Tech Tip: Troubleshooting the Alternator Charging System

Squealing noises under the hood… headlight bulbs that blow out repeatedly or dim… dash voltmeter gauge out of range—either too high or too low. 

These are common indicators that something may be wrong with the charging system. If any of these symptoms are evident, then follow these five steps to pinpoint the issue:

  1. Tech-Tip-Helpful-HintPerform a visual inspection under the hood. Look at the belt tension and condition. Next, check electrical connections and cables for corrosion and proper tightness. Finally, make sure the alternator is mounted properly.
  2. Visually inspect and test the batteries. Inspect the batteries for any signs of physical damage, cracks in the casing, loose terminals or leaking fluid. Clean, repair connection and replace any damaged batteries found. Ensure that all batteries used in the same circuit are of the same manufacturer, CCA rating type and age.  Then test the batteries using a standard battery test. Remember that realistic testing, as well as successful operation, requires a fully charged battery, capable of supplying the starting system’s current needs.
  3. Measure system voltage. With the engine running, use a voltmeter to measure voltage at the battery. If voltage is greater than or equal to 13.8 volts, you can move on to Step 4. However, if it is less than 13.8 volts, then you should measure voltage at Alternator B+ and the alternator case. (Note: If the model is insulated, you have to use a ground stud to get a reading.) At that point, if the voltage falls between 12.6 and 13.7 volts, you need to replace your alternator. If it’s greater than or equal to 13.8 volts, a voltage drop test needs to be performed. (Learn how to perform a voltage drop test with a video on our YouTube page.)
  4. Test alternator output. You can use either an automated tester or a manual process to determine the output of the alternator. If the alternator doesn’t pass this test, then it needs to be replaced.
  5. Troubleshoot using the service manual. If you’ve passed through the first four steps, then the alternator is working as designed and your problem is likely caused by something external to the alternator.                                              

Troubleshooting_GuideTroubleshooting Supplement
Visit the Technical Support page at to download a troubleshooting supplement that contains diagram on the steps outlined, including the battery test, battery load test and alternator load test.

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