Alternator Notes

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Testing an Alternator

Use a multi-meter (voltage meter) and trun on the engine.

  • Open your hood and so you have clear access to the car battery. Locate your alternator and check to be sure the alternator belt isn’t loose.

  • Turn your multi-meter to the 20 V setting.

  • Start the engine. At this point, check to make sure the alternator pulley / belt is spinning properly without slipping.

  • There are two ways to test the voltage coming from the alternator. If you can access to the alternator without getting your body parts or clothing caught up in the spinning pulleys, then place the positive multi-meter probe (red) and touch it to the red terminal connector coming out of the alternator. Now take the negative (black) multi-meter probe and touch it to a metal part of your car frame or the negative terminal on the car battery. Don’t ground to the alternator itself, it could short-out. You should now be getting a reading on your multi-meter display.

    If you don’t have good access to your alternator, then you can simply test the alternator by touching the positive multi-meter probe to the positive terminal on your battery and the negative probe to the negative terminal on your battery.

If the alternator is working well, your multi-meter should read somewhere around 14 volts (usually 13.8-14.2). If it is reading excessively higher than 14 volts (greater than 15 volts) the voltage regulator on your alternator may be faulty. If it is lower than 13-14 volts, it is possible that your engine idle speed is too low. Try revving up the engine to 2000 RPM or higher and take a reading. If the voltage is still too low, check to be sure all the connectors on your alternator are tight and that the alternator belt is not slipping. If it is still not putting out sufficient power, then the alternator’s voltage regulator could be bad or the alternator itself may need a replacement.

A car alternator needs to be putting out between 13.8 and 14.2 volts to effectively charge a 12 volt car battery. If it is putting out too much voltage (over 15 volts), battery acid will leak out of the battery. When a battery is near fully charged the alternator will stop putting out sufficient voltage to charge the battery and the meter will read only the voltage coming from the battery itself. In this case, you can leave your lights on with the engine off for ten minutes to drain your battery a little.

It can also be helpful to test at the battery terminals with the engine off and then turn the car on and test again at the terminals. If your battery is more or less fully charged, it should read at around 12-13 volts with the car off. If you’ve run your car for a long time and while the car is running the voltage reading is in the 13-14 volt range, but then you shut the car off and the battery voltage instantly drops to well below 12 volts (10 volts or under; 9 volts or under in freezing weather conditions), it is likely that your battery needs maintenance or replaced.

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