John Deere stx38 - Manual (Page 124)

4 - 40
3/21/97
CIRCUIT OPERATION AND DIAGNOSIS ELECTRICAL
IGNITION CIRCUIT OPERATION
STX38 (SN 210001— ) & STX46
Function:
To create a spark that ignites the fuel/air mixture in the
engine.
Operating Conditions:
To produce a spark, the key switch (S1) must be in the
START or RUN position. The operator can be ON or
OFF the seat—seat switch (S4); however, the PTO
switch (S2) must be OFF and the brake switch (S3)
must be ENGAGED (brake pedal applied) to start the
engine. Once the engine is running, the operator must
be ON the seat to engage the transaxle (brake pedal
must also be released) or operate the PTO.
Theory of Operation:
The ignition system is an electronic magneto design.
Ignition timing is controlled by the ignition module (A1)
and is NOT ADJUSTABLE.
Battery (G1) current is always present at key switch
(S1) terminal “B”. When the optional fuel shut-off
solenoid (Y2) is installed, it becomes energized when
the key switch (S1) is in either START or RUN position,
via terminal “A”, unseating the plunger and allowing
fuel flow into the venturi of the carburetor. When the
key switch is turned OFF, current flow is broken and
internal spring force closes the solenoid plunger,
stopping fuel flow.
As the flywheel turns in the cranking circuit, a magnet
in the flywheel produces current in the primary coil of
the ignition module (A1) by electromagnetic induction.
When the primary current builds to its highest level, the
ignition module (A1) induces high voltage current into
the secondary coil. This high voltage current then flows
to the spark plug (E1) and jumps the spark plug gap
and creates a spark that ignites the fuel/air mixture,
causing the engine to start and run.
When the key switch (S1) is in the START or RUN
position, the key switch eliminates the path to ground
for ignition current, allowing spark generation to
continue.
The engine is SHUT-OFF by grounding the ignition
module (A1) through: (1) the key switch (S1) or (2) a
combination of the brake switch (S3) and seat switch
(S4) interaction—if the operator gets OFF the seat with
the PTO switch (S2) ENGAGED and the brake switch
(S3) NOT ENGAGED (brake pedal released) and/or
the transaxle is ENGAGED.
When the brake pedal is RELEASED, the brake switch
(S3) plunger closes the two terminals marked “NC”
(normally closed) which creates a path to the seat
switch (S4) for the ignition module (A1) current. When
the operator is OFF the seat, the seat switch (S4) (a
normally closed switch) completes the path to ground
for the ignition module current, preventing high current
induction into the secondary coil which prevents the
spark plug (E1) from firing, killing the engine.
The PTO switch (S2) is used to prevent the engine
from starting if the PTO switch is ENGAGED. With the
PTO switch ON and seat switch (S4) NOT ENGAGED
(operator OFF the seat) a path to ground from the
ignition module (A1) is created, preventing spark
generation to ignite the fuel/air mixture to run the
engine.