John Deere stx38 - Manual (Page 114)

4 - 30
3/21/97
CIRCUIT OPERATION AND DIAGNOSIS ELECTRICAL
CRANKING CIRCUIT OPERATION
STX38 (SN 210001— ) & STX46
Function:
The cranking system is used to energize the starting
motor.
Operating Conditions:
In order to crank the engine; the key switch must be in
the START position, the PTO switch must be OFF, and
the brake must be ENGAGED. The operator DOES
NOT have to be on the seat to crank the engine.
Theory of Operation:
The starting motor has a separate starter solenoid.
Current flows from the battery (G1) positive terminal to
the starter solenoid (K1), fusible link (F1), and key
switch (S1). With the key switch in the START position,
current flows to the PTO switch (S2) and the optional
fuel shut-off solenoid (Y2), causing the solenoid
plunger to unseat and allow fuel to flow into the venturi
of the carburetor.
The PTO switch (S2) is used in the neutral start circuit
to prevent the engine from cranking IF the PTO switch
is ENGAGED. When the PTO switch is OFF (PTO
disengaged), current flows to the brake switch (S3).
The brake switch (S3) is located under the right side of
the chassis and aligned with the brake pedal bell crank.
When the brake pedal is in the ENGAGED position, the
bell crank depresses the brake switch (S3) plunger and
the two unmarked terminals (normally opened) are
closed, providing a path for current to flow to the starter
solenoid (K1). With the small windings being
energized, the solenoid plunger closes the large main
terminals, supplying high battery current to the starter
motor (M1), engaging the bendix drive with the engine
flywheel, cranking the engine until the ignition circuit
fires (see Ignition Circuit Operation), igniting the fuel/air
mixture and starting the engine. Once the engine
starts, the operator releases the key and key switch
(S1) moves into RUN position. This breaks the current
flow to the starter solenoid (K1) plunger small windings,
opening the large main terminal and cutting off high
battery current to the starter motor.
The brake switch (S3) is also used to prevent the
engine from cranking if the drive train is engaged. This
is accomplished through the clutching linkage in the
gear models and the neutral return linkage in the hydro
models working in conjunction with the brake pedal
operation. With the brake pedal engaged, the brake
switch (S3) operation described above is repeated