Toro Groundsmaster 4500-D, Service Manual

Page: 47 / 278

Rev. D
Groundsmaster 4500--D/4700--D Hydraulic System (Rev. A) Page 4 -- 9
Traction Circuit
The traction circuit piston pump is a variable displace-
ment pump that is directly coupled to the engine fly-
wheel. Pushing the top of the traction pedal engages a
hydraulic servo valve which controls the variable dis-
placement piston pump swash plate to create a flow of
oil. This oil is directed to the front wheel and rear axle
motors. Operating pressure on the high pressure side of
the closed traction circuit loop is determined by the
amount of load developed at the fixed displacement
wheel and axle motors. As the load increases, circuit
pressure can increase to relief valve settings: 5000 PSI
in both forward and reverse. If pressure exceeds the re-
lief 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 mow (four
wheel drive) or transport (two wheel drive).
Traction circuit pressure (forward and reverse) can be
measured at test ports located on the traction circuit hy-
draulic tubes of the machine.
The traction pump and wheel motors use a small
amount of hydraulic fluid for internal lubrication. Fluid is
designed to leak across pump and motor parts into the
case drain. This leakage results in the loss of hydraulic
fluid from the closed loop traction circuit that must be re-
placed.
The gear pump section (P3) that supplies oil to the steer-
ing and lift/lower circuits also provides charge oil for the
traction circuit. The gear pump is driven directly off the
traction pump. It provides a constant supply of charge oil
to the traction circuit to make up for oil that is lost due to
internal leakage in the traction pump and motors.
Charge pump flow 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 by a relief valve located in the
filtration and charge control manifold. Charge pressure
can be measured at the charge circuit pressure test port
on the filtration and charge control manifold.
The 4WD/2WD control manifold provides hydraulic flow
management in a variety of situations. Pressure testing
of this control manifold can be performed at a test port
on the manifold.
1. Four wheel drive (mow) and two wheel drive (trans-
port) operation is controlled by a solenoid valve (SV) lo-
cated in the 4WD/2WD control manifold. When in
transport (2WD), the solenoid valve (SV) is energized
and directs charge pressure to shift control valves PD1
and PD2. The shifted valves block hydraulic flow to the
rear axle motor in both forward and reverse directions
when in transport (2WD).
2. A pressure reducing valve (PR1) in the 4WD/2WD
control manifold lowers pressure to the rear axle motor
when the machine is operated in reverse. The lower
pressure prevents rear tire scuffing and provides added
pressure to the front wheels during reverse operation.
3. An adjustable relief valve (RV5) in the 4WD/2WD
control manifold reduces rear axle motor pressure
created in down--hill, dynamic braking conditions.
To enhance traction control, the lift/lower circuit is
equipped with a counterbalance system. An adjustable
counterbalance valve (RV1) located in the power down
and traction assist control manifold transfers cutting unit
weight to the machine to improve traction. Additionally,
the counterbalance system has a secondary relief valve
(RV2) that provides additional cutting unit weight trans-
fer when traction circuit pressure increase is sensed by
directional valve (PD3) located in the power down and
traction assist control manifold.
Hydraulic System