Chapter 8 Ignition; Magneto Ignition System (Points) - Tecumseh ECV100-120 - TECHNICIAN S Handbook

GENERAL INFORMATION
The ignition systems used on Tecumseh engines are either solid state capacitor discharge modules or magneto
ignition systems. The basic functional difference is that the solid state modules are triggered by an electronic switch
(SCR). Magneto ignition systems rely on the mechanical action of opening and closing a set of moveable contact
points to trigger when the spark will occur.
The solid state ignition system consists of a flywheel magnet and key, charge coil, capacitor, a silicon controlled
rectifier, pulse transformer, trigger coil, high tension lead, and a spark plug. Everything except the flywheel magnet,
key and the spark plug are located in a encapsulated ignition module. This solid state (CDI - Capacitive Discharge
Igntion) module is protected by epoxy filler from exposure to dirt and moisture. This system requires no maintenance
other than checks of the high tension lead and spark plug.
The Tecumseh magneto ignition consists of a stator assembly made of laminations, a coil, contact points, condenser,
a permanent magnet mounted in the flywheel, high tension lead, and a spark plug. The coil is sealed by epoxy filler,
and the points and condenser are sealed from dirt and moisture by a crankshaft seal and cover gasket.
OPERATION
SOLID STATE IGNITION SYSTEM (CDI)
As the magnets in the flywheel rotate past the charge
coil, electrical energy is produced in the module. The
energy is stored in the capacitor ( approx. 200 volts) until
it is released by an electrical switch (SCR). As the magnet
continues to rotate, it travels past a trigger coil where a
low voltage signal is produced. This low voltage signal
closes the SCR switch, allowing the energy stored in the
capacitor to flow to a transformer where the voltage is
increased from 200 volts at 200 RPM to 22,000 volts at
3000 RPM. This voltage flows through the high tension
lead to the spark plug where it arcs across the electrodes
and ignites the air-fuel mixture (diag. 1).

MAGNETO IGNITION SYSTEM (POINTS)

As the flywheel turns, the magnets that are mounted in
the wheelpass the coil mounted on the stator. As the
magnet's North Pole enters the area of the center leg of
the stator, a magnetic field is concentrated through the
laminations to the magnet's South Pole. This causes a
generation of current flow in the coil's primary winding.
The ignition points are closed (diag. 2).
As the flywheel continues to rotate, the North Pole
approaches the last leg of the lamination stack. The
magnetic field through the center leg reverses, producing
a large change in the magnetic field, and a high current
in the primary side of the coil (diag. 3).
At this time, the contacts open and the primary current
stops flowing. This change in current causes a voltage in
the primary, which induces a high voltage in the
secondary winding of the coil. The voltage travels through
the spark plug wire, to the spark plug and jumps the gap
of the plug to ignite the air/fuel mixture.

CHAPTER 8 IGNITION

PATH OF MAGNETIC
LINES OF FORCE
MAGNET
FLYWHEEL
ROTATION
Main Menu
1
POINTS
2
CLOSED
POINTS OPEN
3
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