How Do You Test a Magneto Coil with a Multimeter

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Did you know that a magneto is one of the oldest parts of general aviation aircraft? It is a technology which is over a hundred years old. 

Would you like to know if your magneto coil is faulty or not? Look no further because this guide got you covered. Read further to find out more helpful information about a magneto coil, how it works, and the best way to test it.

What is a Magneto, by the way? 

First things first, a magneto is a compact and dependable electrical generator utilized in small gasoline engines that do need batteries, such as RC model airplanes, outboard motors, jet skis, mopeds, dirt bikes, and lawn equipment.

These devices produce a powerful short electric pulse instead of a continuous current. That’s the reason why magnetos are perfect for putting the spark in a spark plug. That’s what drives internal combustion and powers the engine.

Due to their size and reliability, magnetos are widely utilized in airplanes, not to mention they were the main power source behind the ringer in the first telephones.

How Does it Work?

The concept behind a magneto is the opposite of an electromagnet. 

You see, an electromagnet employs electricity that passes through a coil to create a magnet. On the other hand, a magneto utilizes a magnetic field within the coil’s vicinity, referred to as the armature that creates an electric current. 

Hence, a magneto is composed of three vital components. 

  1. Armature – You can see this in the shape of a “U.” It has a main coil of thick wire and a minor coil of thin wire covered around it in layers. 
  1. Flywheel – It has two powerful magnets used to make a magnetic field around the armature
  1. Electric control unit – Normally, at least a capacitor, and a breaker interrupt the electromagnetic field and lead the resulting electric current away from the magneto to where it’s required.

To create electricity, either the coil must move, or the flywheel must rotate between the magnet’s poles. That explains why old telephones had a hand crank. On every rotation, an electromagnetic field is established in the armature’s coils.

Further, a cam on the electric unit makes contact along with the armature, interrupting the field and making electrical voltage within the main coil. The higher tension of the secondary coil related to the primary coil increases the current’s voltage because it is directed to a spark plug.

Then, the cam breaks contact with the armature. The electromagnetic field reproduces another pulse of electricity. The whole process takes only a few seconds. Amazing, right? 

However, for the engine to run properly, a magneto should be installed, which enables the firing to be timed properly to the compression stroke of the pistons. What’s more, the sparkplug should ignite the air or fuel when it’s compressed in the chamber to produce combustion and direct the piston toward.

In big engines, a distributor is used to time the electrical charges to every spark plug. A recent advance is the usage of small computers to create more dependable timing. 

How to Test a Magneto Coil with a Digital Multimeter?

Now that you understand what a magneto is and its principle, you may like to proceed with testing it. 

  1. Detach the flywheel shroud. You will normally see three bolts on it. However, a few engines made in the middle of the 1970s have four bolts.
  1. Find the magneto. Keep in mind that it is the only component, which has unprotected copper windings. It will connect right away to the spark plug with a rubber casing. Slowly pull that rubber casing off your spark plug. 
  1. Grab your digital multimeter and set it into the Ohms setting. You can also use an ohmmeter in this scenario. Then, manually set the button or dial on the multimeter to the 40 k range. We don’t recommend using the auto-ranging feature because it is unreliable and irrelevant with testing a magneto. 
  1. Touch one probe tip to the engine block and the other to the metal clip located inside the rubber casing. That typically connects to the spark plug. Take note that the readings here are presented in thousands of Ohms or “k” Ohms. 

A decent reading would typically range from 3 k to 15 k. A much higher reading implies a poor connection.

  1. For the next step, make sure you check the connection of the HV or high voltage wire to the coil and the spark plug if you receive a reading that is greater than 15 k. You can also retest after you make sure all the connections are clear and uninterrupted.
  1. Ultimately change the magneto coil if your digital multimeter reads O.L. That clearly suggests an internal short in the magneto.

Are you testing for a 2-cylinder engine? It will help if you test every cylinder to guarantee you don’t have any bad connection in the high voltage wires. If your magneto produces the correct ohms, you need to check if the spark plug is running correctly.

Bear in mind that a wear reading could suggest a magnet that has lost its strength. Ensure you check with your local small engine repair shop to figure out if the magnet could be recharged before you decide to replace the magneto totally. 

Final Thoughts

Knowing your device or vehicle is essential for safety. Spend time with a professional and skilled mechanic to find out more about the inner workings of your machine. 

You don’t need to be a professional technician or a techy person to do this test. As long as you have a reliable and efficient digital multimeter in your hand, you are always good to go. 

We hope this short discussion about testing your magneto coil with a digital multimeter has presented you with some practical insights. What are your thoughts about this article? Please share your thoughts with us by leaving your comments below. We’d like to know about your insights too!

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