How do you test the alternator in a 99 Chevy Malibu?

You will need a digital multimeter that can measure DC voltage for an accurate reading. Open the hood and connect the positive and negative multimeter leads to the positive and negative battery posts. Turn on the multimeter and make note of the battery voltage you have before starting the engine. The battery needs to be fully charged to get an accurate alternator reading. A fully charged battery should read 12.68 volts. Assuming the battery is healthy and fully charged, start the engine and let it idle. Make note of the multimeter reading. A healthy alternator should read 14.2 Volts. Now with engine still running, turn on the headlights and turn on the heater blower on high. Multimeter reading should stay above 13.8 volts. If you are below these numbers you may have a tired alternator.