How to Test an Alternator by Disconnecting the Battery

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Proper testing of an alternator requires the use of a multimeter or battery tester. An alternator produces direct current (DC) but can also produce alternative current (AC), which can damage the various electrical components of your vehicle, including the computer. Disconnecting the battery while the engine is running does allow you to identify if the alternator is weak, but poses potential harm to your engine. Use this method if no other alternative exists and practice extreme caution while doing so.

Things You'll Need

  • Socket wrench
  • Protective glasses
  • Rubber gloves
  • Start the vehicle and engage the emergency brake.

  • Open the hood and locate the battery. Stand to the side of the battery while disconnecting it to avoid the moving parts of the engine (fan and belts).

  • Turn off all unnecessary components, such as the lights, radio, and A/C, to limit the drain on your alternator. Allow the vehicle to warm up to operating temperature.

  • Put on protective glasses and rubber gloves. Disconnect the negative lead from the battery and set it to the side.

  • Run the engine uninterrupted for 15 to 30 minutes. Do not rev the engine or drive, if possible. A healthy alternator will allow the vehicle to run.Reconnect the negative lead to the battery if no interruptions or issues are noted.

  • Check all the fuses and reconnect the battery if there is an interruption in the vehicle running. If all of the fuses are fine, take your vehicle to an auto shop to have your alternator professionally tested. You likely have a bad alternator.

Tips & Warnings

  • Doing this repeatedly could cause a good alternator to go bad as you are placing a greater strain on the alternator.

References

  • Photo Credit Hemera Technologies/AbleStock.com/Getty Images
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