John-deere stx38 User Manual download pdf (Page 118)

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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.

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John-deere stx38 User Manual Page 118

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