Reply To: Magneto Testing

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Dear Andy,

OK, I think at this point we need to discuss some facts about magnetos, their construction, operation and testing.

Copper is a superb conductor of electricity. But over long lengths of wire they offer some resistance to an electrical current and this can be measured with an ohms test. Its a very basic tenant of electrical engineering. Most of the magnetos on horticultural machines are of the rotating magnet type, so it will be this I cover here.

The magnets on the flywheel or armature rotate past the soft iron core of the double wound coil. As the north pole passes the core, lines of magnetic force flow in one direction through the core inducing an electrical current in the primary LT windings. The enamelled copper wire used for these windings is about 1mm thick and consist of 350-400 amp turns around the core. The HT secondary coil consisted of around 20,000 turns of enamelled copper wire as fine as a human hair and these are wrapped around the primary windings. The electrical current flow in the primary windings cause the secondary windings to become saturated by magnetic lines of force. As the magnet passes from north to south the direction of the magnetic flow in the core reverses, known as flux reversal. At this instant the primary voltage is at its highest, at this moment the points open and the lines of magnetic force collapse. As this collapse cuts through the secondary windings, a very high voltage is produced in them. The electrical charge wants to find a way to ground and does so through the spark plug via an arc, which ignites the compressed fuel/air mixture.

An important component in the system is the capacitor, also known as the condenser. Its function is to momentarily store electrons as the points open. These electrons are attracted to the hot side of the capacitor because of its proximity to ground. Without it the points would arc and burn out very quickly. The charge held in the capacitor gives an added boost primary coil. When the points close again, the capacitor discharges back through the primary, which induces an additional voltage in the secondary. One of the main points I wished to impress upon in my original and subsequent posts is the vital importance of the capacitor in the system. By using a more modern component, the magneto’s performance can be often improved significantly. Would look at fitting one if an apparently good magneto does not produce a consistent 6mm spark.

The magneto has a hard time on an engine, it is subject to heat (lots), vibration, oil and moisture. If the points are clean and have been set correctly, the most usual fault is ‘shorting’ between the insulated parts of the instrument. The primary windings are pretty robust and rarely fail. A test on them and the rest of the primary circuit can be done with a multimeter as described in my original post. The resistance in the primary winding will be low, but discerable and can be measured with a multimeter set to the correct range.

Due to the fine wire in the secondary windings having to deal with very high voltages. Also because in older mag’s the insulating covering was shellac (beetle wings), which is organic and can rot. The wires can arc between each other, or break. This will severely reduce, or even stop the sufficiently high voltage to produce a spark at the plug. So the first test on a suspect magneto system for the home mechanic should be for the spark gap in air. If this is not sufficient, move onto checking the components for visible problems. Still no spark, then start to do resistance tests. If the HT secondary coil is faulty, it will have an ohms reading outside the manufacturer’s published range in the service or workshop manual. For older magnetos, this information is not available, so look for 3000-7000 ohms. A lower reading indicates a partial break in the windings, a higher one a short caused by wires fusing together. Again the correct ohms range should be set on the multimeter.

Did read again what George has written on his Ignition webpage. To quote “If you test one with a multi meter & find a circuit through the windings it still doesn’t mean it will function properly, some don’t show a circuit & still make a spark”. He unfortunately does not mention resistance and appears misleadingly to suggest that the circuit through the secondary windings is either closed or open when tested with a multimeter. It is very clearly not as simple as that. The circuit should be closed, but will have resistance within it. This can be measured to give an indication of a fault. I can only presume from George’s advice that if he did use a multimeter, he had not set it to the correct range to test with. Which is very unfortunate.

I shall return now to the purpose of my original post. It was for someone who has some mechanical skill, but little knowledge of magnetos. To be able to repair one in their home workshop without resorting to expensive specialists. I have done this many times with a 100% success rate. Wish to make my experience and knowledge available to others to do the same. I have some concerns with your regular pronouncement that a multimeter used CORRECTLY cannot test a magneto to those posting enquiries on the forum. This may have put people off attempting to fix one themselves. Old pedestrian machines are reducing in value as those willing or able to keep them going become fewer in number. A younger person wanting to have a go may have been put off by what you I believe, have incorrectly stated. The machine they may have come across, due to what you have said, would be deemed as having to have an uneconomic repair. It was then weighed in, a tragic loss in my view. The university of life is a highly valuable thing to have. When you get to a certain vintage, we all have one. I will always listen to other’s points of view and if I think it expands my knowledge base, will take it on-board. As you have never owned a multimeter, am not sure if you would know how to use one correctly. In that case, I do not believe you are best qualified to make an informed decision on their use. Have read your other replies to posts, can see that your vast experience of horticultural machines and mowers is invaluable and very much respect it. But also think magnetos are not your strongest suit.

Best wishes,

Grahame