How To Test an Alternator with A Multimeter?

A failing alternator can leave you stranded on the side of the road. Here’s how to make sure your alternator is working properly, with tips from an expert. Is there a test for a bad alternator? Your multimeter will tell you if your alternator is working as it should.

Your multimeter will tell you if the alternator’s electricity is not getting to the battery. Your multimeter will help you do a voltage drop test if you have a cable problem or a dirty connection.

Similar to testing your car battery, a multimeter can be used to quickly determine if your car’s alternator is adequately keeping the car battery at the correct charge and also running the electrical system at the correct voltage.

In most cases, car batteries have an open-circuit voltage of between 12.45 and 12.6 volts. However, your car alternator operates at a slightly higher voltage than the battery voltage for a number of reasons. For one, a phenomenon known as voltage drop causes the voltage at the alternator to decrease as it travels through the hundreds of feet of wiring that snake through a vehicle.

This means that the voltage at the alternator can be 14.5 volts, for example. The voltage at the battery may be 14.2 volts, but way out at the rear tail light bulb, that voltage may be as low as 13.5 volts due to a voltage drop.

For this reason, the operating voltage measured on the battery by each vehicle may differ, but they should all be higher than the open-circuit voltage of the battery. In most cases, you will see a voltage of 14.00 to 14.5 on most vehicles when the vehicle is running and all accessories are off. Now that you know that, it’s time to test the alternator.

How To Test an Alternator with A Multimeter

Before reading any further, we recommend that you consult your car’s owner’s manual for specific instructions on how to check the condition of your alternator. The manufacturer’s suggestions may differ from the instructions below.

Related Posts: What Is an Alternator in Car and How Does It Work?

How Do You Test an Alternator?

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

  1. Get a multimeter.
  2. Set your multimeter to voltage and ensure it’s adjusted to 20 DC volts, or if your voltmeter does not have incremental settings then simply set it to DC volts.
  3. Make sure your alternator’s positive and negative terminals are clean.
  4. Put the multimeter’s each probe to the correct terminal, touching negative to negative and positive to positive. Your multimeter and battery are probably both color-coded, so the negative terminal and probe will be black while the positive terminal and probe are red.
  5. Simply check the voltage of your battery when the car shut off. Take note of that number; it should be somewhere around 12.6.
  6. Then, start the car and check the voltage at the battery again while the vehicle is running. Ideally, it should be in between 14.2 and 14.7. “That’ll tell you that the alternator is producing voltage.”
  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.
  8. Turn the lights, radio, fog lights, fan, and other electronics on, making sure the voltage reading doesn’t go below 13.
  9. When you shut off the car, make sure the reading is above 12.6.

Note: – With the engine off, the 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.

If any of these readings are off, you likely have an issue with your alternator and may want to visit an auto repair shop.

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. While a good alternator can keep the engine running without a negative wire, that has never been a good test.

In the days before computers, you could pull it off without damaging anything. Nowadays you risk burning every electrical device in your vehicle. The second you disconnect the battery; the voltage regulator triggers the alternator to deliver maximum power.

Without a backup battery in the circuit, the alternator can deliver up to 150 volts, depending on the engine RPM. When the smoke clears, this “simple test” could end up costing you several thousand dollars in new electronics.

How to Test an Alternator with A Multimeter?

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, the 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 the Alternator with a Screwdriver?

Testing the Alternator with a 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 the Alternator with a 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 (The 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.

Conclusion

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.