Ignition & Seat Switch Circuit Operation - Sabre 1338 Gear Technical Manual

ELECTRICAL
IGNITION & SEAT SWITCH CIRCUIT
OPERATION
Function:
To create a spark that ignites the fuel/air mixture in the
engine. The ignition coil and module will produce a
spark at the spark plug unless the ignition & seat switch
circuit of the tractor chassis wiring acts to ground the
coil primary and stop the engine.
This circuit is used as a safety interlock to prevent the
engine from starting, or to stop the engine while
running, if the operator attempts to use unsafe actions.
This circuit is independent of the cranking circuit, but
shares some components.
Operating Conditions:
To produce a spark the following conditions must be
met:
• The ignition switch must be in the start or run
position.
• The blade drive lever must be disengaged (back) if
the operator is off the operator's seat.
• The park brake switch must be engaged (brake
applied) if the operator is off the operator's seat.
Once the engine is running, the operator must be ON
the seat to release the brake pedal, or engage the
Blade Drive, or the ignition module will be grounded
and the engine will stop.
Theory of Operation:
The ignition system is an electronic magneto design.
Ignition timing is controlled by the ignition module and
is NOT ADJUSTABLE.
Battery current is always present at ignition switch
terminal "B". When the fuel shut-off solenoid (16
Horsepower engine only) is installed, it becomes
energized when the ignition switch is in either START
or RUN position, via ignition switch terminal "A",
unseating the plunger and allowing fuel flow into the
venturi of the carburetor. When the ignition switch is
turned OFF, current flow is broken and internal spring
force closes the solenoid plunger, stopping fuel flow.
4 - 12
www.servicemanualall.com
As the electric starter turns the flywheel, a magnet in
the flywheel produces current in the primary coil of the
ignition module by electromagnetic induction. When
the primary current builds to its highest level, the
ignition module induces high voltage current into the
secondary coil. This high voltage current then flows to
the spark plug and jumps the spark plug gap and
creates a spark that ignites the fuel/air mixture, causing
the engine to start and run.
The ignition module will stop producing current to the
spark plug if the 940 White wire from the coil is allowed
to ground to the mower chassis. The engine is shut off
by grounding the ignition module in one of two ways:
1. Through the ignition switch when key is moved to
OFF position.
2. Through the brake switch, or blade drive switch, if
the operator tries to get OFF the seat with the
blade drive ENGAGED or the park brake NOT
ENGAGED.
When the brake pedal is RELEASED, the brake switch
creates a path for the ignition module to the seat
switch. When the operator is OFF the seat, the seat
switch (normally closed) completes the path to ground
for the ignition module current, preventing high current
induction into the secondary coil which prevents the
spark plug from firing, killing the engine.
The blade drive switch is in parallel with the blade
switch, and will also ground the ignition module when
the blade drive is engaged with no operator on the
machine, killing the engine.
The blade drive and brake switches are also used to
prevent the engine from cranking if the blade drive
lever is ENGAGED or park Brake is NOT ENGAGED
when ignition switch is moved to START. (See
Cranking Circuit Operation Section)
The operator's seat switch has an interlock built into it's
connector which prevents the mower from being used
with the seat switch disconnected. An internal spring
will short the two cavity connectors together when
disconnected from the seat switch. This has the effect
of grounding the ignition module when the blade drive
lever is engaged or the brake pedal is released, stalling
the engine.
DIAGNOSTICS
3/5/97