The alternator in your car serves two very basic yet crucial goals. The first is to provide enough electricity to maintain a 12-volt charge in your automobile's battery so there is ample power to start the vehicle the next time you want to use it after turning it off. The second is to provide enough excess power to run the vehicle accessories such as the headlights, wipers, horn, turn signals and other accessories. Understanding how to diagnose a faulty alternator relies on your ability to determine if there isn't enough charge coming from it, or if there is too much charge coming from it.
Start your car and allow it to idle for several minutes so the alternator comes to normal averaged operating temperature.
Turn on your digital multimeter to measure at least 14 volts. While the automobile continues to run, open the engine compartment hood and touch the black test probe of your multimeter to the black (negative) terminal of the battery and then touch the red test probe of your multimeter to the red (positive) terminal of the battery. Make note of the voltage being produced by the alternator. The voltage with the car warmed up should measure between 13.5 volts at the lower limit, and shouldn't go above 14.2 volts at the higher limit. If the voltage is over 14.2 then the alternator may have a faulty voltage regulator. If under 13.5 volts, there may be either a faulty internal voltage regulator or worn out alternator components.
Perform a "load test," if possible. Turn on the vehicle's headlights set to bright and check the voltage once again using your multimeter. If the alternator isn't operating properly, or it's internal voltage regulator is faulty, the measured voltage will drop below 13 volts.
Run your car for an additional 5 to 10minutes and watch the car's headlights. If they become dim and continue dimming further as the car runs, turn the car and headlights off immediately. Measure the battery power once again while it isn't running. If the battery voltage has dropped below 12 volts, then the alternator has failed and isn't maintaining the battery voltage necessary.
