How To Test Your Car Alternator

One of the most important, but often overlooked components of any vehicle is the alternator. The alternator is the device inside the car that allows the battery to recharge while driving, consisting of essentially a small generator powered by the rotational movement of the axles. In this article, we’ll look at several methods to test or check your alternator.

Of course, not everyone who is testing an alternator is looking to repair or replace it. Some enthusiasts upgrade to a high output alternator to enhance their vehicles, by adding LED lights, subs, or hydraulics, in order to draw a higher power load. If this is the case for you, you’ll still want to test your alternator, to find out where you’re at and what next steps should be taken.

How to Test

Testing an alternator is a fairly simple process, and as long as you have some basic tools and an understanding of basic mechanical and electrical safety, you should be able to get through it.

For this project, you’ll need a multimeter or voltmeter. A multimeter measures other properties like resistance, while a voltmeter simply measures voltage. For this project, you will only need to measure voltage, so a voltmeter will be a cheaper option if you have to buy this tool.


  • Start with the car turned off. If you are using a multimeter, set it to DC Volts above 15
  • Connect the multimeter’s black cable to the negative terminal and the red cable to the positive terminal.
  • Check for an ideal reading of 12.6 or close to that with the vehicle not running
  • Start the vehicle, and watch for a reading of around 14.5
  • If you are getting a reading of over 14.7 while the car is on, the battery is being overcharged, and if it reads under 14.2, the battery is being undercharged.
  • Turn on the lights, fans, radio, and any other electronics in the vehicle. Watch the reading and make sure it doesn’t get below 13
  • Turn off the car, and see if the reading is above 12.6. It should be slightly higher due to the mechanical activity

If any of these readings are off by more than a few points or you feel that the process isn’t going as it should, you likely have an alternator issue that will need to be corrected. If you’re testing your alternator to find out about specific equipment, electronics, or upgrades to your vehicle, the process and numbers you are looking for will vary depending on what you are doing.

Likely, the issue here will be that these upgrades are drawing too much power from the battery, which puts strain on the alternator and can lead to breakdowns and stranded vehicles, among other concerns. This is where the high output alternator will come into play and allow you to modify your vehicle the way you want to by giving you more voltage to work with.

Less Technical Methods

Let’s say that the previous method seems a little too technical, or you don’t have a multimeter. Not everyone’s comfortable popping the hood and using these tools, but luckily there are some simpler methods you can use to see if you have a problem.

1. Test the Radio

With the car on, locate a low-power AM radio station with no interference. Give the gas pedal a push, and rev the engine a few times. If the sound changes, whines, or otherwise modulates with the pedal, then the alternator is very likely not functioning correctly. This works due to the same differences in voltage that can cause an irritating whining sound from the car audio linked to the alternator.

2. Listen to the Alternator

With the car running, open the hood and locate the alternator. Alternators contain bearings, which can make noises or squeaking sounds when something is going wrong, which will get more noticeable as electronics are turned on. If you hear any strange or unusual noises coming from your alternator, definitely have it checked out.

3. Check the Alternator Gauge

Some vehicles will have a gauge on the dashboard that reads the alternator output for you. Look for the word “Volts” or a battery icon next to a gauge if you can’t locate it. With the car running at 2000 RPM, try turning on the electronics and see how the gauge reacts. If it is higher when the engine is running versus when it is off, the car is probably at least charging the battery.

4. Call an Auto Parts Store

One last resort trick you can use is to head to an auto parts shop and explain your problem. If you’re lucky, they will often do a reading for free, since it’s quite simple with the right tools and experience.

Alternators are a critical component to upgrade when modifying or improving a vehicle. Additionally, they can become faulty or need repairs. Make sure you don’t end up stranded or worse, and keep your alternator testing up to date.

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