How to Test a Car's Alternator

The alternator in your car provides power to charge the battery and run all of your power hungry accessories. If the alternator fails, it's a matter of hours, or even minutes, until your car stops working altogether. Knowing how to properly test your car's alternator will save you time and money and help to keep your car running properly.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• A MicroVAT or DTAC testing system.

1. Clean the positive and negative battery terminals to ensure a good connection with the tester's clamps.

2. Connect the positive and negative clamps to the appropriate battery terminals. The clamps must make firm contact with the battery terminals or the test results will be inaccurate or the tester may not work at all.

3. Turn the tester on and choose the option for "full charging system test." This will test your battery, starter and alternator and provide the most accurate picture of the problem.

4. Follow the directions on the tester's screen. These instructions will include cranking the vehicle after the battery test is performed, holding the engine at a certain RPM range and turning on certain accessories.

5. Read the results displayed on the tester's screen. The tester should scroll through the results, starting with the battery, then the starter and finally the alternator. An alternator test result showing diode ripple problems indicates a bad alternator.

Tips & Warnings

• You don't need anything fancy to take the corrosion off of your battery terminals, any soda or even hot water will do the job nicely.
• There are other ways to determine if the alternator in your car is failing, though some of them can be unsafe. The best option is to visit an auto parts store and ask if they can test your alternator; most places will do this for free.