Hitachi ZW180 Technical Manual page 339

In Neutral
Pressure oil maintained at constant pressure by the
regulator valve flows to the transmission control valve,
and is divided into oil passage (a) to proportional
solenoid valve (1) and oil passage (b) to modulation
spool (2).
Pressure oil does not flow from oil passage (a) to oil
passage (c) as the electric signal to be transmitted
from the controller to proportional solenoid valve (1)
is stopped in neutral. As oil passage (d) between oil
passage (b) and the clutch piston is blocked by
modulation spool (2), clutch pressure is not raised
and becomes neutral.
1
1 - Proportional Solenoid
Valve
2 - Modulation Spool
COMPONENT OPERATION / Drive Unit
a
From Charging
Pump
Filter
To Clutch
b
Piston
T3-9-33
Regulator Valve
c 2
d
T4GD-03-09-013