How to Test an Alternator for Overchargingby Arthur Barnhouse
The alternator is vital part of your car or truck's electrical system. When the vehicle is running, the alternator provides a constant charge to the battery, as well as to other accessories. Without the alternator, the battery will eventually discharge. However, if the alternator isn't working correctly, it may send too great of a charge to the battery, which is known as overcharging. This condition is dangerous to your car's battery and electrical system. You can test to see if the alternator is overcharging, using a simple voltmeter.
Start your vehicle and open the hood. Be aware of the moving parts within the engine compartment, as you do not want to get your hands or tools anywhere near them.
Locate the vehicle's battery. It may be obscured by protective shields, the air filtration or intake system or a fuse box. Whatever the case, you need to remove everything that covers the battery. You must have access to both the negative and positive terminals of the battery.
Turn the digital voltmeter on and adjust it to the proper settings, if necessary. The voltmeter must be set to "DC" and 12 volts.
Connect the clamps or leads of the voltmeter to the battery. You must connect positive to positive and negative to negative. The positive lead is usually red or yellow, while the positive terminal on the battery will be marked with a plus sign. The negative lead on your voltmeter is black, while the negative battery terminal is marked with a minus sign.
Examine the voltmeter. If you've hooked it up properly, you should see a reading somewhere between 13.6 volts and 14.3 volts. A reading higher than 14.4 volts warrants further testing by a professional. If your alternator is found to be overcharging, you will need to have it replaced.