The alternator is the foundation of your vehicle's charging system. If the alternator is not functioning properly then the battery will not be charged and it will eventually go dead. The symptoms of a bad or faulty alternator include a flicking alternator LED light on the dashboard, diminished performance of electric accessories in the vehicle and a weak or dead battery. The same symptoms may indicate a faulty LED light or a bad battery, so it is important to know how to test the alternator to determine the source of the problem.
Open the hood and locate the alternator and the alternator belt.
Test the tightness of the alternator belt by pressing down on it with your finger. If the belt gives more than 1/2 inch it is too loose. The alternator may actually be good but the loose belt prevents the alternator from charging the battery. Check the condition of the alternator belt as well. If it is worn and cracked it needs to be replaced. Alternator belts should be replaced every two years or 25,000 miles.
Start the engine and visually inspect the alternator belt. The belt should be moving uniformly without skipping or missing.
Connect a voltmeter to the battery terminals. Connect the red voltmeter probe to the positive battery terminal and the black probe to the negative battery terminal.
Check the voltage on the meter. The proper voltage should be between 13.8 to 14.2.
Step on the accelerator and increase the engine's rpm to 1,500. Check the voltage meter again. It should now read between 14.2 to 14.6. A reading less that 14.2 indicates a bad diode.
Turn off the headlights while maintaining 1,500 rpm. Do this for a minute. If the voltage meter goes above 14.8 , the alternator is overcharging the battery. This indicates that the alternator regulator is broken.
