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| Illustration 1 | g02132394 |
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Proportional reducing valve (power shift solenoid) (1) Solenoid (2) Valve body (3) Line (pilot oil flow) | |
The proportional reducing valve for the power shift pressure is located on the main pump housing. The proportional reducing valve is a solenoid operated control valve. The proportional reducing valve controls the flow of pilot system oil to the main pump regulators. The main pump regulators are used for horsepower control.
The solenoid receives a pulse width modulated signal (PWM signal) from the machine ECM. The PWM signal that is sent from the machine ECM causes the proportional reducing valve to regulate the pilot pressure at the main pump regulators. The engine speed is an input to the machine ECM which helps the machine ECM to determine the PWM signal that is required for the proportional reducing valve.
The PWM signal is a variable signal. The PWM signal allows the main pumps to be controlled more precisely. The reduced pressure that is created by the proportional reducing valve is called power shift pressure (PS). The output flow of the main pumps is controlled in accordance with the power shift pressure. The power shift pressure is used to control the maximum allowable hydraulic pump output when the engine is under load.
Reference: For more information concerning the pump regulators, refer to Systems Operation, "Pump Control (Main Hydraulic) (Main Pump Regulator)".
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| Illustration 2 | g03408675 |
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Power shift pressure OFF (1) Solenoid (2) Valve body (4) Spring (5) Spool (6) Passage (Drain port) (7) Passage (Power shift pressure output) (8) Passage (Pilot oil supply from pilot pump) | |
The proportional reducing valve consists of solenoid (1), spring (4), and spool (5).
When the machine is operating, oil from the pilot pump enters the proportional reducing valve through line (3). Pilot oil flows from line (3) to passage (8). When solenoid (1) is not energized by the PWM signal, spool (5) remains stationary. Spring (4) keeps spool (5) shifted upward. Pilot oil in passage (8) is blocked from entering passage (7) by spool (5). The oil that is present in passage (6) and passage (7) drains to the hydraulic tank. There is no power shift pressure on the main pump regulators.
Power shift pressure is not needed when the engine speed is maintained during a hydraulic operation. When the engine is not under load, there will be no PWM signal to the proportional reducing valve.
Power Shift ON (Full Pressure)
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| Illustration 3 | g03408699 |
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Full Power shift Pressure (1) Solenoid (2) Valve body (4) Spring (5) Spool (6) Passage (Drain port) (7) Passage (Power shift pressure output) (8) Passage (Pilot oil supply from pilot pump) | |
When the engine is under load during a hydraulic function, the output flow of main pumps is too high. The pump regulators control the output flow of the main pumps. Power shift pressure that is created by the proportional reducing valve is used by the pump regulators to reduce the main pump output. The engine speed is detected by the machine ECM. The machine ECM sends a PWM signal to the proportional reducing valve in order to increase the power shift pressure. The increase in power shift pressure at the pump regulators reduces the output of the main pumps. Reducing the main pump output allows the engine to run at an optimum speed.
Solenoid (1) is energized by the PWM signal which causes spool (5) to shift downward against spring (4). When spool (5) is shifted downward, passage (7) is blocked from passage (6). Passage (7) is connected to passage (8). Pilot oil flows from passage (8) to passage (7). Pilot oil flows from passage (8) to passage (7), and then to the pump regulators. Power shift pressure that is in the main pump regulators reduces the output of the main pumps.
Power Shift ON (Reduced Pressure)
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| Illustration 4 | g03408702 |
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Decreased Power shift Pressure (1) Solenoid (2) Valve body (4) Spring (5) Spool (6) Passage (Drain port) (7) Passage (Power shift pressure output) (8) Passage (Pilot oil supply from pilot pump) | |
The power shift pressure that is controlled by the proportional reducing valve is variable. The power shift pressure depends on the PWM signal that is sent from the machine ECM. When the engine is under lighter loads, the PWM signal will decrease. A decrease in the PWM signal will produce a lower power shift pressure that is proportional to the load on the engine.
Note: Similarly, when the engine is under higher loads, the PWM signal will Increase. An increase in the PWM signal will produce a higher power shift pressure that is proportional to the load on the engine.
As the PWM signal decreases, solenoid (1) creates less force to shift spool (5) downward. Spring (4) overcomes some of the force that is created by solenoid (1). Spool (5) shifts upward. When spool (5) moves upward, the flow of pilot oil from passage (8) to passage (7) is restricted. The restriction of pilot oil flow from passage (8) to passage (7) reduces the pilot oil pressure that is in passage (7). The pilot oil pressure that is delivered to the main pump regulators is reduced. The reduction of the pilot oil pressure to the main pump regulators allows the main pump flow to increase.