Testing A Dynamo and Checking Output

How To Test A Dynamo and Check Output?

Although an alternator in many vehicles has replaced the dynamo, it was common among older models. This is still a significant portion of the vehicles available on our roads. Some drivers prefer the comfort and durability they offer. However, their dynamo can easily cause problems, and understanding on how to test a dynamo and check output is absolutely critical to ensure you have peak performance.

The dynamo is the core component of the car’s charging system. It powers many electrical components such as your headlights and stereo system. The dynamo also ensures your battery is fully charged and that you don’t have problems while starting your vehicle. It is thus crucial that you test the status of your dynamo to ensure it is delivering the best performance.

When should you test the Dynamo?

You can test your dynamo immediately you suspect the electrical components on your vehicle aren’t working well. Some indicators will show you there might be a problem. These include;

  • When the dynamo doesn’t turn, the vehicle dynamo should begin turning when you start the car engine. It should also adjust the drive belt’s tension to ensure it is in place. Your output could be low, or the dynamo could be damaged if it remains stationary in this instance.
  • Your car fails to start or releases a cracking noise: your car will not start correctly when the output from your dynamo is low. It will release cranking noises before shutting down or failing to start completely. This means you should test the status of your dynamo and its output.
  • The headlights get dimmer with time: The vehicle’s headlights will dim with time if the dynamo doesn’t have proper output. It means it doesn’t power the components well and can only sustain it for short periods instead of continuously. This means you have to test the dynamo.

Methods to test a dynamo and check output

There are various ways of testing a dynamo and checking output depending on the extent of the damage. You can tell the extent based on how your dynamo has been performing and the severity of its problems.

#1. Checking dynamo output with tester or voltmeter

Performing the Tests with A Voltmeter

Measuring the voltage across the battery terminals shows you whether the dynamo is undercharging or overcharging it. It will be based on the results you get from your tests. A tester or voltmeter will give you better and more accurate results. The voltmeter setup will also depend on the earthing system on your vehicle. However, this shouldn’t be a problem as it will work the same regardless of a positive or negative sign on the earth system.

First, start your engine and let it run for a while. You can now check the output readings on your digital voltmeter to determine whether the output is sufficient. Performing the test again with additional electric connections in case you get unsatisfactory results ensures you correct any errors that might interfere with your results.

Find a strong cable and link the terminals on your dynamo. They are easy to identify as they are marked D and F. Ensure they are tightly in place and start the engine once more. Leave it in idle mode and let it get to about 900 rpm. You can now perform the test again by connecting the voltmeter’s positive clamp to the D terminal of the dynamo. Connect the second clamp to the earth and monitor the readings.

Checking Output

The correct readings for an effective voltmeter should be between 13.5V to 14.5V. This shows that your dynamo works well and can power your electric components and charge your battery. However, if the readings are lower than 13V, it shows a significant problem with the dynamo. You will need to repair it or replace it with a new one. Getting an alternator instead will also help fix any engine problems.

Testing With a Circuit Tester

A circuit tester can also be used for this test if you don’t have a voltmeter. Here, you should ensure the vehicle runs slightly over 2000 rpm. Connect your circuit tester to the vehicle’s electric components, such as the headlights. It should sustain a 4V reading to show the dynamo is working well. If this drops at any point within your test, the main cause points to a faulty dynamo or poorly installed field coils. You can check on these first before repairing or replacing the dynamo. This is a simplified process for testing a dynamo and checking output.

#2. Testing The Dynamo Cables

Testing the dynamo cables for faulty connections or power leaks is also an important aspect while testing the dynamo output. First, connect a binding cable to the D and F terminals on your dynamo to begin. You can now start your engine and let it warm up until you get about 900 rpm reading.

You can now connect the positive clamp of the voltmeter to the D terminal on the dynamo. Additionally, connect the other clamp from your voltmeter to the F terminal. Ensure that you are keen on these cable connections to get the proper results and better readings.

Checking Output and Evaluation

You can now check on the readings on your voltmeter to evaluate the performance of your dynamo. The readings on a properly working dynamo should be over 14V. It shows the power supply is consistent and that it is working well. The dynamo will be charging as well as it should be.

However, if the reading falls below 13V, there is an error with your dynamo. It also shows there might be a fault with the control box and that it too should be repaired or replaced depending on the extent of the damage.


Regularly testing a dynamo and checking output ensures you have the best performing car components. It also lets you know the best time to replace or repair your dynamo. You can also evaluate the option of upgrading to an alternator for your car’s charging system  once you have these results. Visit your local mechanic in case you encounter any challenges.