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STEERING PUMP
The steering/brake pump is mounted on the rear of the
hoist system pump. The drive shaft mates with an
internally splined coupling inside the hoist pump when
the pump is bolted in place.
The pump is a pressure compensated piston type with
system operating pressure controlled by an unloader
valve. The pressure compensator is adjusted to 3325
psi (22.9 MPa). The pump mounted unloader valve
maintains system pressure between 2750 psi (19.0
MPa) and 3025 psi (20.9 MPa).
Oil flow from the pump is routed through a high pres-
sure filter then to the bleeddown manifold valve where
the flow is directed to the steering circuit (accumula-
tors, flow amplifier, etc.) and to the brake apply system
components mounted in the hydraulic components
cabinet behind the cab.
L4-16
Operation
The driveshaft (35, Figure 4-10) runs through the cen-
terline of pump housing (37) and valve plate (48). The
cylinder barrel (45) is splined to the drive shaft.
A ball bearing (33) supports the outboard end of the
driveshaft and a bushing type bearing (47) supports the
inboard end. The pump cylinder barrel is carried in a
polymerous (journal type) bearing (41).
The valve plate has two crescent shaped ports (inlet
and outlet). Pumping pistons (44) in the cylinder barrel
are held against a swashblock (28) by a shoe retainer
(17). The shoe retainer is held in position by the fulcrum
ball (40) which is forced outward by retainer springs
(13). The springs act against the pump cylinder barrel
(45) forcing it against the valve plate while also forcing
the piston shoes against the swashblock.
The semi-cylindrical shaped swashblock limits the pis-
ton stroke and can be swiveled in arc shaped saddle
bearings (21). The cradle is swiveled by the control
piston (14).
High Altitude Operation
The steering pump on later model trucks incorporates
an adjustable stroke control device (25, Figure 4-10).
If the truck is operating at high altitude locations (10,
000 feet (3,050 meters) above sea level or greater), it
may be necessary to adjust the pump stroke to reduce
oil flow to eliminate pump cavitation problems.
Steering Circuit
L04031
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Troubleshooting
