How do you know if it is the battery or the alternator that has a problem if the battery won't hold a charge and you hear a clicking sound when you try to start a 2002 VW Jetta?

Answer 1

Using a volt meter, check the voltage while the vehicle is not running. If the voltage is below 11 or so, the battery is either not fully charged or is incapable of holding a charge. Jump start the vehicle, (make certain that you do not cross connect the battery. ALWAYS RED TO POSITIVE, BOTH VEHICLES!) and after the vehicle is running, remove the jumper cables and put the meter across the battery again. If you are maintaining anything around 13.5 V, the alternator is probably ok. More complete tests are available at some of the auto parts stores around. Out here it's Auto Zone, Checker Auto or Kragen Auto. If the alternator is holding the voltage at about 13.5 V, run the car for about 15 minutes. Shut it off, let the car sit for a couple minutes then check the battery voltage again. It should not be below 12 V. There is also a possibility that you just have dirty battery terminals. Sometimes the terminals will corrode from the battery acid. To check the terminals, put a volt meter lead on the battery post and another right next to it on the associated terminal, then have someone attempt to start the car. If the voltage is anything other than zero you have a bad connection between the battery terminal and the post. Just take it apart using a battery terminal puller, then thoroughly clean the battery post and terminal using a special brush designed for that use.

AnswerHere's an easier way to tell if it's the alternator or battery. Get a jump and then disconnect the positive side battery cable. If the car continues to run it's more than likely the battery because the battery is mainly used to start the car. The alternator provides all the power to run everything in the car and the car won't stop until you turn it off. It is best to have it checked to be sure. Most parts stores like Autozone, Advance Auto will check it for free.