How to diagnose a car alternator problem

Written by ehow contributor
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Your car's alternator is what powers your vehicle. If it malfunctions, your car won't run. A faulty alternator, however, will show signs of its impending demise. Recognizing those indicators can prevent you from being stranded. Follow these steps to diagnose a car alternator problem.

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Things you need

  • Voltmeter
  • Tubing or heater hose

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  1. 1

    Verify that the "alternator" bulb on the console panel is working. The bulb should light up when the engine starts. Failure to do so may indicate an alternator problem.

  2. 2

    Check for worn belts that may cause low output on the part of your alternator. This prevents the battery from fully charging, particularly when headlights or air conditioning are used. Low or no output may also be from bad alternator/battery connections, battery terminals or poor ground connections.

  3. 3

    Examine the alternator belt for cracks or glazing when the engine is cold. Adjust any looseness in the belt per your service manual's instructions.

  4. 4

    Inspect your battery's voltage and terminal connections while the engine is idling. Using a voltmeter, look for a reading between 13.8 and 15 volts. If the reading dips to 12.8 to 13 volts, your alternator may be malfunctioning.

  5. 5

    Test your connections by hooking a circuit with a light bulb inside from power to ground or vice versa to see if the bulb lights. If it does light, then the connections between your battery and its components are working properly. Next check the wiring from the battery through your alternator. A lit bulb indicates the battery wiring is good. The problem, therefore, is most likely in your alternator.

  6. 6

    Listen for a loud, grinding noise caused by the rotor failing within the alternator. Put one end of the heater hose or tubing next to your ear while moving the other end close to the alternator. The volume will increase if your alternator is faulty.

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