How to check alternator output

Written by arthur barnhouse
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How to check alternator output
A voltmeter is a valuable tool for checking a vehicle's charging system. (The Readout image by Cinneman from

The alternator in your car or truck is an essential part of your vehicle's engine. Without a working alternator, the battery would run out of power and you would soon find yourself stranded. Rather than finding yourself in need of a tow truck, you can test your alternator in order to determine whether it is providing a good enough charge to sustain your car's battery. Thankfully, performing this voltage output test is a simple task that can be accomplished in less than ten minutes.

Skill level:
Moderately Easy

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Things you need

  • Digital voltmeter
  • Friend or family member

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  1. 1

    Purchase a digital voltmeter. You can find this device at most automotive supply stores for a fairly affordable price (less than £26). A digital voltmeter is often easier to use and provides a more accurate reading than its analogue counterpart.

  2. 2

    Park your vehicle on level ground and pull the emergency brake. Leave the ignition switch on, as the engine must be running in order to get a reading of the alternator's output. Open the bonnet of the car and ask a friend or family member to get in the driver's seat and wait for intsructions from you.

  3. 3

    Adjust the settings on your voltmeter, if necessary. It must be set on the "DC" scale, and it must be adjusted for 12 volts.

  4. 4

    Remove any guards or plastic shields from the battery. In order to test the output of your alternator, you need to access the positive and the negatives terminals on the battery. While you're working under the hood, be sure not to touch any moving parts of the engine.

  5. 5

    Place the red, positive lead or clip of your voltmeter on the positive terminal of the battery, then place the black, negative lead or clip of your voltmeter on the negative terminal.

  6. 6

    Give the cue to your friend or family member. He or she needs to step lightly on the accelerator, causing the engine to rev to approximately 2,000 to 2,500 RPMs. If your car doesn't have an RPM gauge, revving the engine to a fast idle will suffice.

  7. 7

    Check the digital display on your voltmeter. If your alternator has a proper voltage output, you should see a reading between 13.5 and 14.4 volts. If you note a reading significantly higher than 14.4 volts or lower than 13.5 volts, your alternator should be checked by a professional, as it may need to be replaced. A reading of less than 12 volts is a definite indicator that you alternator is not working correctly, or that there is a short circuit somewhere in the charging system.

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