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How to check alternator amp output

Updated March 23, 2017

The correct ampere output from your alternator is important as amperes are the current that powers the electrical items in your car. Items such as heater-fans, wipers and headlights use high amperes and, when you turn them on, your alternator has to adjust its output to meet the energy requirements. If you find that your lights are dim or your heater-fan isn't going fast enough, you need to check the alternator ampere output.

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  1. Check the amperes your alternator produces if it operates correctly. Look in your car's manual and go to the specification section: it's usually at the back. Turn to the page for your alternator details. It tells you the minimum and maximum amperes from the alternator. The minimum is when you are using items that consume small amounts of energy, such as your car stereo. The maximum is when you turn on major energy consuming items. Make a note of the two figures.

  2. Wear protective gloves as you need to be in close contact to the car battery to check the ampere output from your alternator. Ensure you don't have loose clothing as you need to start your car's engine.

  3. Open the car's bonnet and secure it. Turn on your car's engine and let it idle. Ensure you don't have energy consuming items turned on such as lights and air-conditioning as you need to check the minimum alternator ampere output.

  4. Use a multimeter and set it to read amperes. Put the metal prong on the end of the black wire that extends from the multimeter onto the negative battery terminal. The negative battery terminal has a black cable attached to it and is labelled "Neg." Put the metal prong on the end of the red wire onto the positive terminal of the battery. The positive terminal has a red cable attached to it and is labelled "Pos."

  5. Look at the display on the multimeter. It reads the minimum amperes you made a note of earlier. It doesn't matter if it's 10 per cent higher or lower than the minimum specification. If the reading is more than 10 per cent lower, get the alternator checked by a professional. If it's more than 10 per cent higher, check to make sure no electrical equipment is turned on and if it isn't, get it checked by a professional.

  6. Turn on energy consuming equipment in your car: air-conditioning, wipers and lights are good. The more you turn on the more amperes are needed to power them so your alternator adjusts the output, if it's operating correctly.

  7. Measure the alternator amperes output using your multimeter as before. The reading is near the maximum amperes you made a note of earlier. If the reading is more than 15 per cent below the maximum, check you've turned on as many item as possible. If you have, then you need to get the alternator checked. If you haven't, turn on more equipment and check the reading again. If there's little or no change then get your alternator checked.

  8. Turn off the car's engine. Close the car's bonnet.

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Things You'll Need

  • Protective gloves
  • Multimeter

About the Author

Stephen Benham

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.

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