Toro 08161SL Service Manual Model 30412 Groundsmaster 4000 D Preface Publication

Traction Circuit: 4WD (Mow)
The traction circuit piston pump is a variable displace-
ment pump  that is directly  coupled to  the engine fly-
wheel. Pushing the traction pedal engages a hydraulic
servo valve  which controls the  variable displacement
piston pump swash plate to create a flow of oil. This oil
isdirectedtothefrontwheelandrearaxlemotors.Oper-
ating pressure on the high pressure side of the closed
traction circuit loop is determined by the amount of load
developedatthefixeddisplacementwheelandaxlemo-
tors.  As the  load increases,  circuit  pressure can  in-
crease to  relief valve settings:  4000 PSI (274  bar) in
forward and 5000 PSI (343 bar) in reverse. If pressure
exceeds the  relief setting, oil  flows through  the relief
valve to the low pressure side of the closed loop traction
circuit. The traction circuit provides operation  in either
4WD (mow) or transport (2WD).
Forward Direction
When the transport/4WD  switch is in the  4WD (mow)
position and the traction pedal is pushed in the forward
direction, oil from the piston pump passes through the
traction manifold. Oil flow from traction manifold port M1
drives the front  wheel motors in  the forward direction
and then returns to the hydrostat. Oil flow from traction
manifold port M2  is routed to the P1  port of the 4WD
manifoldwhereitisdirectedtothePD1cartridgeandout
ofthe manifold to drivethe rearaxle motor in theforward
direction. Oilreturning fromthe rearmotor re--entersthe
4WD manifold at the M2 port. Flow passes through the
PD2cartridge,throughtheCVcheckvalve,outmanifold
port P2 and back to the hydrostat.
When going down a hill, the tractor becomes an over--
running load that drives the wheel and axle motors. In
this condition, the rear axle motor could lock up as the
oil pumped from the motor increases pressure as it re-
turns to the hydrostat. To prevent rear wheel lock up, an
adjustable relief  valve (RV)  in the  4WD manifold  re-
duces rear axle motor pressure created in down hill, dy-
namic braking conditions.
Traction circuit pressure (forward and reverse) can be
measured at test ports on the sides of the machine.
The  traction  circuit  pump  and  motors  use  a  small
amount of hydraulic fluid for internal lubrication. Fluid is
designed to leak across traction pump and motor com-
ponents into the case drain. This leakage results in the
lossofhydraulicfluidfromtheclosedlooptractioncircuit
that must be replaced. The charge circuit is designed to
replace this traction circuit leakage.
Reverse Direction
The traction circuit operates essentially the same in re-
verse 4WD (mow)  as it does in  the forward direction.
However,theflowthroughthecircuitisreversed.Oilflow
from the hydrostat is directed to the front wheel motors
and also to the 4WD manifold. The oil to the front wheel
motors drives them in the reverse direction and then re-
turns to the hydrostat through the traction manifold. The
oil to the 4WD manifold enters the manifold at port P2
and flows through pressure reducing valve (PR) which
limits the down stream pressure to the rear axle motor
to 650 PSI (45 bar) so the rear wheels will not scuff the
turf during  reverse operation.  This reduced  pressure
flows through the PD2 cartridge and out port M2 to the
rear axle motor.Return oilfrom therear motor re--enters
the4WDmanifoldatportM1,flowsthroughthePD1car-
tridge,exitsthemanifoldatportP1andreturnstothehy-
drostat.
The gear pump section that supplies oil to the steering
and lift/lower circuits also provides oil for the charge cir-
cuit. This gear  pump is driven  directly off the traction
pump. It  provides a  constant supply  of charge  oil to
make up for oil that is lost due to internal leakage in the
traction pump and motors.
Pump flow for the charge circuit is directed through the
oil filter and to the low pressure side of the closed loop
traction circuit. A  filter bypass valve allows  charge oil
flow to  the closed loop  if the filter  becomes plugged.
Charge pressure is limited to 250 PSI (17 bar) by a relief
valve located in the oil filter manifold. Charge pressure
can be measured at the charge circuit pressure test port
on the oil filter manifold.
A flow divider is incorporated into the traction circuit and
is located in the traction manifold. When in 4WD (mow),
the operator can momentarily engage this flow divider
when low traction situations  could lead to wheel spin.
Depressing the flow divider switch energizes the sole-
noid valve in the traction manifold. This energized sole-
noid valve causes traction pump hydraulic flow to split
between the front  wheel motors (approximately  45%)
and rear axle motor (approximately 55%) to reduce the
chance that excessive flow goes to a spinning wheel.
Groundsmaster 4000--D
Page 4 -- 11
Hydraulic System
Product Specification
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