How To Check the Car Alternator

Alternator is a mechanical device which converts mechanical energy into electrical energy and produces alternating current as a result. The common type of alternator uses a rotating magnetic field and a stationary armature, but a few models have a moving armature in combination with a fixed magnetic field.

In theory, all AC generators which you see every where can be used to produce alternating current, provided that you attach a moving source of mechanical energy to its rotor. However, the term ‘Alternator’ refers to the electric machine present in the automobiles which uses a small part of the mechanical motion produced in the engine and converts it into electric current, which is then used to charge the car batteries.

The conventional method of checking an alternator is to take off the terminals of the battery and measure the voltage produced by the alternator when the engine is running. However, due to the complex electronic circuits present in the modern cars, this method is not feasible since the voltage spikes generated while taking off the terminals may damage the electronic circuitry. Standard diagnostic methods and equipment should be used to safely test the performance of an alternator.


  • 1

    Turn the transmission in parking mode and apply parking brakes. Turn the ignition to ‘OFF’ mark and wait about 10-15 minutes for all circuits to turn off completely. Arrange a voltmeter and set its knob to DC voltage, and if there are ranges present, choose the 5-50 range.

  • 2

    Apply both ends of the voltmeter to car battery; it should display somewhat around 12.5 volts. If the battery is not charged properly, fire up the ignition for about half an hour to charge the battery completely.

  • 3

    Touch the test light cable to the negative terminal of the battery. Connect its other end to the charging post which is present on the back of the alternator. If the light does not turn on, it may be due to a broken wire, fix the faulty wire before moving forward.

  • 4

    Now turn on the engine and measure the voltage around the terminals of the battery. It should be in the 13.8 to 14.5 range.

  • 5

    Turn on the headlight, the radio and the indicators lights together. If the terminal voltage drops below the 13.9 mark, it is due to a weak or faulty alternator. You may either get it fixed or replace the complete alternator.

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