How to Test an Alternator on Car?

Alternators are one of the most important parts of the engine. It is usually attached to the front of the block and is responsible for keeping the battery charged while driving on the road. Without this, the battery will die quickly and you will be stranded. Think your alternator is broken? Here are some tips from the experts on how to check your alternator’s health.

Before testing the alternator, here are some signs that your alternator may be weak:

  • The battery dashboard light is on.
  • The engine cranks slowly or does not turn at all.
  • Batteries, headlights and interior lights seem weak.
  • Alternator is making rumbling or squeaking.
  • The attempt to start the car failed.
  • The above symptoms can also be a sign of a weak car battery.

How to Test an Alternator with A Voltmeter?

Do not test the alternator by disconnecting the negative battery cable. Use a voltmeter instead for easy and safe testing.

If the battery is new and the car still won’t start, the alternator may be bad. If you want to test the alternator by disconnecting the negative battery cable, don’t. A good alternator might keep the engine running without the negative cable, but that was never a good test.

In the pre-computer days, you could take it apart without damaging anything. Today, there is a risk of frying all the electrical equipment of the car. Second, remove the battery. A voltage regulator tunes the alternator for maximum power output.

Without a battery acting as a buffer in the circuit, the alternator can output up to 150 volts depending on engine speed. When the smoke clears, that “easy test” could cost you thousands of dollars in new electronics.

Step 1: Do the voltmeter test

  • Get a cheap voltmeter.
  • With the engine off, the battery voltage should be between 12.5 and 12.8 volts. If not, charge the battery with a battery charger before retesting.
  • Start the engine and check the voltage increase. If the numbers are high, the alternator may be good. (Detecting open or shorted dynamo diodes requires more sophisticated test equipment.)
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Step 2: If the alternator passes

  • If the meter leads are connected to the battery terminals and the meter reads between 13.8 and 15.3 volts (engine running, lights and accessories off), the alternator is working properly.
  • If the alternator is working properly, the dead battery may be due to the computer module not turning off when the vehicle is turned off. If the alternator passes the voltmeter test, take the vehicle to a shop and pay a professional mechanic to find and repair the faulty module.
how to test alternator

How To Test an Alternator with A Multimeter?

The following steps describe how to test the alternator with a multimeter.

  • Get a multimeter.
  • Set the multimeter to more than 15 DCV (DC Volts).
  • Make sure the positive and negative terminals of the alternator are clean.
  • Connect the black cable of the multimeter to the negative terminal and the red cable to the positive pole.
  • Look for an ideal alternator reading of approximately 12.6.
  • Start the car and look for a reading between 14.2 and 14.7.
  • A reading above 14.7 means the battery is overcharged, numbers below 14.2 means the alternator is undercharging the battery.
  • Turn on lights, radios, fog lights, fans, and other electronics until the voltage reading does not fall below 13.
  • Make sure the coefficients are above 12.6 when turning off the car.
  • If any of these readings are off, there is probably a problem with the alternator and a visit to the auto repair shop is recommended.

How to test Alternator with Screwdriver?

Testing the alternator with a screwdriver is a simple process that you can easily do at home. You don’t need to take your car to a mechanic or auto repair shop just yet. The process involves several steps, but before you begin, make sure your car is turned off and parked on level ground to prevent it from tipping over.

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To test your alternator with a screwdriver, you need to follow a few simple steps.

Step 1: Put the key in the ignition

The first step is to simply put the key in the ignition. Do not start your car. Just put it in the slot and leave it there.

Step 2: Open the hood of your car.

To test the alternator with a screwdriver, you need to open the hood. Place the lock on the underside of the hood and pull it up. You get access to alternator for testing.

Step 3: Find your car’s alternator

The alternator is a large winding device that is belted over the other engine parts. Depending on your vehicle, you may also see an extra belt, pulley, or gear near the alternator. The main part of this device is connected to the battery through a cable. If you can’t find the manual, ask someone to help you or make one yourself.

Step 4: Put the Screwdriver Near the Alternator

Now that you know where the alternator is, test it with a screwdriver. With the key in the ignition, place a screwdriver near the alternator.

The screwdriver should be close enough to touch, but not so close as to damage other parts of the engine.

When you touch the tip of your screwdriver to the alternator nut near the pulley, you should feel a magnetic pull on the screwdriver.

This means that the alternator is working and there is nothing to worry about. However, if you don’t feel a strong magnetic force, there may be something wrong.

Note: The car key must be in the ON position (on the dashboard) for this test to work.

Now you should know how to test with the screwdriver when the alternator does not work and what are the possible reasons for this problem. So, if you have problems with your alternator, be sure to take your vehicle to an auto body shop for maintenance.

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How to Test Alternator by Disconnecting Battery

Another dangerous way to check the alternator is to completely disconnect the battery from the car. With the car running, disconnect the battery and see if the car continues to run. If so, that means the alternator is still producing at least enough voltage to keep the engine running. If not, it means that the alternator is not producing enough voltage.

Testing a car’s alternator by disconnecting the battery is a simple process. Here is a guide on that topic:

  • Step 1: Testing should always be done in an open and safe area. For example, a corridor is available to perform this test. Make sure children and pets are at a safe distance before starting.
  • Step 2: The vehicle should be checked several times to make sure it is in a stable position. After being satisfied with the position, the test begins by opening the hood of the car.
  • Step 3: Sit in the driver’s seat.
  • Step 4: Once you’re in the driver’s seat, you need to start your car’s engine. If your car’s alternator has a problem, you will have problems starting the car.
  • Step 5: Once the car engine starts, you need to get out of the driver’s seat.
  • Step 6: In this step, the car battery must be disconnected while the car is still in running position.
  • Step 7: Check if the car can drive normally.

Caution: This method is not recommended as it can seriously damage the alternator and leave the car in a worse condition than before. But it works if you’re in a pinch.


We hope this article was very useful for you and helped you to solve the alternator problem. As mentioned above, there are several other ways to test your alternator and you can have your mechanic check it out. But this simple process will give you a good idea if your alternator is working properly.