Komatsu 930E-2, Shop Manual

Page: 396 / 739

Oil pressure from the bleeddown manifold is routed
back to the pump and sensed by the “ACC” port on the
unloader valve. A steering pressure switch, mounted
on the bleeddown manifold is connected electrically to
a “Low Steering Pressure” warning lamp mounted in
the cab and warns the operator if system pressure
drops below 2300 psi (15.9 MPa).
The bleeddown manifold performs many functions,
including distribution of the steering circuit oil supply
to the steering system accumulators (5), the flow am-
plifier valve (3), and to the brake apply system.
Two accumulators, pre-charged with nitrogen, are
mounted on the left frame rail behind the front tire. They
provide a sufficient oil supply to allow the operator to
steer the truck off the haul road and park in a safe area
if a loss of steering circuit oil occurs. A steering circuit
bleeddown solenoid mounted on the bleeddown mani-
fold is electrically activated when the key switch is
turned OFF and bleed the hydraulic oil back to the tank
to release the pressurized oil when the truck is parked.
If a loss of steering pressure occurs during opera-
tion, the truck must be stopped immediately. The
pressure in the accumulators allow the operator to
steer the truck for a short period. Do not attempt
further operation until the problem is located and
corrected.
The bleeddown manifold and accumulators supply oil
to the flow amplifier valve. The flow amplifier directs oil
flow to the appropriate ports on the steering cylinders
as determined by inputs from the steering control unit
as the operator steers the truck (see Figure 4-2).
Oil entering the steering control unit is blocked until the
steering wheel is turned in a desired direction. The
valve then directs oil to the flow amplifier which in turn,
provides a high volume of oil to the steering cylinders.
Hydraulic oil at the opposite ends of the steering cylin-
ders flows back through the flow amplifier, and bleed-
down manifold to the hydraulic tank.
Oil flow to the brake apply system flows through a
check valve which will prevent loss of oil in the brake
circuit if a loss of oil occurs in the steering circuit. (Refer
to Section “J” for a detailed description of the brake
apply system hydraulic circuit and components.)
COMPONENT DESCRIPTION
STEERING CONTROL UNIT
The steering control unit (Figure 4-2) is mounted inside
the cab and is directly coupled to the lower end of the
steering column.
Operation of the steering control unit is both manual
and hydraulic in effect. This component incorporates
a hydraulic control valve; steering effort applied to the
steering wheel by the operator actuates the valve,
which in turn directs hydraulic oil through the flow
amplifier valve to the steering cylinders to provide the
operator with power steering.
FIGURE 4-2. STEERING CONTROL UNIT
“T”: Tank Port
“L”: Left Steer Port
“P”: Pressure (Supply) Port
“R”: Right Steer Port “LS”: Load Sense Port
1. Valve Assembly
L4-2 Steering Circuit L04031