies the current that generates the magnetic field, thus regulating the current that is charging the battery. As the voltage of the battery approaches full charge, the regulator cuts back on the output of the alternator. An alternator can not start charging with a 100% dead battery, although once going, it sometimes can generate an output if the battery is removed. Never try this, the voltage from the alternator can rise, and damage the vehicle's electrical system.

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