How to Test an Alternator on Car?

how to test alternator with voltmeter

Your alternator is arguably one of the most important parts of your engine. Usually found attached to the front of the block, it’s responsible for keeping your battery charged as you drive down the road. Without it, your battery would quickly run out of juice, leaving you stranded. Think your alternator might be failing? Here’s how you can check on its health yourself, with tips from an expert.

Before you test your alternator, here are some signs that your alternator might be weak:

  • Your battery light is illuminated on the dashboard.
  • Your engine is cranking slowly or won’t crank at all.
  • The battery, headlights, and/or interior lights seem weak.
  • The alternator is making rumbling or squeaking sounds.
  • You’ve tried and failed to jump-start your car.
  • The above symptoms could also be signs of a weak car battery.

How to Test an Alternator with A Voltmeter?

Don’t test an alternator by disconnecting the negative battery cable. Use a voltmeter instead to conduct a simple, safe test.

If you have a newer battery but your car won’t start, you may have a bad alternator. If you’re tempted to test an alternator by disconnecting the negative battery cable, don’t do it. A good alternator may indeed keep the engine running without the negative cable, but this was never a good test.

In the pre-computer days, you could pull it off without damaging anything. Today, you risk frying every electrical device in your vehicle. The second you disconnect the battery; the voltage regulator pegs the alternator to put out maximum power.

With no battery in the circuit to act as a buffer, the alternator can put out up to 150 volts, depending on engine rpm. When the smoke clears, that “simple test” could end up costing you several thousand dollars for new electronics.

Step 1: Conduct a Voltmeter Test

  • Get a cheap voltmeter.
  • With the engine off, battery voltage should be between 12.5 and 12.8 volts. If it’s below that, charge the battery with a battery charger before you conduct the test again.
  • Then start the engine and check for increased voltage readings. If you see higher readings, chances are the alternator is good. (More sophisticated testing equipment is needed to detect an open or shorted alternator diode.)

Step 2: If Your Alternator Passes

  • If you connect the meter leads to the battery terminals and the meter shows that it’s in the 13.8 to 15.3-volt ranges (engine running, lights, and accessories off), that means the alternator is working as it should be.
  • If the alternator is functioning correctly, it’s likely that your dead battery was caused by a computer module that isn’t shutting down when you turn off the car. If your alternator passes the voltmeter test, get your vehicle into a shop and pay a professional mechanic to find and correct the misbehaving module.
how to test alternator with voltmeter

How To Test an Alternator with A Multimeter?

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

  • Get a multimeter.
  • Set your multimeter to DCV (DC Volts) above 15.
  • Make sure your alternator’s positive and negative terminals are clean.
  • Put the multimeter’s black cable to the negative terminal and the red cable to the positive terminal.
  • Look for an ideal alternator reading of around 12.6.
  • Start the car, and look for a reading of between 14.2 and 14.7.
  • A reading of over 14.7 means the battery is being overcharged, while a reading under 14.2 means the alternator is undercharging the battery.
  • Turn the lights, radio, fog lights, fan, and other electronics on, making sure the voltage reading doesn’t go below 13.
  • When you shut off the car, make sure the reading is above 12.6.
  • If any of these readings are off, you likely have an issue with your alternator and may want to visit an auto repair shop.

How to test Alternator with Screwdriver?

Testing Alternator with Screwdriver is a simple process that you can easily do at home. You don’t have to take your vehicle to the mechanics or auto repair shop just yet. The process involves a few steps, but before you start make sure that the car is off and parked on level ground where it won’t roll away.

To test Alternator with Screwdriver you will need to follow simple steps:

Step 1: Insert Key into Ignition

The first step is to insert your key into the ignition simply. Don’t start the car up; just insert it into the slot and leave it there.

Step 2: Open Your Car Hood.

In order to test the alternator with a screwdriver, you have to open your hood. Find the latch on the underside of the hood and pull it up. This is how you can access your alternator for testing.

Step 3: Find the Alternator of The Car

The alternator is a large coil-shaped device with a belt attached to it that goes over other engine parts. Depending on your vehicle, you may see additional belts, pulleys, or gears near the alternator as well. The main part of this device is hooked up to a battery through a cable. Ask someone to help you out or get yourself a manual if you can’t find it.

Step 4: Put the Screwdriver Near the Alternator

Now that you know where to find your alternator, use your screwdriver to test it. Put the screwdriver near the alternator when your key is in the ignition.

You have to hold the screwdriver close enough that you can touch it, but not so close that you’ll damage any other parts of the engine.

Touch the tip of your screwdriver’s blade to the alternator nuts located near the pulley and you must feel the magnetism that attracts the screwdriver.

This means that your alternator is working and you don’t have to worry about it. But if you don’t feel a strong magnetic pull, you might have a problem.

Note: Your car key must be in the “On” position (Dashboard lights up) for this test to work.

Now you should know how the alternator will test with a screwdriver when it is not working and the possible causes of this problem. So, make sure that you take your vehicle to the auto repair shop for maintenance and repairs if you have issues with the alternator.

How to Test Alternator by Disconnecting Battery

Another, riskier method to check the alternator involves disconnecting the battery from the car altogether; With the car running, disconnect the battery and see if the car continues to run. If it does, that means the alternator is still producing sufficient voltage to at least keep the engine firing. If it doesn’t, that means the alternator isn’t producing enough voltage.

Testing the alternator of the car by disconnecting the battery is a simple process. Here is your guide in that regard:

  • Step 1: You should always conduct the test in an open and safe place. For example, you can utilize the driveway for making this test. Before you start, ensure that children and pets are at a safe distance.
  • Step 2: You should check several times to ensure that the vehicle is in a stable position. If you are satisfied with its position, the testing starts with opening the car hood.
  • Step 3: Now get to the driving seat.
  • Step 4: Once you are in the driving seat, you should initiate the car engine. In case there are issues with the car alternator, you will face some issues in starting the car.
  • Step 5: After the car engine starts, you need to come out of the driving seat.
  • Step 6: At this stage, you need to disconnect the car battery while the car is still in a running position.
  • Step 7: Check if the car can run as usual.

Please note: We wouldn’t recommend using this method, as it could cause some serious damage to the alternator and leave your car in worse shape than it was before. But it can work if you’re in a pinch.


We hope that you find this article very useful and that it helped you to solve your problem with the alternator. As mentioned above, there are a few other ways to test your alternator and you can ask your mechanic to check them. But using this simple process will give you a pretty good idea of whether or not your alternator is working properly.

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