Traction Circuit: Low Mode - Toro Groundsmaster 4500-D Series Service Manual

Traction Circuit: LOW Mode

Forward Direction
Groundsmaster ® 4500-D/4700-D
19245SL Rev A
The traction circuit piston pump is a variable displacement pump that is directly
coupled to the engine flywheel. This pump utilizes an integral electro−hydraulic
servo piston assembly that controls the rate and direction of hydraulic flow.
Pressing the traction pedal rotates a potentiometer that provides an input to the
machine TEC controller. The controller in turn sends a corresponding PWM
(Pulse Width Modulation) output to the electronic pump control to rotate the
pump swash plate accordingly to control pump output and direction. The swash
plate angle sensor is attached to the pump to provide the position of the swash
plate. Traction circuit oil is directed to the dual displacement 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 wheel and axle
motors. As the traction load increases, circuit pressure can increase to relief
valve settings: 350 bar (5075 PSI) in forward and 360 bar (5225 PSI) in reverse.
If traction circuit pressure exceeds the relief setting, oil flows through the piston
pump relief valve to the low pressure side of the closed loop traction circuit.
Front wheel and rear axle motors are positive displacement, two speed variable
motors. The dual displacement wheel and axle motors allow operation in
either LOW (mow) or High (transport) speed. The motors are spring biased to
maximum displacement for LOW speed and are hydraulically shifted to minimum
displacement for High speed. The rear axle motor includes a flushing valve
that bleeds off a small amount of hydraulic oil for cooling of the closed loop
traction circuit. The charge circuit replenishes oil that is bled from the traction
circuit by the flushing valve.
The Smart Power TM feature prevents the engine from slowing down in heavy load
conditions (e.g. cutting tall grass) by automatically decreasing the traction speed
if necessary. With a reduced traction speed, the cutting blades can continue to
be at optimum speed.
An optional traction circuit flow divider is available that equally splits traction pump
flow between the front wheel motors and rear axle motor to prevent excessive
circuit flow going to a spinning wheel. If equipped, the front traction manifold
which includes the flow divider is mounted to the right side of the front frame.
The piston pump, front wheel motors and rear axle motor use small amounts of
hydraulic fluid for internal lubrication. Fluid is designed to leak across traction
pump and motor components into the case drain. This leakage results in the loss
of hydraulic fluid from the closed loop traction circuit that must be replaced. The
charge circuit is designed to replace this traction circuit leakage. The gear pump
section that supplies oil to the steering and lift/lower circuits also provides oil
for the charge circuit.
Gear pump flow for the charge circuit is directed through the oil filter and to the
low pressure side of the closed loop traction circuit. Charge pressure is limited to
18 bar (260 PSI) by a relief valve located in the piston pump.
With the armrest console Auto and Low mode control switch in the LOW mode
position, the solenoid valve in the two speed shift manifold is not energized. The
front wheel motors and rear axle motor are at their maximum displacement when
in LOW speed providing a slower traction speed for mowing conditions.
Page 5–15 Hydraulic System: Hydraulic Flow Diagrams