How To Test An Alternator (3 Best Ways)

Your vehicle’s alternator is an essential part of your car. It is located at the front of the vehicle engine, and it’s essential to help keep your battery going when you drive on the road.

If you have a faulty alternator, your battery can quickly drain, leaving you stranded.

Are you worried that your alternator might break soon? How would you know if it’s still in good working condition?

It can be difficult to tell whether or not your alternator is still capable of running effectively as there are many factors at play. In this article, we have offered several methods of testing an alternator.

Bad Alternator Symptoms

Before we get into the steps to test an alternator, Here are some bad alternator symptoms you should look out for.

  1. The battery light on the car’s dashboard is on.
  2. The engine runs slowly or refuses to crank at all.
  3. Weak headlights, a low battery, and/or dim interior lights.
  4. There is a rumbling or squeaking noise coming from the alternator.
  5. Despite your efforts, you are still unable to jump-start your car.

The symptoms mentioned above could be indicators of a weak car battery. Therefore, you should follow the following steps to find out how to test an alternator to know if yours is bad.

How To Test An Alternator With A Multimeter

How To Test An Alternator With A Multimeter

The following steps address how to test an alternator with a multimeter

  1. Get a multimeter.
  2. Check that your multimeter’s DCV (DC Volts) setting is 15 or higher.
  3. Make sure the positive and negative terminals on your alternator should be clean and in good working condition.
  4. Attach the black cable to the negative terminal of the multimeter and the red cable to the positive terminal.
  5. Get an alternator that has a reading of around 12.6.
  6. Start the car and check that the reading is between 14.2 and 14.7.
  7. When a reading is greater than 14.7, it is an indication that the battery is being overcharged. In contrast, a reading lesser than 14.2 is an indication that the alternator is rather undercharging the battery.
  8. Ensure the lights, radio, fog light, fan, and other electronic components are turned on. The voltage reading shouldn’t be below 13 volts.
  9. Make sure the temperature of your car does not fall below 12.6 degrees after turning it off.
  10. Most likely, your alternator has a problem if one of these readings is incorrect, which would require automotive repair service.

How To Test An Alternator With A Voltmeter

How To Test An Alternator With A Voltmeter

Here are the steps on how to test an alternator with a voltmeter.

1. Get A Voltmeter

Almost any auto part shop sells a voltmeter for less than $20. Do not be concerned about purchasing an expensive voltmeter for this purpose. Use an inexpensive voltmeter; it will do the job.

2. Check The Battery First

When your battery charges the alternator, it creates a moderate amount of friction that can rotate the alternator at speed enough to fill the battery with electricity.

Like everything else, batteries aren’t perfect, and if they don’t get charged, they won’t have enough power to start your engine, and consequently, you will not be able to test the alternator.

You should check the battery first before determining the condition of the alternator, particularly if it’s cold outside or the battery is old.

If the battery is the problem, the alternator may be working fine.

Here’s how to test your battery:

  • Turn off the car. You should verify the engine is shut down before you connect the Voltmeter.
  • Lift the hood.
  • Take the Voltmeter and attach it to the battery. Connect the green probe wire to the battery’s positive terminal, then attach the black probe wire to the negative terminal of your battery. Never let the metal connectors come in contact with your skin, as it could be potentially dangerous.
  • Check the Voltmeter; a battery that has a voltage reading of over 12.2 will serve as a good starting power source for the alternator.
  • The battery should be fully charged before setting the alternator to test, or you can use an alternative method to test the alternator if it is producing insufficient voltage.

3. Start The Vehicle And Rev The Engine To 2500 RPM

Your battery will draw power when you do this, causing your voltage regulator to send your alternator into high gear.

4. Keep The Engine In Operation And Test The Battery Using The Voltmeter

If you want to make sure your alternator is functioning properly, check the voltage meter and open an electrical diagram.

If when you turn on the engine, the voltage goes up to at least 13v, and if after that the voltage doesn’t go down or go up more than 0.5v even with your RPM’s jumping up and down, then you have a working and reliable alternator.

On the other hand, when the engine turns on, and there aren’t any changes in the voltage, or it later drops down below 13v, you may be having a problem with your alternator because it is not working correctly.

To properly check, keep repeating this process, but keep the radio station on and turn on lights and other appliances like AC.

In order to charge the alternator, the battery voltage needs to be at or above 13 volts, as long as the engine is running at 2500 PM and all other accessories are on.

How To Test An Alternator By Monitoring Your Alternator

How To Test an alternator by monitoring your alternator

The following steps address how to test an alternator by monitoring it.

1. Check The Alternator Gauge

First, get a voltage/amp gauge to test the alternator’s voltage output. Next, start the engine and let it run at 2,000 RPMs to see how much power it’s generating.

Then shut down all devices that put an extra strain on the alternator, such as your heater or AC or headlamps or any other device which could drain power from the system.

To tell if your alternator is charging your battery, you should check the voltage gauge.

If the voltage needle stays higher while the engine’s running than it does when the engine’s not in use, this indicates that the alternator is well charged and therefore going to be able to keep power running to the rest of your car’s systems.

2. Listen To The Alternator While The Engine Is Running

If your bearings are faulty, you might hear odd sounds coming from your car’s rear end.

And if you throw in a lot of electrical components that draw power all at once, the sound is going to get louder and weirder as time goes on.

3. Turn On The Radio And Rev Your Engine

Check the radio first. Start your engine and tune your radio to a much lower number on the AM band, but without any music.

If you hear a humming or distorted noise whenever you turn on the engine, you might have a problem with your alternator.

4. Find An Auto Parts Store That Will Test Alternators For No Charge

All shops will want you to buy the alternators they are selling. In order to get more business, each of them will try to persuade potential buyers to try their alternators before they buy them by offering a chance to do so at no cost.

Carefully pull out your alternator from its socket and bring it back so you can rebound up to 60 miles.

Check out this video for a more detailed guide on how to test an alternator.

Conclusion

These simple methods should help you in knowing whether your alternator might be on its way out.

It’s important to recognize symptoms and signs of a bad alternator when they appear so that you can begin the process of getting it replaced before there’s too significant an impact on your vehicle.

If you learn that your specific car requires alternator repair, check out our article on alternator repair to see which options are available to you and how much a new alternator cost.

Scott Greene is a seasoned automotive technician for over 5 years and has been deep into advanced automotive diagnostics for a couple of years. He Loves writing about Automotive Diagnostics and Repair, Trouble codes, Buyer guides for various car parts and accessories, and lots more. 

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