AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

How To Test a Magneto Coil With a Multimeter?

In modern vehicles, there are a wide range of areas that can be concerning due to the countless sensitive components they consist of. Older vehicles and engines have an additional problem to think about: magnetic coils. Magnetic coils are critical components found in the ignition system of small aircraft, tractors, lawnmowers, and motorcycle engines, among other machines.

A magneto is an electrical generator that utilizes a permanent magnet to produce periodic, strong pulses of current, rather than generating a constant supply of it. Through its various coils, it can supply strong pulses of current to a spark plug that ignites compressed gasses in the ignition control system of an engine. To accomplish this, there are five key components that work in tandem to make a magneto function properly.

Five Key Components of a Magneto Coil

To begin, the armature is the first component of the five. It is U-shaped, and it is positioned close to the flywheel, where the two magneto ignition coils are wrapped around. When the engine flywheel rotates past the armature, it gets periodically supplied with current by the magnets. This current builds up until a maximum is reached, causing the electronic control unit to activate a switch and make the contacts points open. This sudden release delivers a strong electric current into the spark plugs, firing up the engine.

Symptoms of a Bad Magneto Coil

If there is a faulty magneto coil, you may experience the following:

  • The engine warning light illuminates the dashboard.
  • You may experience difficulty starting the engine.
  • There is more distance traveled by gas.
  • There is a lack of acceleration power.

How To Test a Magneto Coil With a Multimeter

To test a magneto coil, take off the rubber casing and set the multimeter to Ohms (Ω). Then, you must set the Ohm range to 40k. Place the multimeter probes at the magneto copper coil and position the metal clip under the rubber casing. Keep in mind that any value above or below the 3k to 15k range indicates that the magneto coil is faulty. To better understand the process, the following is a detailed description:

The first step consists of detaching the flywheel shroud from the entire setup. The flywheel shroud is a metal casing that encapsulates the magnet and is held together with three bolts. Once the shroud has been removed, you will see the magneto coil. As the magneto coil has wires protected by a rubber casing, the rubber casing must be removed. At this point, set your multimeter dial and measure the resistance of the coil. Do not opt for auto-ranging, as this can produce very unreliable results.

To measure the resistance of the coil, you want to measure the primary and secondary coils. In order to do this, place the red multimeter probe on the U-shaped winding of the primary coil, and the black probe on a metal surface. Measuring the secondary coil requires that you place one of the probes on the U-shaped metal winding, and the other on the rubber casing at the other end of the magneto. Lastly, check the multimeter for readings and evaluate your results. If you have any questions, refer to the manufacturer’s manual.


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