Starting circuit operation – John Deere 318 User Manual

Page 304

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The function of the starting circuit is to crank the
engine by energizing starter motor (M1). PTO
switches (S2 and S3), transmission neutral switch
(S5), and brake switch (S4) are used as interlock
safety switches within the starter circuit. For the
starter motor to energize, the following conditions
must be met:

• Front and rear PTO switches in OFF position.
• Hydrostatic Control Lever in N/STOP position.
• Brake pedal depressed.
• Key Switch in “START” position.

NOTE: Depressing the brake pedal required only on

later models:
316 (S.N. 596121— )
318 (S.N. 600305— )
420 (S.N. 595881— )

For all other machines, depressing the brake
pedal is not necessary for starter operation.

PTO switches (S2 and S3) are used in the starting
circuit to prevent the engine from cranking while the
PTO is engaged. Each PTO switch contains two sets
of contacts; one for the PTO circuit (lower contact)
and the other (upper contact) as a safety interlock for
the starting circuit. When the PTO switch is in the ON
position (PTO engaged), the PTO switch interlock
contacts are open. When the PTO switch is in the
OFF position (PTO disengaged), the PTO interlock
contacts are closed, allowing current to flow to
transmission neutral switch (S5).

The transmission neutral switch is used in the starting
circuit to prevent the engine from cranking when the
transmission is in gear. When the hydrostatic control
lever is in the forward or reverse position, the
transmission neutral switch contacts are open. Moving
the hydrostatic control lever to the N/STOP position
closes the contacts, allowing current to flow to brake
switch (S4).

NOTE: Brake switch equipped on machines: 316

(S.N. 596121— ), 318 (S.N. 600305— ),
and 420 (S.N. 595881— ) only. For all
other machines, current flows from the
transmission switch, directly to terminal “S2”
on key switch (S1).

The brake switch will prevent the engine from
cranking unless the brake pedal is depressed or park

brake is engaged. Depressing the brake pedal closes
the brake switch contacts, allowing current to flow to
terminal “S2” on key switch (S1).

The key switch initiates current flow through the
starting circuit with the use of two sets of contacts.
Both contacts are open when the switch is in the
OFF position. When the key switch is turned to the
START position, both contacts close. If all the
interlock switch contacts in the start circuit are closed,
current from the battery positive terminal flows
through the 25 amp circuit breaker (F3) to terminal
“B” on the key switch. Current flows across the
closed contacts, out terminal “A” to the 20 amp fuse
(F1). From fuse (F1), current flows through the PTO,
transmission, and brake (if equipped) switches and
flows back to terminal “S2” of the key switch. The
current flows across the key switch contacts, then out
terminal “S1” to starter solenoid (K2), energizing the

NOTE: For machines 316 (S.N. —362983), 318

(S.N. —364137) and 420 (S.N.
—360009), a bendix drive starter motor is
used. These starter motors use a remote
mounted starter solenoid relay.

Starter solenoid (K2) contains two coil windings called
the pull-in and hold-in windings. Current flowing
through these coils produce a strong magnetic field
which pulls a plunger inward and closes solenoid
main contacts (A). Because the starter is a shift type
(solenoid mounted on starter), the plunger also moves
the starter drive gear outward to mesh with the
flywheel ring gear.

When the solenoid contacts close, high current from
the battery flows across the solenoid contacts to
starter motor (M1), causing it to turn. Because the
pull-in windings are grounded through the starter,
current will flow through the pull-in windings only as
long as the solenoid main contacts are open. When
the solenoid contacts close, both ends of the pull-in
windings have the same voltage. This causes the
current to stop flowing through the pull-in windings.
Because the hold-in windings are grounded directly to
the chassis, current continues to flow through the
hold-in windings. This keeps the solenoid energized
(main contacts closed; starter drive engaged) until the
key switch is turned to the RUN or OFF position.

MX,159024020,1 -19-16MAY95

Component Location and Operation/Theory of Operation

TM1590 (17MAY95)


316, 318 & 420 Lawn and Garden Tractors





This manual is related to the following products:

316, 420