On Car Alternator Testing.


Be aware of the dangers of acid splashes and of explosive gas build up when working near batteries especially when charging and testing and also take care not to short out the battery.

  1. Before attempting to test any alternator it is essential to ensure that the battery is at least a 75% charge, and also the battery is in good condition and has no dead or shorted cells. Even when working perfectly an alternator cannot charge a battery which is almost completely flat.
  2. You should also examine all starter cable connections and earth braid connections to ensure they are clean and tight and in good condition. The connections at the rear of the alternator should also be checked as these are a frequent source of trouble particularly on cars which use spade connectors for the power connections which come from the battery.
  3. For all these tests you will need either a 0-20 volt multimeter or a LED type charging system tester. Suitable low cost digital multimeters and testers can be bought from Maplin or Tandy.
  4. If the charge is low Charge the battery for a few hours - overnight if possible. If no hydrometer is available, switch the charger off and allow the battery to de-gas for at least half an hour then check the battery voltage. A voltage of 12.8 volts offcharge is generally taken as indicating that no more charging is required.
  5. With the battery in the car you can now perform a cranking voltage test. This is a very simple test to check if the stater is not drawing excessive current or if the battery has a dead or shorted cell. Disconnect the coil to prevent the engine starting. If the battery has removable tops remove them. Connect the volt meter across the battery ( use the 0-20 volt scale ) and measure the battery voltage across the terminals while an assistant turns the engine over on the starter. The battery voltage while cranking should not be less than about 10.5 volts. A low reading indicates either a problem with the battery or the starter is drawing too much current. A reading of about 6 volts indicates a dead cell in the battery usually accompained by gas or boiling of the cell - beware of acid splashes and explosive gas build up. A reading of 6 volts which then rises to 8 after a few seconds is also a sure sign of a dead cell. A reading of 9 volts could indicate a stater problem.
  6. With coil lead still disconnected you can also test the eath lead voltage drop. Connect the voltmeter between the battery negative terminal and the engine cylinder block. Use an low voltage scale to measure the voltage drop while an asistant cranks the engine on the starter. The voltage drop should not be more than 0.250 volts.
  7. Now reconnect the coil connect the voltmeter across the battery terminals. Start the engine and allow it to idle. Increase the engine speed to about 2500 RPM. -- the voltage should now read between 13.5 to 14.5 volts. A voltage below 13.4 indicates the alternator is not charging or performing poorly, over 14.8 volts and the battery and other electrical and electronic components will be damaged.
  8. Now repeat the above test with head light dip beams and heated rear window switched on, the voltage may drop slightly but as long as the voltage doesn't drop below 13.4 volts the alternator is coping with the load. With the engine still at 2500 RPM switch off the lights, heated rear screen and any other electrical loads, the battey voltage should not climb above 14.8 volts.
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