- 1). Test your alternator with a voltmeter as your vehicle is idling. If the voltmeter goes to 12.8 volts or less, the alternator is creating strong resistance in the electrical system, suggesting failure.
- 2). Listen to the alternator as the vehicle idles. If the alternator makes a loud hum or whining sound, it may be on the verge of failure.
- 3). Drive your vehicle around your neighborhood or within easy access to a repair facility. If your alternator is going bad, the vehicle will shut down after 20 to 30 minutes. The alternator, as it fails, will drain the battery, causing all electrical systems to power down.
- 4). Look at the dashboard when you turn the headlights or radio to the "On" position. If the alternator is going bad, your dashboard lights will appear dimmer than usual. The alternator will not be able to recharge the battery at a sufficient rate to power the electronics if it is not working properly.