How to Diagnose an Alternator

The alternator is an essential part of your car's electrical system. It recharges the battery while the car is running and it also powers the lights and other electrical accessories. If your car has trouble starting or if is seems underpowered, there may be a problem with either the alternator or the car battery. Alternators eventually wear out and need to be replaced, however, it is important to know if the alternator or the battery is the source of the problem. A third possibility is a loose alternator belt.

Things You'll Need

• Voltmeter

Instructions

1. Make a checklist to diagnose a faulty or bad alternator. The typical symptoms of a bad alternator include hard or slow cranking when you attempt to start the engine, dim headlights, a drained battery, and a dashboard indicator light that stays on.

2. Open the hood and locate the alternator and alternator belt. They will be in front of the engine.

3. Press down on the alternator belt with your fingers. If the belt gives more than a 1/2 inch it is too loose. A loose belt prevents the alternator from charging the battery. Tightening the belt may correct the problem.

4. Attach a voltmeter to the battery. Connect the red lead wire of the voltmeter to the positive terminal on the battery. Connect the black lead wire of the voltmeter to the negative terminal of the battery.

5. Turn the engine on an inspect the voltmeter. If the alternator is in good condition the voltmeter should read between 14 and 14.2. Some alternators will generate between 13.8 to 15.3 volts with the lights or electric accessories turned off. If the voltmeter is lower than 13.8 or higher than 15.3, this indicates a problem with the alternator.

6. Turn on the lights and other electric accessories in the car. The voltmeter should be 13 volts.

References

• Ecofix: Alternator And Generator Repair
• Aa1 Car: Charging System Checks