How to Diagnose Alternator Faults
- 1). Remove the cover from the battery with the engine off and identify the positive and negative posts. The positive has a "plus" sign next to it and a red cable connection. The negative has a "minus" sign next to it and a black cable connection.
- 2). Attach the red lead of the voltmeter to the positive battery post and the negative lead to the negative battery post. Turn the voltmeter on. A reading will appear while the engine is off; this is an indication of the voltage in the battery itself, which should be between 12.5 and 12.8 if the battery is in good condition.
- 3). Crank the engine and take another look at the voltmeter. While the alternator is working to supply power to the battery, the voltmeter reads its voltage instead of the battery. The reading should be somewhere between 13.6 and 14.3 volts if the alternator is producing sufficient power.
- 4). Visually inspect the alternator and the battery cable connections. Loose or corroded connections prevent electrical flow from the alternator to the battery, which makes the alternator work too hard to provide the battery with voltage.
- 5). Listen for noises coming from the alternator while the car is running. Noises can be an indication of bad bearings and shorted diodes. Take the alternator to a mechanic or auto parts store for further diagnosing.