Ignition circuit operation – John Deere 318 User Manual

Page 306

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The function of the ignition circuit is to produce spark
across the gap of spark plugs (E1 and E2). The
circuit is a battery ignition type that fires both spark
plugs simultaniously, thus eliminating the need for a
distributor. The ignition circuit automatically stops the
engine anytime the operator rises off the seat for
more than one second when tractor is in gear or if
PTO is engaged.

When the key switch (S1) is turned to the RUN or
START position, current flows from the positive
terminal of battery (G1), through circuit breaker (F3),
across key switch contacts at terminals “B” and “A”,
to fuses (F1 and F2). From fuse (F2), current flows
across the contacts of seat switch (S6), through time
delay IC (E), to switch transistor (D) located inside
the TDC module. As long as current from the IC
flows to the transistor, the transistor is “switched on”.
In this state, the transistor completes the path to
ground for TDC ignition relay coil (B). The ground
path allows current to flow from fuse (F1), through
the relay coil to ground. This energizes the relay coil
which closes relay contacts (A).

Current from fuse (F1) then flows across ignition relay
contacts (A), out the TDC module to ignition coil (T1).
The current flows through the coil primary windings,
then through ignition module (A2) to ground.

Current flowing through the primary windings
produces a magnetic field around the primary and
secondary windings.

NOTE: For machines (S.N. —420000), ignition points

(E3) are used. The ignition points are
actuated by a pushrod that rides on a
camshaft lobe.

The ignition module receives a signal from a trigger
ring containing permanent magnets (the ring rotates
with the engine crankshaft). This signal causes the
ignition module to “break” the circuit, momentarily
stopping current flow through the primary windings,
and cause the magnetic field to collapse across the
secondary windings. The collapsing magnetic field
induces high voltage in the secondary windings of the
ignition coil. The induced voltage flows from one end
of the secondary windings, through the two spark
plugs (jumping the plug gaps), then back to the
opposite end of the secondary windings. The engine

block completes the circuit between the two spark

When the operator rises from the seat, the seat
switch contacts open, causing current to stop flowing
to the time delay IC. If the operator does not return
to the seat within approximately one second, the time
delay IC stops current flow to transistor (D). The
transistor will “switch off”, causing current through
relay coil (B) to stop flowing and de-energize the coil.
With the coil no longer energized, relay contacts (A)
will open. Current stops flowing to the ignition coil,
thus stopping the engine.

NOTE: Driving the machine over rough terrain can

cause the seat switch contacts to momentarily
open and close. When this happens, the time
delay IC allows the engine to operate without

If the operator returns to the seat within
approximately one second, current flow is
re-established to the time delay IC before it has a
chance to “time out” and stop current flow to the
transistor. Current flow is NOT interrupted to the
ignition coil and the engine is allowed to continue

When operator is off the seat, current to the ignition
coil can still be maintained through the neutral start
(interlock) circuit. For current to flow through the
interlock circuit, the key switch must be turned to the
RUN or START position, the hydrostatic control lever
in the STOP position, the PTO switch(es) in the OFF
position (PTO disengaged), and park brake engaged
(later models only).

With these conditions met, current flows from terminal
“A” of the key switch to fuse (F1). From fuse (F1),
current flows through the interlock contacts of the
PTO switch(es), transmission neutral switch, and
brake switch to terminal “S1” on the key switch. From
terminal “S1”, current flows to transistor (C) located in
the TDC module. As long as current from the
interlock circuit flows to transistor (C), the transistor is
“switched on”. In this state, the transistor provides an
alternate path to ground for relay coil (B). The
energized relay closes the relay contacts, allowing
current to flow to the igniton coil.

MX,159024020,3 -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