How to Test the Alternator Current Overcharge

Updated March 23, 2017

Current is measured in amperes and is effectively the energy produced from your car's alternator that's required to power electrical items in your car. Alternators automatically adjust their output current, depending on the number of electrical items turned on in your car, but they do have a maximum output rating. If an alternator produces more current than is required it can blow fuses and potentially damage electrical equipment. You can test if the alternator is producing the correct current, or is overcharging, using a multimeter.

Check the maximum current your alternator produces, if it's operating correctly. Get the information from your car's manual. It's usually in the specification and technical section at the back of the book. It gives two figures: minimum and maximum, you only need to make a note of the maximum figure so you can test if your alternator current is overcharging.

Open your car's bonnet, secure it and then turn on your car's engine. Turn on as many electrical items in your car as you can; wipers, lights, and the heater-fan are good examples of items that require a lot of current to operate. The alternator adjusts its output current to power the devices you have turned on, if it's working correctly.

Put on a pair of lightweight rubber gloves. You need to use the multimeter and attach the sensors onto the battery terminals so the gloves protect you from getting an electric shock if you accidentally touch one of the terminals.

Set your multimeter to measure amperes. This is the current that flows from the alternator. Put the end of the black wire from the multimeter onto the negative battery terminal. The wire has a metal pole on the end and the battery terminal is labelled "Neg." Put the end of the red wire from the multimeter onto the positive battery terminal labelled "Pos."

Read the ampere display on the multimeter. It reads the same or similar to the figures you noted earlier from the car's manual. The reading is fine if it's the same or slightly less than the figure you noted. If the reading is 5 per cent, or more, higher than the figure you noted then the alternator current is overcharging. Get the alternator checked out by a professional as it could damage your electrical equipment and blow fuses.

Turn off the electrical equipment in your car then turn off the engine. Close the car's bonnet.

Things You'll Need

  • Light-weight rubber gloves
  • Multimeter
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About the Author

Stephen Benham has been writing since 1999. His current articles appear on various websites. Benham has worked as an insurance research writer for Axco Services, producing reports in many countries. He has been an underwriting member at Lloyd's of London and a director of three companies. Benham has a diploma in business studies from South Essex College, U.K.