Chapter 8 Ignition; General Information; Operation; Solid State Ignition System (Cdi) - Tecumseh ECV100 - 120 Technician's Handbook

3 to 11 hp 4-cycle l-head engine

Table of Contents

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 Ignition) 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 wheel, pass 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.