Alternator Symptoms

• Suspect a faulty alternator if your lights begin to dim while driving or you have trouble starting your car, according to AA1Car.com. These are among several symptoms of a failing alternator.
  
  Check your alternator fs your car has an alternator dash light and it flickers when you start the car. If your battery goes dead easily, the alternator may be going bad, and it should be replaced.
  
  Check the belt that turns the alternator for wear. If the belt is frayed or worn out, replace the belt and the alternator may begin working properly again.
  
  

Check Battery

• Sometimes when alternator issues are suspected, the problem is with the battery. Check the surface voltage to determine whether the battery is the root problem, according to 2CarPros.com.
  
  Connect a voltage meter to the positive and negative terminals of the battery while the car is not running. For most accurate results do this in the morning after the car has sat overnight. The voltage meter should read between 12.5 and 12.8 volts. If the meter reads much less than this, the battery is beginning to fail and the alternator may be in good working order. Replace the battery to solve the problem.
  
  

Alternator Voltage Testing

• Once the battery and belt have been ruled out, test the alternator itself. Perform a voltage output test on the alternator in the same way you tested the battery. Connect the terminals on the voltage meter to the corresponding terminals on the battery.
  
  Start the car while performing this test to determine the voltage being created by the alternator. The reading on the meter should be between 13.6 and 14.3 volts. If the reading is off the mark, then it is likely that the alternator needs replacing, according to 2CarPros.com.
  
  

Amperage Testing

• Test the amperage output of the alternator with a helper. Connect the amperage meter to the battery in the same way the voltage meter was connected.
  
  Have your helper start the engine and turn on all available electrical accessories in the car---lights, radio, hazard lights, wipers and anything else that requires electricity. Tell them to rev the engine slightly, holding at about 1,200 rpm.
  
  With all of this going on, check the amperage meter. The alternator should be producing near the maximum amps it was designed for. If the alternator is a 90-amp model, the reading should be around 88 amps. Stop the test shortly after taking the reading, as the alternator is not designed to operate for long periods at maximum output.