EngineMan
Lawn Addict
- Joined
- Jun 12, 2012
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Testing the magneto
The magneto, like the ignition coil on an automobile, contains two windings:
A primary with a few turns of heavy wire.
A high voltage secondary with thousands of turns of super fine wire.
In an automobile, the battery supplies the primary current; in a magneto, the magnet on the flywheel
moving past the core at high speed acts as a generator and induces current in the primary.
As the magnets spin past the pole pieces of the magneto core, the points are closed and current builds up
in the low voltage winding (and flux builds up in the core). At or slightly before Top Dead Center
(TDC), the current (and flux) should be maximum and at this instant the points open. The flux then
collapses (and the condenser (capacitor) across the points acts as a snubber allowing the current to
bypass the open points and preventing arcing at the point contacts). This rapid decrease in flux results in
coupling of the stored energy to the turn high voltage winding and results in up to 10,000 V or more at
the spark plug.
(For EE types, this is somewhat similar in basic operation to the fly-back converter in a switch-mode
power supply except that the moving magnet supplies the input power instead of the rectified AC line
and the points act as the switch instead of a power transistor.)
The secondary will always be accessible for testing but the primary of an electronic ignition may be not
be due to the electronic components:
Secondary: 3 K ohms (maybe a little higher but not open). Much lower would indicate a shorted
winding.
Primary (if non-electronic and accessible): very low - guessing less than an ohm.
Wires can break due to corrosion or vibration. This would result in an open winding - infinite resistance.
Shorts can develop between adjacent windings or to the core. This may be detectable as reduced
resistance but without knowing exactly what it should be, there is no way of knowing if a slight
discrepancy represents a problem or just slight variations in design or manufacturing.
A more complete test would involve checking the 'Q' or doing what is called a 'ring' test and even more
for an electronic ignition. This requires special equipment. Therefore, it is best to swap in a known good
unit. They are not that expensive.
Electronic ignition - There is likely a single potted module which includes the circuitry and
ignition coil. If anything goes wrong with this module, replacement is the only option. Once the
wiring and resistance of the secondary has been checked, there are really no addition tests that can
be performed on an electronic ignition module without special equipment. A defective ignition
module will have to be replaced.
So replacing with a known good or new one seems to be the only way of knowing for sure..!
The magneto, like the ignition coil on an automobile, contains two windings:
A primary with a few turns of heavy wire.
A high voltage secondary with thousands of turns of super fine wire.
In an automobile, the battery supplies the primary current; in a magneto, the magnet on the flywheel
moving past the core at high speed acts as a generator and induces current in the primary.
As the magnets spin past the pole pieces of the magneto core, the points are closed and current builds up
in the low voltage winding (and flux builds up in the core). At or slightly before Top Dead Center
(TDC), the current (and flux) should be maximum and at this instant the points open. The flux then
collapses (and the condenser (capacitor) across the points acts as a snubber allowing the current to
bypass the open points and preventing arcing at the point contacts). This rapid decrease in flux results in
coupling of the stored energy to the turn high voltage winding and results in up to 10,000 V or more at
the spark plug.
(For EE types, this is somewhat similar in basic operation to the fly-back converter in a switch-mode
power supply except that the moving magnet supplies the input power instead of the rectified AC line
and the points act as the switch instead of a power transistor.)
The secondary will always be accessible for testing but the primary of an electronic ignition may be not
be due to the electronic components:
Secondary: 3 K ohms (maybe a little higher but not open). Much lower would indicate a shorted
winding.
Primary (if non-electronic and accessible): very low - guessing less than an ohm.
Wires can break due to corrosion or vibration. This would result in an open winding - infinite resistance.
Shorts can develop between adjacent windings or to the core. This may be detectable as reduced
resistance but without knowing exactly what it should be, there is no way of knowing if a slight
discrepancy represents a problem or just slight variations in design or manufacturing.
A more complete test would involve checking the 'Q' or doing what is called a 'ring' test and even more
for an electronic ignition. This requires special equipment. Therefore, it is best to swap in a known good
unit. They are not that expensive.
Electronic ignition - There is likely a single potted module which includes the circuitry and
ignition coil. If anything goes wrong with this module, replacement is the only option. Once the
wiring and resistance of the secondary has been checked, there are really no addition tests that can
be performed on an electronic ignition module without special equipment. A defective ignition
module will have to be replaced.
So replacing with a known good or new one seems to be the only way of knowing for sure..!
