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How To Diagnose And Fix An Alternator Under 30 Mins


Alternator Diagnosis

The alternator is responsible for powering most of your vehicle's electronic components and charging the battery while driving. The alternator works by converting mechanical energy into electrical energy and supplies the components with direct current (DC). When the alternator is running, its rotor shaft spins a set of magnets, which creates a magnetic field inducing an alternating current (AC) which is converted to direct current (DC) by the rectifier to charge the battery and run your car's electrical systems.


Although an alternator lasts a long time, heat, overuse, and exposure to water and dirt can cause it to wear out. Without a working alternator, your car might face several issues, like dim lights, difficult starts, engine stalling, dead battery, warning lights on the console, etc. which are often mistaken for battery problems. Therefore, it is necessary to inspect the alternator to find out the problem at hand.


How To Diagnose A Faulty Alternator?

The best way to test the alternator is by using a multimeter or with a battery tester. It helps test the charging system (alternator, alternator voltage regulator) of the vehicle and determine whether the alternator is working correctly or not. Here are a few ways to determine if your alternator is failing:

1. Using A Multimeter (Under 30 Mins)

1. Using A Multimeter  (Under 30 Mins)

Step 1: Test The Battery First

  • First, ensure that your engine is turned off and then grab a multimeter and check the voltage on the battery.
  • Before connecting the multimeter probes to the battery, make sure that the terminals are clean to avoid inaccurate readings.
  • Set the Multimeter to 20 DC volts and plug the red or positive cable of the meter to the positive terminal in the battery and the black or negative cable to the battery’s negative terminal.
  • You must see a reading close to 12.4 volts and above. This shows that the battery has enough charge and can start the engine and the alternator. If the reading is lower, then your battery might not be charged.



Step 2: Test The Alternator With The Engine Turned On

  • With the battery charged and tested for power, start the engine and read the voltage on the battery with the multimeter. Make sure none of the accessories are switched on while you check the reading.
  • You must see a hike in the voltage reading, around 14.3 volts and above. You don’t have to worry if the readings fluctuate with varying RPMs.
  • Keep the vehicle running for a few minutes and then turn it off to check the battery voltage again. If you see that the voltage reading has increased since the vehicle was running, then the alternator is working fine and is charging the battery well.


Step 3: Stress Test The Alternator

  • Load test the alternator by starting the engine and turning on various accessories, like radio, lights, air conditioning, etc.
  • If the alternator is working fine, it will charge the battery properly and the battery will show a reading above 13 volts on the multimeter. If the reading is below 13 volts or higher than 15, your alternator might be faulty.


2. Using A Battery Tester (Under 15 Mins)

2. Using A Battery Tester  (Under 15 Mins)

Battery Testers are designed to test not only the battery but also the charging system of the vehicle. A battery tester also helps you get more information about your vehicle compared to a multimeter.

  • Plug the tester terminals to that of the battery, making sure the positive is connected to positive and negative to negative (negative = black, positive = red).
  • Switch the battery tester to voltage mode. It should show a reading above 12.4 volts.. If it reads less, the alternator has not charged your battery, or the vehicle is parked for long.
  • Switch the tester in CCA (cold cranking amps) mode and input your battery’s CCA rating number (mentioned on the battery) into the tester. Compare your battery's CCA rating to the reading displayed. If the reading is low, then your battery might be unable to start the vehicle, which can be due to a bad or failing alternator.
  • Finally, with your battery tester in “Charge Testing” mode, start the vehicle and see the reading. It should fall around 14.3 volts. Any lower reading indicates a problem with your alternator.
  • Many modern smart battery testers have more functions to analyze both the battery and the charging system. Select the required test and follow the instructions given. At the end of the test, the device will inform you which system is faulty - battery or alternator. 


Diagnosing an alternator is important as most of the time the battery is considered faulty and replaced only to find the newly replaced battery dies very soon. Diagnosing the issue will confirm which system is at fault and ensures the right component is replaced. The alternator is susceptible to harsh weather conditions which can cause it to fail after prolonged use. Once you have determined your alternator is bad, you can replace it with a new or a re-manufactured alternator, which is as good as their OEM counterparts.