In this post, you’ll learn how to test a magneto coil with a multimeter and evaluate its efficiency.
If you have a magneto-based ignition system in your motorcycle or lawnmower, with time, you may face a low spark in the plug, causing failure in ignition. It indicates a fault in the magneto, and it’s time to examine your magneto coils.
Let’s explore further…
Bad magneto symptoms
A vehicle cannot produce enough spark required for ignition, or there is no spark.
Difficulty in starting the engine.
Lack of acceleration as the engine loses power.
The dashboard engine check light glows.
How to test a magneto coil with a multimeter?
We measure resistance between different components of a magneto coil using a multimeter. The purpose is to evaluate the efficiency of magneto coils and wires if they produce current and carry it to the spark plug for ignition.
To test a magneto, you should have a multimeter and the recommended resistance range of your magneto model to compare with the multimeter readings.
There are three components on the magneto coil that you have to test in different ways.
- The metallic core is an A-shaped iron frame.
- A kill switch terminal is a small piece of metal coming out of the rubber casing of the coils. The kill switch is grounded with kill wires to shut down the magnetic field and spark.
- HT lead or spark plug cable that carries current from the secondary coil to the spark plug for ignition
Test 1: Primary coil or kill terminal test
The first test is carried out on the magneto coils’ metal base (iron core) and the kill switch terminal.
Set up multimeter
Set your multimeter to the resistance mode and adjust the range in ohms. You can also adjust it for auto-ranging. Auto range resistance setting will measure any readings between ohms and kilo-ohms.
Connect probes to testing points
Turn on your multimeter. Attach one probe of the multimeter anywhere on the coil base (iron frame). Put the other probe on the kill switch terminal.
You can place any probe on either component because resistance is non-directional.
To get an accurate reading, you can use an alligator clip at the end of probes to firmly hold the components of the coil.
If the multimeter reading is constantly changing, it could be due to layers of rust formed on a component, hindering a good connection. Remove it by scratching the rusted part with a pin or metal.
If the multimeter displays OL, there is no continuity in the coil base and the kill switch. It could be due to a blown-up magneto or a short circuit. In such cases, the only option is to replace it with a new one.
Compare readings with the recommended range.
Compare the multimeter reading with the resistance range of your specific magneto model.
Coils in different engines will have different resistance ranges, and you can find your magneto model resistance in the manufacturer’s manual.
If the multimeter measures resistance below or above the range, your coils are not efficiently working.
Usually, a primary working coil will have 2-8 ohm resistance, but a good coil will have 3-5 Ω resistance.
Test 2: Secondary coil resistance test
This test is carried out to measure the efficiency of the secondary coil components. We measure resistance between the iron base of the magneto coil and the end of HT lead, going from the secondary coil to the spark plug.
Please turn on the multimeter and set it to the resistance setting in the kilo-ohms range. As the secondary coil is larger in terms of size and producing current, that’s why its resistance is also higher as compared to the primary coil.
Attach one probe to the magneto iron core (metal base) and the other probe to the end of the spark plug wire inside the cap.
Read and Compare Multimeter Readings
If the multimeter displays OL, there is no continuity, and the current cannot flow to the spark plug due to a short circuit or dead component.
If the multimeter shows a continuously changing value, try to scratch the probe inside the spark plug cap and on the iron core to remove rust. Unstable reading is due to rust hindering a proper connection in probes and components or the windings are loose.
After measuring the resistance between the spark plug and the iron core, detach the probe from the iron core and place it over the kill wire terminal. This resistance must be the same as in the previous reading.
Evaluate Multimeter Reading:
After you get resistances, compare it with the resistance range of your magneto model, available in the manufacturer manual; otherwise, you can find it online.
A working secondary coil will have a resistance between 2-15 kΩ. While one having resistance between 3-8 kΩ is considered a good coil. A coil with resistance above or below the range might be overused, rusted, or faulty.
This is how we test a magneto coil to evaluate its efficiency in producing current and carrying it to the spark plug for ignition.
What is a magneto?
A magneto is a small generator in the engine that produces pulses of high-voltage alternating current, which is used in ignition through the spark plug.
A fixed magnet on the rotating flywheel generates a changing magnetic field that drives electrons in the magneto coils and produces current.
Magnetos are mainly used in piston aircraft engines, tractors, marines, lawnmowers, and motorsports like snowmobiles and motorcycles.
How does magneto work?
Structure and Components of Magneto
A magneto coil consists of two copper wire coils. The first one is a primary coil made of around 70 loops of thick wire.
Over the primary coil is a large secondary coil made of thin wire and thousands of loops. Both coils are fixed around an iron core, and each component is.
The primary and secondary coil in magneto is essential in generating a current that is used to produce a spark and acts as a transformer.
Also, there is a tiny insulated trigger coil, also known as an initiator, made of copper windings wrapped around a small iron piece.
Each time the rotating magnet passes through the trigger coil, the magnetic field pushes electrons of the coil toward the primary coil.
Working of Magneto
As we start the engine, a magnet fixed on the flywheel rotates with it, and each time the magnet passes the electronic coil, it generates an electric charge.
The flywheel rotates quickly, and the magnet passes the trigger coil multiple times a second. The trigger coil also feels the magnetic field from the rotating magnet.
Each copper atom has a free electron, and a wire consists of billions of atoms, which means billions of free electrons in a wire; that’s why copper is an excellent conductor.
A magnet attracts the free electrons and pulls them in a uniform direction.
As the identically charged particles repel, the free electrons in the coil will repel each other. Each time magnet passes through these electrons. Its magnetic field will drive electrons in a uniform direction. The motion of electrons produces an electric current.
After the current moves from the trigger coil to the primary coil, it generates a magnetic field. The magnetic field in the primary coil also influences the secondary coil.
As a result, the flow of electrons in the secondary coil increases multiple times due to its larger size. This is how the magneto acts as a transformer by magnifying a low voltage in the primary coil to a higher voltage in the secondary coil.
From the secondary coil, the current is carried to the spark plug via HT (high tension) lead. All processes repeat when the flywheel rotates, and the magnet comes near the coils in each rotation.
This is how flywheel magneto generates pulses of high voltage alternating current for the ignition system.
How to test a magneto on a motorcycle?
First, check if there is a spark in the spark plug. Remove the spark plug, insert a screwdriver tester inside the spark plug cable and put your finger on top. Crank the bike engine(run). If the spark plug has a spark, the tester screwdriver will light(glow). Now we’ve to check the motorcycle coil.
We perform a resistance test using a multimeter to test a motorcycle magneto. Remove the plug/connector from your motorcycle coils’ power and trigger lead.
Set the multimeter to resistance mode ‘Ω,’ and connect both probes to power and trigger terminals on the coil. If your motorcycle coil(magneto) is fine multimeter should read 3-5 ohms.
Now set the multimeter to kiloohms and connect one probe to the power/trigger terminal(lead) and the second probe inside the spark plug (inside the spark plug cap).
A fine magneto should read around 10 kΩ between the power/trigger lead and the spark plug. If the multimeter displays OL or 1, it means there is no continuity, and it is a reason for no spark while ignition.
How do I know if my magneto is bad?
If your bike engine cannot produce enough spark required for ignition, or there is no spark, it’s a symptom of a bad magneto.
A faulty magneto generates a low-power spark or cannot carry to the spark plug due to a damaged or burnt component, short circuits, or faulty wiring in the coil or the spark plug cable.
Is a magneto AC or DC?
A magneto coil is a mini generator in engines that produce pulses of alternating current due to a continuously changing magnetic field.
Magneto is used to provide ignition power instead of auxiliary power to operate lights and other heavy loads such as air conditioners or cooling fans.
What is the difference between a magneto and a.c generator?
Magneto and AC generators produce current, but their working principle differs. In a magneto, a fixed magnet on a flywheel rotates around the coils to create a magnetic field, generating a current.
Inside an AC generator, a coil rotates around a magnet to produce a constantly changing electromagnetic field, converting mechanical energy to electric current.
What does a condenser do in a magneto?
A condenser is a capacitor in a magneto coil that prevents power loss when current passes from one component to another and also aids in collapsing the magnetic field.
If you are facing issues in your vehicle’s ignition, you have to test magneto coils, as they are responsible for producing sparks for ignition.
Generally, we use a multimeter to measure resistance between the magneto components. By measuring resistance, it’s easier to determine the functionality of magneto and the condition of winding and secondary wires in coils.
Hopefully, this post was helpful to you in understanding how to test a magneto to measure its efficiency and how a magneto works to generate current.