How to Test a Car Alternator with a Multimeter

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Imagine you are on the road or adventuring when all of a sudden, your car breaks down, or your dash lights go off in the middle of nowhere. Such an experience can be the worst and most frustrating. The possible causes of such problems could be the failure of your car’s alternator. You can avoid experiencing these problems by regularly testing the alternator. 

A car alternator is a significant element of the charging system of your vehicle when your car is moving. It generates electric power to the car’s other components. The alternator produces alternating current and is responsible for charging the car’s battery and providing electrical power for other electrical systems, such as entertainment systems, horns, lights, instrument panels, among others. 

The alternator works by converting the car’s engine mechanical energy into electrical energy through electromagnetism. Some of the power is channeled to recharge the battery while the rest is distributed to other systems. This is what keeps your car moving. 

Signs of problems with the alternator

Since the alternator distributes power to most electrical systems in the car, if it has issues, then you are doomed to encounter various issues with the operation of your vehicle. Here are some of the signs you should look out for to know if your alternator has problems. 

  1. You may experience the headlights becoming dimmer than usual. This can be risky, especially when driving at night or in foggy weather conditions. Also, the lights in your instruments panel may fail to turn on, causing you not to see your speed and other data, which can lead to accidents due to high speed.
  2. When you encounter the failure of your car’s air conditioner or the cabin heater, it can be an indication of a faulty alternator.
  3. If you see a battery-shaped cautioning light once you turn on the ignition, it is an indication your charging system has a problem. Though this warning may not necessarily mean your alternator has a problem, in some cases, it could be. Nevertheless, you need to conduct further testing to be sure.
  4. Experiencing whining sound coming from the alternator.

If you are experiencing any of these symptoms with your vehicle, it is essential to test the alternator. There are many devices you can use to do the test. Nonetheless, a multimeter is the best tool.

A multimeter

A multimeter is an electronic device used to measure resistance, voltage, continuity, current, and other values. You can use a multimeter to carry out simple tests as well as sophisticated tests to detect any faults in electronic items. Hence, they are preferable to other testing devices. Multimeters are of two different types, analog, and digital multimeters. 

Analog multimeters

These are multimeters that use analog techniques to display measurement readings. They have graduated scale and a needle that moves over it, marking the values. Analog multimeters are great because you can use them to detect slow voltage changes. They have high sensitivity and slow resistance, which makes them exemptional.

Digital multimeters

These are electronic meters whose measurements readings display on an LCD screen. They are common because they are small in size and rea quite easy to operate. Digital multimeters are preferable due to their high accuracy, high digital resistance, and the fact that they are easy to read.

Digital multimeters are preferable than analog multimeters mainly because they do not have moveable parts, thus, making them not to wear out quickly. However, they are more expensive than analog multimeters.

To test if the alternator is working well, you need to measure the voltage, resistance, and voltage. Using a multimeter is pretty straightforward, and, in this article, we will focus on using a digital multimeter. 

Measuring the voltage of an alternator using a multimeter

  1. Turn off your vehicle’s engine and open the hood to get clear access to the battery.
  2. Trace the alternator and confirm that the belt is not loose.
  3. Connect the probes in the correct sockets. That is, the black probe to the common port while the red or positive probe in the socket labeled V.
  4. Turn your meter’s dial to Volts setting and select AC, since an alternator produces alternating current.
  5. It is recommendable to connect the probes parallel to the points in which you measure voltage.
  6. Hold the black probe to the first point of the test of the wiring.
  7. Turn on the meter
  8. Hold the red probe to the other point of the test.
  9. Do the reading and record it.

Measuring current flowing from the alternator with a multimeter

  1. Turn off your vehicle’s engine.
  2. Attach the probes in the correct sockets.
  3. Turn your meter’s dial to measure current, and select the highest range. That is the Amp setting.
  4. To measure current, you should insert your meter in the circuit in series.
  5. Turn on the meter.
  6. In case you realize the range is extremely high, set it to a lower range.
  7. Take and record the reading.
  8. Once you finish measuring current, remove the setting from Amps to V. This is to prevent damaging your multimeter when switched on accidentally.
  9. Switch off the meter.

Measuring the resistance of the alternator using a multimeter

  1. Turn off the engine of your car.
  2. Insert the probes correctly. The black probe onto the common socket and the red probe on the socket labeled V.
  3. Turn your dial and change the setting from Volts to Ohms.
  4. Set the range to the lowest Ohm. In most cases, the range is usually 200 ohms.
  5. Touch the tip of a probe at the end of each component in the test.
  6. Take the reading and record it.
  7. If your meter displays “I,” then the resistance is higher than the meter can display. Change the dial to a higher range and repeat the above procedure until you see a value displayed.


Conclusively, once you detect a minor problem with electric systems in your car, it is essential to test your alternator to detect the problem and fix it before it magnifies. With your multimeter, you do not need to wait for a mechanic to check it for you, you can do it yourself following the able steps. And as the saying goes, ‘a stitch in time saves nine.’

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