John-deere stx38 User Manual download pdf (Page 124)

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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.

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John-deere stx38 User Manual Page 124

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