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| Illustration 1 | g00302943 |
Selector and pressure control valve (1) Speed selector spool (2) Modulation relief valve (3) Load piston (4) Pressure differential valve (5) Direction selector spool (6) Ratio valve (7) Outlet passage to the torque converter (8) Inlet passage from the transmission oil pump | |
The selector and pressure control valve is fastened to the planetary transmission. The selector and pressure control valve is completely enclosed by the transmission case.
Speed selector spool (1) and direction selector spool (5) are connected to the transmission selector lever by a linkage arrangement.
(1) The speed selector spool sends oil to No. 3 clutch, No. 4 clutch, and No. 5 clutch (speed clutches).
(2) The modulation relief valve controls the maximum pressure in the system.
(3) The load piston controls the rate of pressure increase in the clutches.
(4) The pressure differential valve maintains a pressure differential between the speed and direction clutches. This valve prevents movement of the machine if the engine is started with the transmission selector lever in a forward or reverse speed.
(5) The direction selector spool sends oil to No. 1 clutch and No. 2 clutch (direction clutches).
(6) The ratio valve controls the maximum oil pressure to the torque converter. The ratio valve is also called the Torque converter inlet relief valve.
Operation
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| Illustration 2 | g00304103 |
The engine is running and the transmission is in NEUTRAL. (1) Priority valve (2) Modulation relief valve (3) Load piston (4) Pressure differential valve (5) Direction selection spool (6) Ratio valve (8) Inlet passage (9) Valve body (10) Orifice (11) Orifices (12) Priority valve (AA) Speed clutch pressure (P1) (BB) Torque converter inlet pressure (LL) Return oil | |
Transmission in Neutral (Running Engine)
When the engine is started, the transmission charge pump pulls oil from the transmission case reservoir. The pump sends the oil through a filter to priority valve (12), and then to the selector and pressure control valve for the transmission.
When the transmission selector lever is in the NEUTRAL position, speed selection spool (1) is in the position that is shown in the illustration 1.
Direction spool (5) can be in either the FORWARD or REVERSE position.
The position of speed selection spool (1) opens No. 3 clutch to oil from the transmission pump. No. 4 and No. 5 speed clutches are opened to the reservoir. The position of direction selection spool (5) opens the right end of pressure differential (8) to the reservoir.
Oil from the pump flows to modulation relief valve (2). The oil fills the chamber around the modulation relief valve. The oil flows through an orifice in the valve spool which opens the poppet valve. The oil fills the slug chamber at the left end of the valve spool.
Also, pump oil flows through the flow control orifice to direction selection spool (5). The oil can then flow to ratio valve (6) and pressure differential valve (4) .
The oil also flows from the direction selection spool to speed selection spool (1). From the left end of the speed selection spool, the oil flows to the No. 3 clutch.
The oil that flows to the pressure differential valve flows through orifice (10) in the valve spool. Oil starts to fill the chamber at the left end of the valve spool.
When the transmission control lever is in NEUTRAL, orifices (11) cannot send oil to the reservoir because the passage around the orifices is filled with pump oil. This allows the differential valve to move a small amount to the right, and to the SET position. In the SET position, orifices (11) are closed by the valve body.
In this position, no oil is sent to the direction clutches.
Operation of the Modulation Relief Valve
Pump oil that flows to the pressure differential valve also flows through an orifice at the right end of load piston (3). The right end of the load piston is closed to the reservoir by the position of pressure differential valve (4). The flow of oil to load piston (3) is restricted by the orifice.
When No. 3 clutch is full of oil, the pressure in the speed clutch circuit starts to increase. The increase is felt in the slug chamber at the left end of modulation relief valve (2) .
When the pressure in the speed clutch circuit is at the initial setting of the modulation relief valve, the modulation relief valve moves to the right. The movement to the right allows extra oil to flow to the lubrication circuit through the torque converter circuit.
The pressure that is felt by the modulation relief valve is also felt on the right end of load piston (3). The orifice in the supply passage to the load piston causes a short delay in the pressure increase at the right end of the load piston.
The pressure increase at the end of the load piston moves the piston to the left. The movement of the piston to the left increases the force of the springs. The increased force of the springs moves modulation relief valve (2) to the left also. The movement to the left causes a pressure increase in the slug chamber. The increased pressure moves the modulation relief valve back to the right.
The clutch pressure increases gradually because of the left and right movement of the modulation relief valve, and the movement to the left of the load piston. The gradual increase in pressure is called modulation.
The movement to the left of the load piston stops when the load piston moves to the drain passage. At this time, the modulation stops. As oil flows out of the drain passage, oil comes through the supply passage to the load piston chamber. This keeps the load piston in position without any further movement.
Pressure in the system will be limited by the spring force on the modulation relief valve. At this time, the valve is opened in order to allow oil flow to the lubrication circuit through the torque converter circuit.
Operation of the Ratio Valve
Ratio valve (6) for the torque converter controls the maximum pressure to the converter. Preventing damage to the torque converter components in a cold engine start (cold oil) is the primary purpose of the ratio valve.
The pressure to the torque converter that is caused by the restrictions to flow is felt against the left end of the valve spool. The whole diameter of the valve spool is pushed by the pressure.
Pressure from the speed clutch circuit is felt in the slug chamber of the valve spool.
Only the diameter of the slug is pushed by the pressure.
The pressure that is needed for moving the valve to the right is less than the pressure needed for moving the valve to the left. Pressure on the left end of the valve moves the valve to the right. Pressure in the slug chamber moves the valve to the left.
When the inlet pressure to the torque converter gets to the maximum, the valve spool moves to the right. The movement to the right allows the extra oil to flow to the reservoir. When the pressures are again in balance, the valve moves back to the left.
All oil that is not used for the clutches flows to the ratio valve for the torque converter.
Neutral to First Speed Forward (Running Engine)
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| Illustration 3 | g00304104 |
The engine is running and the transmission is in NEUTRAL to FIRST SPEED FORWARD. (1) Speed selection spool (2) Modulation relief valve (3) Load piston (4) Pressure differential valve (5) Direction selection spool (6) Ratio valve (9) Valve body (10) Orifice (11) Orifices (12) Priority valve (AA) Speed clutch pressure (P1) (BB) Torque converter inlet pressure (DD) Direction clutch pressure (P2) (LL) Return oil | |
When the transmission selector lever is moved to FIRST SPEED FORWARD, speed selector spool (1) and direction spool (5) move. The position of the spools is shown in illustration 2.
The position of direction selector spool (5) opens a passage to No. 2 clutch and a passage from No. 1 clutch to the reservoir.
The position of speed selector spool (1) opens a passage to No. 5 clutch to pump oil. At the same time, No. 3 clutch and No. 4 clutch are opened to the reservoir.
When the shift from NEUTRAL to FIRST SPEED FORWARD is made, the No. 3 clutch is opened to the reservoir. The pressure in the system decreases. Springs move modulation relief valve (2) toward the left.
Pressure differential valve (4) moves until the oil from the right end of load piston (3) can go to the reservoir. This allows the load piston to move to the right.
No. 5 speed clutch starts to fill. When the clutch is full of oil, the pressure increases in the system. The pressure increase is felt through orifice (10) and against the left end of the pressure differential valve.
The pressure moves the pressure differential valve until the right end of load piston (3) is closed to the reservoir. The pressure differential valve allows oil to go to the No. 2 clutch which starts to fill.
The pressure differential valve will maintain a 380 kPa (55 psi) pressure difference between the oil to the speed clutch and the oil to the direction clutch. The pressure difference allows the speed clutch to engage before the direction clutch.
When the end of the load piston is closed to the reservoir, the pressure oil starts to fill the load piston chamber again. The load piston starts to move to the right and modulation begins.
The movement of the load piston compresses the springs. The springs move the modulation relief valve to the left. The movement to the left causes the pressure of the pump oil in the slug chamber to increase. The increased pressure moves the modulation relief valve back to the right.
The modulation goes on until the clutches reach full pressure. At this time, the load piston will be completely to the left and the modulation relief valve will allow oil to flow to the lubrication circuit.
Pressure Differential Valve Operation
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| Illustration 4 | g00304105 |
Pressure differential valve operation (1) Speed selection spool (2) Modulation relief valve (3) Load piston (4) Pressure differential valve (6) Ratio valve (9) Valve body (11) Orifices (12) Priority valve (AA) Speed clutch pressure (P1) (BB) Torque converter inlet pressure (LL) Return oil | |
When the engine is started with the transmission selector lever in a forward or reverse speed, the operation of pressure differential valve (4) prevents movement of the machine.
When the engine is started, the transmission charge pump sends oil to modulation relief valve (2) and across the flow control orifice to direction spool (5). From the direction selection spool, the oil can go to ratio valve (6) and pressure differential valve (4). The oil also flows to speed selection spool (1). At the same time, the oil begins to fill the No. 5 clutch.
Oil that flows to pressure differential valve (4) flows through orifice (10). The oil from the orifice then fills the chamber at the left end of the spool. Small orifices (11) are open to the reservoir because of the position of the speed selection spool.
Pressure on the left end of the pressure differential valve cannot increase enough to move the valve to the right. Because the pressure cannot move the valve, oil cannot flow to the direction clutch and the machine will not move.
Before the transmission will engage the transmission selector lever must be moved to the NEUTRAL position in order to reset the differential valve.
Moving the transmission selector lever to NEUTRAL closes the passage around orifices (11) so that oil can not flow to the reservoir. This allows the pressure to increase inside the differential valve and the valve moves to the right and to the SET position. See "Transmission in Neutral (Running Engine) ".
The operation of the hydraulic controls will then be normal when a transmission shift is made.
Note: All machines are equipped with a neutral start switch. The transmission selector lever must be in NEUTRAL before the engine starter can be engaged in order to start the engine.