How to test a magneto coil with a multimeter
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With modern cars, there is no end to where problems can come from.
However, old cars and engines are another component to think about; magneto coils.
Magneto coils are important components in the ignition system of small aircraft, tractors, lawn mowers, and motorcycle engines, among others.
Many people don't know how to check these components for problems, and we're here to help.
In this guide, you will learn the following:
- What is a magneto coil and how does it work?
- Symptoms of a Bad Magneto Coil
- How to test a magneto coil with a multimeter
- And FAQ
What is a magneto coil and how does it work?
Magneto is an electrical generator that uses a permanent magnet to create periodic and strong current pulses, rather than to constantly supply it.
Through its coils, it applies this strong current pulse to the spark plug, which ignites the compressed gases in the engine's ignition control system.
How is this momentum created?
There are five components that work together to make a magneto work:
- Арматура
- Primary ignition coil of 200 turns of thick wire
- A secondary ignition coil of 20,000 turns of fine wire, and
- Electronic control unit
- Two strong magnets are built into the engine flywheel.
The armature is a U-shaped element located next to the flywheel and around which two magneto ignition coils are wound.
According to Faraday's law, any relative movement between a magnet and a wire induces current and flow in the wire.
The engine flywheel has two magnets embedded at a specific point.
When the flywheel rotates and this point passes the armature, magnetic fields from the magnets are periodically applied to it.
Remember that the coils of wire are at anchor, and according to Faraday's law, this magnetic field supplies the coils with electricity.
Here you can see how to route the wire.
This periodic supply of current accumulates in the coils and reaches a maximum.
As soon as this maximum is reached, the electronic control unit activates the switch and the contacts open.
This sudden surge sends a strong electrical current to the spark plugs, starting the engine. All this happens in a few seconds.
Now the magneto may no longer serve its purpose effectively, and coils are usually the culprit.
Symptoms of a Bad Magneto Coil
When the magneto coil is faulty, you experience the following
- The check engine light comes on on the dashboard
- Difficulty starting the engine
- Greater distance traveled by gas
- Lack of acceleration power
If you notice any of these, the problem could be with the magneto coils.
As with testing other electronic devices and components, you will need a multimeter to test these coils.
How to test a magneto coil with a multimeter
Remove the rubber shroud, set the multimeter to ohms (ohms), and verify that the ohm range is set to 40k ohms without autoranging. Place the multimeter probes on the copper winding of the magneto and the metal clamp under the rubber casing. Any value below or above the 3k to 15k range means the magneto coil is bad.
This is only the most basic and most direct description of what you need to do, and more explanation is needed to properly understand the process.
- Disconnect the flywheel housing
The first step is to detach the flywheel housing from the entire setup.
The flywheel housing is a metal casing that covers the magnet and is held in place by three bolts.
Engines made in the 1970s usually have four bolts holding the shroud in place.
- Find the magneto coil
After the shroud is removed, you will find the magneto coil.
Finding the magneto coil shouldn't be a problem, as it's the only component behind the shroud with exposed copper windings or a metal core.
These copper windings (armature) form a U-shape.
- Remove rubber cover
The magneto coil has wires protected by a rubber casing that go into the spark plug. To test this, you must remove this rubber boot from the spark plug.
- Set the multimeter scale
For a magneto coil, you measure the resistance. This means that your multimeter's dial is set to ohms, represented by the symbol omega (Ω).
Instead of autoranging, you manually set the multimeter to the 40 kΩ range. This is because automatic ranging gives very unreliable results.
- The position of the multimeter probes
Now, in order to measure the resistance inside the magneto coil, two things need to be done. You want to measure the primary and secondary coils.
For the primary coil, place the red test lead on the U-shaped winding and ground the black test lead to a metal surface.
To measure the secondary winding, place one of the multimeter probes on the U-shaped metal core (winding), and insert the other probe into the rubber casing on the other end of the magneto.
While this probe is in the rubber housing, make sure it touches the metal clip on it.
Here is a video that shows exactly how to measure the primary and secondary magneto coils.
- Rate results
After the probes have been placed on different parts of the magneto, you check the multimeter reading.
Readings are in kiloohms and should be between 3 kΩ and 15 kΩ, depending on the type of magneto being tested.
Referring to the manufacturer's manual will help you with this. Any reading outside of this range means your magneto coil is bad.
Sometimes the multimeter may display "OL", which means there is an open circuit or short circuit between these two points. In any case, the magneto coil needs to be changed.
In addition to these, there are certain tips that you should pay attention to.
If the multimeter reads above 15 kΩ, the connection between the high voltage (HV) wire on the coil and the metal clip that goes to the spark plug may be the culprit.
If all of this is checked and the magneto shows the correct resistance readings, then the problem could be the spark plug or weak magnets in the flywheel.
Check these components before deciding to replace the magneto.
F.A.Q.
How many ohms should the ignition coil have?
A good magneto coil will give readings of 3 to 15 kΩ ohms depending on the model. Any value below or above this range indicates a malfunction and you may need to replace it.
How to check magneto for spark?
To test the magneto for spark, you use a spark tester. Connect the alligator clip of this spark tester to the magneto coil, try turning on the engine and see if this tester flashes.
How to test a small motor coil with a multimeter
Simply place the multimeter's leads on the "U" shaped metal core and the spark plug's metal clamp on the other end. Readings outside the range of 3 kΩ to 5 kΩ indicate that it is defective.
How do you test a magneto capacitor
Set the meter to ohms (ohms), place the red test lead on the hot connector, and ground the black test lead to a metal surface. If the capacitor is bad, the meter will not give a stable reading.
How many volts does the magneto put out?
A good magneto puts out about 50 volts. When a coil is inserted, this value increases to 15,000 volts and can be easily measured with a voltmeter.
