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How to Tell If an Alternator Is Broken

Updated April 17, 2017

An alternator is a component in your car that changes energy from the engine into AC electricity, which is internally converted to DC electricity used to both charge the battery and run any additional electrical components in the vehicle. Without the alternator your battery will run itself down and die. Your vehicle would no longer run. A multimeter is a wonderful tool that can help you determine if your alternator is functioning properly.

Do not start your car yet. Touch the positive lead on the multimeter to the positive terminal post on your battery and the black lead on the multimeter to the negative terminal post on your battery. The voltage on the battery with the engine off should be between 12 and 12.8 volts.

Insert your key in the ignition and start your vehicle. Allow the vehicle to idle in the parked position. Touch the positive or red lead from the multimeter to the positive or red terminal post on your battery and the negative or black lead on the multimeter to the negative or black terminal post on your battery. With the vehicle idling the voltage should now read between 13.6 and 14.4 volts if the alternator is functioning properly.

Turn on the headlights and other power accessories in the vehicle. Again place the positive lead on the multimeter onto the positive terminal post on your battery and place the negative lead on the multimeter onto the negative or black terminal post on your battery. The voltage, with the engine, lights and power accessories running, should remain around 13 volts.

Plan to replace the alternator if the alternator voltage is too high or too low at any of these measurements.

Tip

Always check your multimeter's leads to make sure which is the positive and which is the negative.

Warning

Wear protective eye wear and gloves. Refrain from wearing loose clothing while working on an automobile. Check battery for acidic residue before starting work and use baking soda to neutralize if necessary. Remove metals or jewelry from your person when working on your vehicles charging system.

Things You'll Need

  • Multimeter
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About the Author

Kyle Anderson attended the University of Tennessee, where he received his B.S. and M.S. in English studies. He has worked as an editor and writer for several print publications and websites.