First, let me explain that there are a lot of people operating under the misconception that the alternator is needed to start a car. It is not. In fact, you can unplug all the electrical wiring to the alternator and still start your car. However, once the car is running, the alternator is needed to keep the battery providing electrical energy to the car.
Next, let me say that the best way to check your car alternator is to remove the alternator and take it to your local parts dealer to have them check it with their tester. They will check your alternator or starter for free. But, for the person who lives in the rural areas, especially for the one who lives several miles from the nearest parts dealer, that may not be feasible, for one reason or another. For those people, here is a couple of ways to check the alternator while it is still in the car:
Electric Meter Testing
If you have a household electrical testing meter, you can find out if the alternator is working by checking the battery while the engine is running. Simply start the car, lay the tester down off the battery but close enough for you to see the face of the meter. Turn the switch to DC. Using alligator clips, connect the positive lead to the positive pole of the battery and the negative lead to the negative pole. Now check to find out how many volts are being put out by the battery. It should be at least 12 volts. If not, replace the alternator. If it is, turn on the heater, radio, lights and windshield wipers. If the voltage being put out is still 12 volts, the alternator is good. If not, replace the alternator. The battery can only put out as many volts as the alternator puts into it.
The Old Fashioned Way
For those of you who do not have an electrical meter, there is another reliable way to check the alternator. Back when our cars had an alternator, or generator, and a voltage regulator, we came up with a way to find out whether it was our alternator or voltage regulator that was bad. It went like this:
First, start the car. If it died, it was the alternator that was bad. If it continued to run, we pulled the light switch on. If it died then, it was the voltage regulator that was bad. Today, you can still use this method, but the voltage regulator is now built into the alternator. So, if the engine fails to run after starting or after turning on the lights, you know for sure the alternator needs replacing.