Testing an Alternator: The Complete Guide | Autance

There are different ways by which problems with the alternator can manifest themselves. These are often dependent on the type…

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Testing an Alternator: The Complete Guide | Autance © Testing an Alternator: The Complete Guide | Autance

There are different ways by which problems with the alternator can manifest themselves. These are often dependent on the type of problem or issue that this part of the automobile has. The good thing is that assessing the presence of an alternator problem is quite easy. All you need is a voltmeter or multimeter plus a few other things and you could already be on your way to diagnosing a problematic alternator. Here is your complete guide to testing the alternator in your automobile.

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First Things First: The Alternator

Whenever we experience issues with the electricals in our vehicles, we always think about two things: the battery and the alternator. The battery produces a type of current known as direct current because it moves electricity only in one direction: from the battery to the car. On the other hand, the alternator derives its name from alternating current. Hence, the alternator is that part of the car which produces an alternating type of current. It produces the kind of electricity that you would have in your house.

Both the battery and the alternator work together to generate electricity for the different electrical parts of the automobile. This can include the lights, horn, entertainment system, and instrument panel, among others. The alternator produces electricity through electromagnetism. It converts the mechanical energy of the car’s engine into electrical energy. The alternator channels some of this energy to recharge the battery. Some of the energy runs the different electrical systems of the car.

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Why Test Your Alternator

It is obvious that the alternator is a critical part of a modern vehicle’s electrical and charging system. It recharges the battery of your car to keep it working. The alternator also powers up the car’s electrical system during operation. Hence, if there is a problem with the alternator, then you will experience a reduction in the functionality of these electrical systems.

You may notice your headlights getting dimmer than before. This can be dangerous when driving at night or in zero-visibility conditions. You may not have power to your air conditioning unit or your car’s cabin heater, making your ride unbearable. The lights on your instrument panel may not turn on, leaving you in the dark as to your speed and other important vehicle data. Your car entertainment system will also not provide you with a remarkable experience. Any component in your car that requires electricity will start to malfunction.

It is for this reason that you should check the condition of the alternator. This is to make sure that it can generate sufficient voltage to operate the different electrical components of your car. Hence, if you see a battery warning indicator light in your instrument panel, it is always a good idea to test the voltage output of your car.

Testing the Alternator with a Voltmeter

One of the easiest ways to test the integrity of your alternator is by using a very simple device called a voltmeter. Here’s how to perform the test.

  • Measure the Base Voltage of the Battery

The very first thing you need is to measure the voltage output of your car’s battery. To do this, make sure that your battery has a full charge.

Get your voltmeter and set it to DC mode at 20 volts. Make sure that your voltmeter has brand new batteries to ensure more accurate readings. Check if the device registers 0.000 volts on the display screen. Be familiar with the color-coded cables of your voltmeter. The red-colored cable is always positive while the black-colored cable is always negative. Depending on the type of voltmeter that you have, the cables may have alligator clips or pencil-like probes. Most mechanics prefer the latter because it is so much easier to use.

Make sure that the battery terminals are free of deposits or any debris. If the terminals are not clean, it can lead to inaccurate voltage readings. Connect alligator clips or the probe tips to their respective battery terminals: positive to positive and negative to negative. Upon contact, you should see a change in the voltmeter display. A fully-charged car battery will often have 12.6 volts. Take note of this value. If it helps, you may want to write it down.

If the base battery voltage reading is between 6.5 and 10.8 volts, you may want to recharge the battery first or replace it with a new one. If there is no issue with your battery, then you may want to have it load-tested. Keep in mind that testing the alternator requires a fully-charged battery.

  • Measure the No-Load Voltage of the Charging System

Get an assistant to start your car’s engine. Let the engine run for a while at idle. Ask your assistant to maintain the speed of the engine at 1500 RPM. Also tell your assistant to switch off all of the car’s accessories. Turn off any vehicle lights, including those in the instrument panel. Turn off the stereo and every other accessory that may run on electricity.

Connect the alligator clips or the test probes on the battery terminals again. You should see an increase in the voltage reading between 0.5 and 2.0 volts. Hence, if your base battery voltage is 12.6 volts, then you should see a reading of anywhere between 13.1 to 14.6 volts.

If the increase is more than 2.0 volts or 14.7 volts on the voltmeter display, it can mean that you have an overcharging condition. This can signal a problem in your car’s alternator or its electrical system.

If there is no noticeable change in the voltmeter reading, it is possible that you have a malfunctioning charging system. This is also true if you notice the voltage output to be lower than your base battery voltage reading. Check that your system’s wires and connectors are in good condition. If the voltage reading remains the same, then you may have a problem with your alternator or the voltage regulator.

  • Measure the Load Voltage of the Charging System

If your charging system passes the No-Load Voltage Test, it’s time to measure the system’s voltage output at full load.

Restart your engine and let your assistant maintain the engine speed at 2000 RPM. Switch on all of your car accessories such as the wipers, headlights, air conditioning, and radio, among others. Reconnect the voltmeter and read the display.

In this test, you will try to determine your system’s charging voltage. Hence, you will subtract the base battery voltage reading from the load voltage reading. It is ideal that you get a charging voltage of at least 0.5 volts higher than the baseline reading. In our example, you should at least have 13.1 volts on the voltmeter display.

Some put the critical levels at 13.0 and 15.0 volts. Anything lower or higher than these values can point to a faulty alternator.

  • Test the Alternator Diode

To check for a bad alternator diode, you may want to reset your voltmeter to the lowest possible AC voltage scale setting. Have your assistant start the car’s engine and let it run for a few minutes at 1200 RPM. Connect the voltmeter probes on their respective battery terminals. You should read “0.00” in the display.

If there is a fraction of a volt in the display (i.e., 0.001 volts), it only means you have a bad alternator diode. Unfortunately, you cannot replace the diode alone. You will need to replace the entire alternator.

Testing the Alternator When You Don’t Have a Voltmeter

Using a voltmeter is one of the most practical and most accurate ways you can determine the condition of your car’s alternator. However, it is not the only test available for you. You can also evaluate your alternator without ever using a voltmeter. Here’s how.

  • Assess the Condition of the Alternator Drive Belt

The alternator draws its energy from the mechanical power of the engine via the drive belt that connects to the alternator. One way you can check for the integrity of the alternator is by evaluating this drive belt. A worn-out drive belt or a loose belt can lead to charging problems.

  • Listen for a Characteristic Sound from the Engine

If you try to listen to your engine while it is running, you may hear a very distinct sound. Mechanics describe the noise as loud clicking sound. It is often an indication of a problematic alternator. Of course, the problem with this approach is that you have to have a good ear.

  • Perform the Headlight Test

It is best to perform the headlight test in a darkened environment so you can evaluate the brightness (or dimness) of your headlight. Switch on your car’s headlights and take note of its brightness. Next, start your engine. You may notice the brightness of the headlight to fade a little bit. After some time, however, it should go back to its original brightness level. This is an indication that the alternator is doing its job of charging your system.

If you are not sure about how to test your alternator, a mechanic should always be a good person to call. Remember, as easy as it is to test an alternator, there could be some things that may require a more professional evaluation.


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