PRINCIPLES OF OPERATION.
HYDRAULIC SYSTEM. The hydraulic system is a closed center system. The oil from the variable displacement
pump supplies pressure to the parts of the system while the engine is running. The main parts of the system are
illustrated and explained in the following paragraphs. Refer to Appendix F for Hydraulic System Schematics.
Hydraulic Variable Displacement Pump Assembly and Lines (Figure 1-45).Mounted under the cab. Shaft
driven by engine. The variable displacement rear section draws oil from the hydraulic tank and provides oil flow for
steering, articulation, wheel lean, and earthmoving components. The constant displacement front section circulates oil to
the reservoir (tank) through the cooler, the filter, and the strainer.
Combination Valve (Figure 1-46). Located at the left of the variable displacement pump. Oil from the
variable displacement pump flows through the combination valve when the engine is running. It then goes to the control
valves for the earthmoving components and to the steering metering valve. A hydraulic pressure unloading valve is
included for cold weather starting.