To answer the question whether the alternator works correctly you just need to use any working instrument of appropriate measurement range, designed for measuring constant voltage and take a number of simple measurements.


We propose to follow the procedure described below:

  • Without switching the engine on, connect the measurement instrument to the battery, as shown on Fig. 1, measure the voltage and remember the voltmeter reading
  • Start the engine and watch if the battery charge indicator light goes off.  Continue the testing even if it doesn’t.
  • Let the engine work for two minutes without energizing the load.  We recommend the delay since in some of the recently constructed cars the alternator does not switch on instantly after the indicator light goes off.
  • Measure the voltage on the battery clamp again.  The measured voltage without any load energized and at slow revolution should be more than 13 V.  
  • Compare the reading with the previous one.  An increase in voltage indicates the battery is charging.  It is not, however, a sufficient basis to assess whether the alternator works correctly.
  • Turn on the defogger setting its speed to the maximum and switch on the rear window defroster, while keeping the engine at about 2000 RPM (approximately two times more than on idle speed). 
  • Measure the battery voltage again. If the alternator works correctly, the voltage should be at least 13.5V.


Once the indicator light has gone off and the measured voltage is as required above, you may conclude the alternator is working correctly. If the voltage is not as required or the indicator light is still on, you may assume the alternator works incorrectly. If you want to try to diagnose the causes of alternator malfunction, it is worthwhile to perform additional procedures and take extra measurements:

  • Check whether the working alternator emits abnormal buzzing caused by damaged rectifying diodes or produces loud working noise because of a damaged bearing
  • Check if the driver belt is sufficiently tensioned. If it is too loose, the alternator will not reach the full current capacity
  • Check whether the rotations of the alternator rotor correspond with the rotations of the alternator belt pulley (applies to alternators with overrunning pulley - the so-called freewheeling alternators). In some damages to the pulley, the alternator rotor is not driven even if the pulley rotates.
  • Take an additional measurement directly on the alternator clamps, as shown on Fig. 4. Compare the reading with the alternator voltage.  If the car electric installation is working property, the difference in the voltmeter readings should not exceed 0.1V.  Any difference in the measured values above that threshold indicates the installation is defective (poor quality of clamps on battery terminals, no ground to the engine shell, incorrectly connected alternator, etc.).


Based on the measurements taken and indications of the indicator light you can tentatively identify the underlying cause of the alternator’s incorrect operation.  This will help you make appropriate decisions and remove the defect. The table listing the symptoms versus their potential causes may prove helpful when diagnosing the problem. 


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