John Deere stx38 - Manual (Page 118)

4 - 34
3/21/97
CIRCUIT OPERATION AND DIAGNOSIS ELECTRICAL
IGNITION CIRCUIT OPERATION
STX30 & STX38 (SN —210000)
Function:
To create a spark that ignites the fuel/air mixture in the
engine.
Operating Conditions:
To produce a spark the key switch must be in the run or
start position, and the operator must be ON the seat or
with the operator OFF the seat, the PTO switch must
be off and the transaxle must be in neutral.
Theory of Operation:
The ignition system is a electronic magneto design.
Ignition timing is controlled by the ignition module (A1)
and is not adjustable. The engine is shut off by
grounding the ignition module through the key switch
(S1) (terminals M and G) and the start relay (K1)
(terminals 87A and 30). The ignition module is
grounded (spark stopped) if the operator gets off the
seat with the PTO engaged or the transaxle in gear.
With the key switch (S1) in the start or run position, the
key switch eliminates one path to ground for ignition
current. The start relay (K1), when energized by the
neutral start circuit or the seat switch circuit, eliminates
the second path to ground so a spark can be produced.
Current flows from the battery (G1) positive terminal to
the fusible link or circuit breaker (F1), and key switch
battery terminal. With the key switch in the run position,
current flows to the seat switch (S4), and PTO switch
(S2). The PTO switch is used to prevent the engine
from starting if the PTO is engaged. With the PTO
switch closed (PTO disengaged), current flows to the
neutral start switch (S3). The neutral start switch is
used to prevent the engine from starting if the transaxle
is in gear.
With the neutral start switch closed, current flows to the
key switch terminal S1, fuse (F2) and start relay coil
terminal 85 and energizes the coil, closing the relay.
Closing the relay breaks the path to ground for ignition
coil current between terminals 87A and 30. Also battery
current from the start relay terminal 85 flows to the fuel
shutoff solenoid (Y2), and energizes the solenoid
allowing fuel flow. An alternate current path is provided
to keep to start relay energized when the4 PTO is
engaged, or the transaxle is in gear. With the operator
on the seat, the seat switch (S4) is closed and current
flows to the start relay coil keeping the relay energized.
As the flywheel turns, a magnet in the flywheel
produces current in the primary coil by electromagnetic
induction. When the primary current is at the highest
level, the ignition module induces high voltage current
in the secondary coil. The high voltage current flows
through the ignition module wire to the spark plug (E2).
The voltage is now high enough to jump the spark plug
gap and a spark is produced.