Pto circuit operation – John Deere 318 User Manual
PTO CIRCUIT OPERATION
The function of the PTO circuit is to energize the
PTO clutch(es) and turn on the PTO lamp(s). Also,
the PTO circuit automatically disengages the PTO
clutch(es) anytime the operator rises off the seat for
more than one second.
To engage the PTO clutch(es), the following
conditions must be met:
• Operator on seat.
• Key Switch at RUN position.
• PTO switches initially at OFF position,
then moved to ON position.
When the operator is on the seat and key switch (S1)
is turned to the RUN or START position, current
flows from the positive terminal of battery (G1),
through circuit breaker (F3), to key switch terminal
“B”. The current flows across the switch contacts to
key switch terminal “A”. From terminal “A”, current
flows to fuses (F1 and F2). From fuse (F2), current
flows across the closed contacts of seat switch (S6)
to pin “9” of TDC module connector (X22).
Inside the TDC module, current flows to time delay IC
(A), then to switch transistor (B). 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 PTO relay coil (G).
NOTE: Front PTO switch (S2) and optional rear PTO
switch (S3) have two sets of contacts each.
One set is used to actuate the clutches and
the other set is used in the interlock circuit
(see Starting Circuit Theory of Operation in
If machine is not equipped with rear PTO, a
jumper wire at 3-pin connector (X5) is used in
place of the rear PTO switch.
With the PTO switch(es) in the OFF position, current
will flow from fuse (F1) to connector (X3) on the front
PTO switch. Current flows across the switch contacts,
then to 3-pin connector (X5). If equipped with optional
rear PTO, current will flow from connector (X5) to
connector (X6) on the rear PTO switch. Current flows
across the switch contacts and back out to connector
(X5). From connector (X5), current flows to the TDC
module, through pin “6” of connector (X23). Inside the
TDC module, current flows through diodes (E and F),
then through the PTO relay coil and transistor (B) to
ground. The interlock circuit current energizes the
PTO relay coil, which closes relay contacts (D). Once
the contacts are closed, current flows directly from
fuse (F1), across the relay contacts to the relay coil.
This current keeps the relay energized (latched) as
long as transistor (B) provides a path to ground.
Once the coil is latched, current from the interlock
circuit is no longer needed to keep the relay
When the front PTO switch is moved to the ON
position, current flowing across the relay contacts will
also flow to connector (X4) on the front PTO switch,
through pin “4” of connector (X23). Current flows
across the switch contacts to front PTO clutch (Y1),
engaging the clutch. Current also flows to front PTO
lamp (P1), turning the lamp on.
NOTE: Operation of optional rear PTO is same as
front PTO, except power for rear PTO comes
from TDC module through pin “8” of connector
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 (B). The
transistor will “switch off”, causing current through the
PTO relay coil to stop flowing and de-energize the
coil. At this point the PTO relay contacts will open,
stopping current flow to the PTO clutch(es) and
lamp(s), thus disengaging the PTO clutch(es) and
turning the lamp(s) off.
NOTE: Driving the machine over rough terrain can
cause the seat switch contacts to momentarily
open and close. If this happens, the time
delay IC allows the PTO(s) 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, allowing
the PTO(s) to continue operating.
NOTE: Illustration shows component ground for
machines (S.N. 475001— ). For machines
(S.N. —475000), the component ground
point is located at right side of pedestal panel.
Component Location and Operation/Theory of Operation
316, 318 & 420 Lawn and Garden Tractors