Now for the three wire theory of operation:
The function of the fuel shutoff solenoid circuit is to
stop engine operation by shutting off fuel flow to the
fuel injector pump. This is accomplished either
manually using the key switch or automatically when
the operator rises from the seat for more than one
second with machine in gear or PTO engaged.
OPERATOR ON SEAT—MACHINE IN GEAR
AND/OR PTO ENGAGED:
When the key switch (S1) is turned to the ON
position, current flows from the positive (+) terminal of
battery (G1), through fusible link (F4) to the key
switch. Current flows across key switch contacts to
25-amp fuse (F1) and 10-amp fuse (F2). From fuse
(F2), current flows across the closed contacts of seat
switch (S6) to pin “A” of TDC module 3-pin connector
Inside the TDC module, current flows to seat time
delay IC (J), then to time delay transistor (H). As long
as current from IC (J) flows to transistor (H), the
transistor is “switched on”. In this state, the transistor
completes the path to ground, allowing current from
the seat switch to flow through relay coil (D),
energizing the relay. This closes contacts (C), which
allows current from fuse (F1), to flow to solenoid (Y3)
hold-in windings (B). The magnetic field produced in
the hold-in windings is not strong enough to move the
armature of fuel shutoff solenoid by itself.
From relay contacts (C), current also flows to pull-in
time delay IC (G) and pull-in relay coil (F). IC (G)
provides a path to ground for relay coil (F), causing
the relay to energize and close relay contacts (E).
This allows high current from the battery to flow
through fusible link (F4), across relay contacts (E), to
solenoid pull-in windings (A). This energizes the
pull-in coil, which pulls the fuel shutoff solenoid
armature in and moves the injection pump linkage to
the ON position.
<u>After approximately one second, IC (G) breaks the
pull-in relay’s path to ground, thus stopping current
flow through the relay coil. The relay contacts (E)
open, causing current to stop flowing to the solenoid
Current continues to flow to the solenoid hold-in
windings. The magnetic field produced by this current
is strong enough to hold the solenoid armature in,
thus keeping the injector pump linkage at the ON
When the operator rises from the seat, the seat
switch contacts open, causing current to stop flowing
to seat time delay IC (J). If the operator does not
return to the seat within approximately one second,
IC (J) stops current flow to transistor (H). The
transistor will “switch off”, stopping current flowing
through relay coil (D), causing relay contacts (C) to
open. Current stops flowing to the solenoid hold-in
windings, de-energizing the fuel shutoff solenoid. With
the solenoid no longer energized, a return spring
moves the injector pump linkage back to the OFF
position, thus stopping fuel flow to the injection pump.
NOTE: Driving the machine over rough terrain can
cause the seat switch contacts to momentarily
open and close. If this happens, the seat 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 IC (J) before it has a chance to
“time out” and stop current flow to transistor (H).
Current flow is NOT interrupted to the fuel shutoff
solenoid, allowing the engine to continue operating.
OPERATOR OFF SEAT—MACHINE IN NEUTRAL
AND PTO DISENGAGED:
NOTE: 332 (S.N. 596723— ) and 430 (S.N.
596048— ); Park brake must be engaged.
When operator is off the seat, current to the fuel
shutoff solenoid can still be maintained through the
interlock circuit. For current to flow through the
interlock circuit, the key switch must be turned to the
ON or START position, the hydrostatic control lever in
the N/STOP position, the PTO switches in the OFF
position (PTO disengaged) and park brake engaged
(332 (S.N. 596723— ) and 430 (S.N. 596048— )
With these conditions met, current flows from fuse
(F1), through the interlock contacts of the PTO
switches, transmission neutral switch, and brake
switch (if equipped) to pin “A” of TDC module 6-pin
connector (X13). Inside the TDC module, the interlock
current flows to transistor (I), causing the transistor to
“switch on”. In this state, the transistor provides an
alternate path to ground for relay coil (D). The relay
coil energizes, closing relay contacts (C), thus
allowing current to flow to the fuel shutoff solenoid
hold-in windings (B).
...Hope this helps.