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| Illustration 1 | g00601571 |
Selector and Pressure Control Valve (1) Speed selection spool (2) Valve body (3) Load piston (4) Pressure differential valve (5) Direction selection spool (6) Ratio valve (7) Modulation relief valve | |
The selector and pressure control valve is located inside the top cover of the transmission.
Speed selection spool (1) and direction selection spool (5) are connected to the control linkage. A cable from the transmission control lever is connected to the control linkage.
| Valve | Function |
| (1) Speed selection spool | This spool directs the oil to a speed clutch. |
| (3) Load piston | This piston controls the rate of pressure increase in the clutches. |
| (4) Pressure differential valve | This valve keeps a pressure differential between the speed and the direction clutches. This prevents the movement of the machine if the engine is started with the transmission in a speed and a direction. |
| (5) Direction selection spool | This spool directs oil to a direction clutch. |
| (6) Ratio valve | This valve limits the maximum pressure to the torque converter. |
| (7) Modulation relief valve | This valve controls the maximum pressure in the system. |
Operation of Transmission Selector and Pressure Control Valve
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| Illustration 2 | g00576473 |
Transmission Hydraulic Control (Transmission in Neutral, Engine Off) (8) Oil filter (9) Pitot tube (10) Lockup clutch selector valve (11) Reducing valve (12) Valve body (13) Speed selection spool (14) Modulation relief valve (15) Load piston (16) Pressure differential valve (17) Direction selection spool (18) Ratio valve (19) Torque converter (20) Relief valve for converter outlet (21) Oil cooler (22) Screens (23) Reservoir (24) Oil pump (A) Torque converter scavenge section (B) Transmission charging section | |
Engine Running (Transmission in Neutral)
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| Illustration 3 | g00576476 |
Transmission Hydraulic Controls (Engine Running, Transmission in Neutral) (12) Valve body (13) Speed selection spool (14) Modulation relief valve (15) Load piston (16) Pressure differential valve (17) Direction selection spool (18) Ratio valve (25) Orifice (26) Orifice (AA) Speed clutch pressure (P1) (BB) Torque converter inlet (HH) Signal for the lockup clutch (LL) Return oil | |
When the engine is started, the transmission charging pump pulls oil from reservoir (23). The pump sends the oil through oil filter (8) to reducing valve (11) and to the pressure control valve.
When the transmission control lever is in the NEUTRAL position, speed selection spool (13) and direction selection spool (17) are in the position that is shown in the schematic.
The position of speed selection spool (13) opens the No. 4 clutch to pump oil. The No. 5 speed clutch and the No. 6 speed clutch are opened to reservoir (23). The position of direction selection spool (17) opens the direction clutches to reservoir (23) .
Oil from the pump flows to modulation relief valve (14). The oil fills the chamber around the modulation relief valve. The oil flows through an orifice that is located in the valve spool. The oil then opens the poppet valve. The oil fills the slug chamber at the left end of the valve spool.
The pump oil also flows to ratio valve (18) and pressure differential valve (16) .
The oil to pressure differential valve (16) flows through orifice (26) in the valve spool. The oil starts to fill the chamber at the left end of the valve spool. When the transmission selector lever is in the NEUTRAL position, orifice (25) can not send oil to reservoir (23) because the drain passage is blocked by the position of speed selection spool (13). This allows the differential valve to move to the SET position. In this position, orifices (25) are closed by valve body (12). The pressure differential valve can not move to the right because the oil drain passage is blocked. In this position, no oil is sent to the direction clutches.
Operation of Modulation Relief Valve
The pump oil that flows to pressure differential valve (16) also flows through an orifice to the right end of load piston (15). The right end of the load piston is closed to the reservoir by the position of the pressure differential valve. The flow of the oil to the load piston is restricted by the orifice.
When No. 4 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 (14). 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. This allows extra oil flow to the lubrication circuit.
The pressure that is felt by modulation relief valve (14) is also felt on the right end of load piston (15). 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. This pressure increase at the end of the load piston moves the piston to the left. The movement to the left increases the force of the springs. The movement also moves modulation relief valve (14) to left. This causes a pressure increase in the slug chamber. This will then move the modulation relief valve to the right.
The left movement and the right movement of modulation relief valve (14) causes the clutch pressure to gradually increase. The movement to the left of load piston (15) will also cause the clutch pressure to gradually increase. This gradual increase in pressure is known as modulation.
The movement of load piston (15) to the left stops when the load piston moves toward the drain passage. At this time, modulation stops. As oil is flowing out of the drain passage, the oil flows through the supply passage to the load piston chamber. This keeps the load piston stationary. The pressure in the system will be limited by the spring force on modulation relief valve (14). At this time, the valve is opened in order to allow the oil flow to the lubrication circuit.
Operation of Ratio Valve
Ratio valve (18) controls the maximum pressure to the torque converter. The primary purpose of the ratio valve is the prevention of damaged components of the torque converter.
The pressure to the torque converter that is caused by restrictions to flow is felt against the left end of the valve spool. The pressure then pushes the whole diameter of the valve spool.
The pressure from the speed clutch circuit is felt in the slug chamber of the valve spool. This pressure pushes only the diameter of the slug.
The pressure that is needed on the left end of the valve to move the valve to the right is less than the pressure that is needed in the slug chamber to move the valve to the left.
When the inlet pressure to the torque converter reaches the maximum pressure, the valve spool moves to the right. This allows the extra oil to reservoir (23). When the pressures are again in balance, the spring force moves the valve to the left.
The oil that is not being used by the clutches flows to ratio valve (18) .
Engine is Running (Transmission in Neutral to Fourth Speed)
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| Illustration 4 | g00576477 |
Transmission Hydraulic Controls (Engine Running, Transmission in Fourth Speed) (12) Valve body (13) Speed selection spool (14) Modulation relief valve (15) Load piston (16) Pressure differential valve (17) Direction selection spool (18) Ratio valve (25) Orifice (26) Orifice (AA) Speed clutch pressure (P1) (BB) Torque converter inlet (CC) Direction clutch pressure (P2) (HH) Signal for lockup clutch (LL) Return oil | |
When the transmission control lever is moved to the FOURTH SPEED, speed selector spool (13) and direction selector spool (17) will move to the positions that are shown in the schematic.
The position of direction selector spool (17) opens a passage to the No. 3 clutch. The position of the direction selector spool also opens a passage from the No. 4 clutch to reservoir (23) .
The position of speed selector spool (13) opens a passage to the No. 5 clutch to pump oil. The position of the speed selector spool also opens the No. 4 and No. 6 clutch to reservoir (23) .
When a shift from the NEUTRAL to the FOURTH SPEED position is made, the pressure in the system decreases. The springs move modulation relief valve (14) toward the left. Pressure differential valve (16) moves until oil from the right end of load piston (15) can go to reservoir (23). This allows the load piston to move to the right.
The No. 5 speed clutch begins to fill. When the No. 5 speed clutch is full of oil, the pressure increases in the system. This pressure increase is felt through orifice (26) and against the left end of pressure differential valve (16). This moves the pressure differential valve until oil from the right end of load piston (15) is closed to reservoir (23). The differential valve will also allow oil to go into the No. 3 clutch. The No. 3 clutch will start to fill. The pressure differential valve will keep a pressure difference between the oil to the speed clutch and the oil to the direction clutch. This allows the speed clutch to engage before the direction clutch. Refer to Testing and Adjusting, "Transmission Clutch Pressure - Test" for more information on the pressure differential valve.
When the end of load piston (15) is closed to the reservoir, the pressure oil starts to fill the load piston chamber. The load piston starts to move to the left and the modulation starts.
This movement of the load piston compresses the springs. The movement also moves modulation relief valve (14) to the left. This causes the pressure of the pump oil in the slug chamber to increase. This then moves the modulation relief valve to the right.
Modulation continues until the clutches reach full pressure. At this time, load piston (15) will be completely to the left and modulation relief valve (14) will allow oil flow to the lubrication circuit.
Starting the Engine (Transmission in a Speed and Direction)
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| Illustration 5 | g00576479 |
Transmission Hydraulic Controls (Starting the Engine With Transmission in a Speed and Direction) (12) Valve body (13) Speed selection spool (14) Modulation relief valve (15) Load piston (16) Pressure differential valve (17) Direction selection spool (18) Ratio valve (25) Orifice (26) Orifice (AA) Speed clutch pressure (P1) (BB) Torque converter inlet (HH) Signal for lockup clutch (LL) Return oil | |
When the engine is started with the control lever in a forward or reverse speed, pressure differential valve (16) prevents movement of the machine.
When the engine is started, the transmission charging pump sends oil to modulation relief valve (14) and across the flow control orifice. From direction selection spool (17), the oil can flow to ratio valve (18) and pressure differential valve (16). The oil flows to speed selection spool (13). The oil then begins to fill the No. 5 clutch.
The oil that flows to pressure differential valve (16) flows through orifice (26). The oil then fills the chamber on the left end of the spool. Orifices (25) are open to reservoir (23) because of the position of speed selection spool (13). The pressure of the pressure differential valve can not reach a high enough pressure in order to move the valve to the right. Because of the lack of pressure, oil can not flow to the direction clutch and the machine will not move.
Before the operator can move the machine, the operator must shift to the NEUTRAL position. This closes the passage around orifices (25) so that oil can not flow to reservoir (23). This allows the pressure to increase inside pressure differential valve (16). The pressure differential valve will move to the right to the SET position. The operation of the hydraulic controls will then be normal when a shift is made to a forward or reverse speed.