TUBE 5T2554 - Caterpillar

When the manual brake release valve is in the OFF position, oil from the charge pumps flows through the brake valve, through the needle valve, and into the parking brake cylinders. The pressure in the parking brake cylinder overcomes the spring forces, and the parking brake releases.Charge pressure acts against the brake pressure switch and opens the circuit to the brake pressure indicator. In this case, the brake pressure indicator de-energizes.When the shift switch is moved to the TRAVEL position, the shift solenoid is energized and charge oil is directed to the shift mechanisms of each propulsion motor. Charge pressure acts against the shift spool. High-pressure oil from the forward circuit flows through the shuttle valve, the shift spool, the shift modulator, and to the head end of the actuator piston. The actuator piston overcomes the spring forces and causes the propulsion motor to shift.When the shift spool moves to the TRAVEL position, the actuation check valve opens and the de-actuation check valve closes. This action modulates pump pressure through the actuation orifice to the actuator piston. When the shift spool moves to the PAVE position, the de-actuation check valve opens and the actuation check valve closes. This action modulates pressure release through the de-actuation orifice to the case drain.When the machine is moving in the TRAVEL mode, both check valves and orifices are alternately active to buffer actuator modulation and improve actuator stability.The balance line connects ports "MA1" on each motor. When the motors are shifted, the balance line improves synchronization of the actuators. This action reduces momentary differential turning during shifting.When the machine is traveling forward, the left forward solenoid and the right forward solenoid are energized. The condition of the forward solenoids is determined by the duty cycle of the signals that are sent from the machine ECM. The machine ECM analyzes the input signals in order to determine the magnitude of the output signals which are sent to the forward solenoids.Charge pressure flows across the direction control valves. This pressure acts against the forward chambers of the servo pistons. The charge oil causes the servo piston to shift. This shift changes the angle of the swashplate in the pump. In this case, the pump generates flow in the forward circuit.Forward circuit oil flows to the forward combination valves and flows to the forward side of the propulsion motors. Forward circuit oil in the forward combination valves closes the makeup valves, and then forward oil acts against the main relief valves. If the pressure differential between the forward circuit and charge circuit reaches 29000 1000 kPa (4206.20 145 psi), the main relief valve opens. This action directs oil from the forward circuit into the charge circuit.Oil in the left and right forward circuits causes the propulsion motors to turn. After turning the motor, the pressure of the forward circuit oil is reduced. Oil at a reduced pressure then enters the reverse circuit. Reverse circuit oil flows to the reverse combination valves and to