992G Wheel Loader and 854G Wheel Dozer Power Train Transmission Hydraulic Control Operation Caterpillar


Transmission Hydraulic Control Operation
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992G Wheel Loader and 854G Wheel Dozer Power Train [SENR1346]
POWER TRAIN
CONTROL GP-TRANSMISSION HYD
992G Wheel Loader and 854G Wheel Dozer Power Train Transmission Hydraulic Control
992G Wheel Loader and 854G Wheel Dozer Power Train Transmission Hydraulic Control Operation
1.1. Starting the Engine with the Transmission in Neutral
2.1. Shifting from the NEUTRAL position to the First Speed Forward position with a Running Engine



Illustration 1g00670970

Transmission Hydraulic Controls in the NEUTRAL position and with the Engine Stopped.

(1) Number 2 clutch solenoid

(2) Number 3 clutch solenoid

(3) Priority valve

(4) Number 1 clutch solenoid

(5) Number 5 clutch solenoid

(6) Number 4 clutch solenoid

(7) Flow control orifice

(8) Lockup clutch solenoid valve (if equipped)

(9) Impeller clutch solenoid valve

(10) Slug

(11) Converter inlet ratio valve

(12) Slug

(13) Modulating relief valve

(14) Direction selection spool

(15) Torque converter

(16) Lockup clutch (if equipped)

(17) Impeller clutch

(18) Selector spool for first speed and for third speed

(19) Pressure differential valve

(20) Load piston

(21) Flow control orifice

(22) Selector spool for second speed

(23) Transmission oil filter

(24) Transmission oil filter

(25) Flow control orifice

(26) Oil pump

(27) Torque converter outlet relief valve

(28) Oil cooler

(29) Magnetic screen

(30) Lubrication passage for transmission

(31) Reservoir

(32) Reservoir

(A) Oil pump output pressure tap

(B) Pump pressure tap

(C) Pressure tap for lockup clutch (if equipped)

(D) Torque converter inlet pressure tap P3

(E) Speed clutch pressure tap P1

(F) Pressure tap for impeller clutch

(G) Direction clutch pressure tap P2

(H) Pump pressure tap

(I) Torque converter outlet pressure tap

(J) Transmission lubrication pressure tap

When the engine is started, the transmission charging section of oil pump (26) pulls oil from reservoir (31) through magnetic screen (29). The pump sends the oil through oil filter (23) to priority valve (3). Some of the oil from the priority valve flows to lockup clutch solenoid valve (8) (if equipped) and to impeller clutch solenoid valve (9). The remainder of the oil flows to the selector and pressure control valve. The selector and pressure control valve is part of the transmission hydraulic controls.

The torque converter charging section of oil pump (26) pulls oil from reservoir (31) through magnetic screen (29). The pump sends oil through oil filter (24) to torque converter (15) .

Starting the Engine with the Transmission in Neutral




Illustration 2g00670975

Transmission Hydraulic Controls with a Running Engine in the NEUTRAL position

(2) Number 3 clutch solenoid. (7) Flow control orifice. (11) Converter inlet ratio valve. (13) Modulating relief valve. (18) Selector spool for first speed and for third speed. (19) Pressure differential valve. (20) Load piston. (AA) Speed clutch oil P1. (BB) Direction clutch oil P2. (CC) Converter inlet oil P3. (DD) Converter outlet oil. (EE) Lubrication oil. (FF) Return oil. (GG) Pilot oil. (HH) Torque converter clutch oil.

When the STIC transmission direction control switch is in the NEUTRAL position, solenoid (2) is activated. The solenoid moves a spool and oil is directed to the end of spool (18). Spool (18) moves down and pump oil flows around the spool to the Number 3 clutch.

The Number 3 clutch is now applied. All of the remaining clutches are open to the reservoir.

The oil from pump (26) flows through flow control orifice (7). The oil then flows to Number 3 clutch, converter inlet ratio valve (11), and pressure differential valve (19) .

The oil to pressure differential valve (19) flows through a small orifice in the valve spool. This oil starts to fill the chamber at the top end of the spool. The pressure in the chamber at the top of valve spool (19) increases.

The increase in pressure moves the valve downward against the force of the springs. The movement of the valve spool closes a passage from the area behind the bottom end of load piston (20) and behind the reservoir. At this time, pressure differential valve (19) is in the position that is shown in the schematic. This allows the pressure in the system to increase.

While the pressure in the chamber at the top of pressure differential valve (19) increases, the valve moves downward. This opens the direction clutch circuit to the flow of pump oil. The valve also closes the bottom end of valve (19) to the reservoir. The pressure in the direction clutch circuit increases. The increase is felt in the spring chamber of valve (19) .

When the pressure in the direction clutch circuit is at the maximum, the combination of the pressure in the spring chamber and the force of the springs moves valve (19) upward. The valve moves upward until the flow of pump oil to the direction clutch circuit is stopped. At this time, the movement of the valve spool stops. Now, the valve spool moves down and the valve spool moves up. This keeps a constant pressure in the direction clutch circuit.

Oil from the pump also flows to modulating relief valve (13). This fills the chamber around the modulating relief valve. The oil flows through an orifice in the valve spool. This oil opens the poppet valve at the top of the valve spool. This allows oil to fill the slug chamber at the top of the valve spool.

When the Number 3 clutch is full of oil, the pressure in the speed clutch circuit begins to increase. The increase is felt in the slug chamber at the top of modulating relief valve (13). When the pressure in the speed clutch circuit is at the initial setting of the modulating relief valve, the modulating relief valve moves down. This allows extra oil to go to the torque converter. At the same time, pump oil flows through an orifice. The oil goes through the orifice to the area that is between the bottom of load piston (20) and the cover on the selector and pressure control valve body. This area is closed to the reservoir by the position of differential valve (19). The rate of flow to the area behind load piston (20) is restricted by the orifice.

The pressure that is felt by the modulating relief valve (13) is also felt behind load piston (20). This is because of the pressure increase in the speed clutch circuit. The orifice in the supply passage causes the oil to flow to the area behind the load piston at a specific rate. As the modulating relief valve moves toward the bottom, the load piston moves toward the top. This causes the pressure in Number 3 clutch to increase gradually.

This gradual increase in pressure is modulation. The load piston moves more toward the top against the force of the springs. This occurs until the area behind the load piston is open to a drain passage. At this time, modulation stops. As the oil flows out of the drain passage, oil continues to fill the area behind the load piston. This keeps the load piston in a position without any further movement.

The operation of the load piston and the operation of the modulating relief valve keeps the system pressure at a constant rate.

Pump oil also flows through flow control orifice (7) to converter inlet ratio valve (11). The pump oil flows through an orifice in the valve spool. The oil then fills the slug chamber. The pressure presses against the diameter of the slug.

The oil pressure that is going to the torque converter is felt against the top of valve spool (11). This pressure presses against the whole diameter of the valve spool.

The pressure on the top of the valve spool (11) that is necessary to move the valve spool downward is less than the pressure in the slug chamber that is necessary to move the valve spool upward.

When the inlet pressure that is going to the torque converter rises to the maximum, the valve spool (11) moves downward. This allows the extra oil to flow to the reservoir. When the pressures are balanced again, the valve spool moves upward.

All of the oil that is not used by the clutches flows to the ratio valve for the torque converter .

Shifting from the NEUTRAL position to the First Speed Forward position with a Running Engine




Illustration 3g00670981

Transmission Hydraulic Controls in the FIRST SPEED FORWARD position

(1) Number 2 clutch solenoid. (2) Number 3 clutch solenoid. (5) Number 5 clutch solenoid. (7) Flow control orifice. (10) Slug. (11) Converter inlet ratio valve. (13) Modulating relief valve. (14) Direction selection spool. (18) Selector spool for first speed and for third speed. (19) Pressure differential valve. (20) Load piston. (AA) Speed clutch oil P1. (BB) Direction clutch oil P2. (CC) Converter inlet oil P3. (DD) Converter outlet oil. (EE) Lubrication oil. (FF) Return oil. (GG) Pilot oil. (HH) Torque converter clutch oil.

When the STIC transmission direction control switch is moved to FORWARD and the speed control switch for the STIC transmission is shifted to first speed, solenoids (1) and (5) are activated and solenoid (2) is deactivated. Solenoid (1) moves a spool. Solenoid (1) also directs oil to the top end of direction spool (14). Spool (14) moves downward and pump oil flows around the spool. The pump oil then flows to the Number 2 clutch.

Solenoid (5) moves a spool. Solenoid (5) also directs oil to the bottom of speed selection spool (18). Solenoid (2) is deactivated. Solenoid (2) also directs oil from the top end of speed selection spool (18) to the reservoir. Spool (18) moves upward and pump oil flows around the spool to the Number 5 clutch.

When the shift to first speed forward is made, the Number 3 clutch is opened to the reservoir. The pressure in the system decreases. Springs move modulating relief valve (13) upward. Springs also move pressure differential valve (19) upward until the large orifice at the top end of valve (19) is closed to pump oil by the valve body.

As the pressure differential valve (19) moves upward, the chamber behind load piston (20) opens to the reservoir. This allows the springs to move the load piston downward. The speed clutch oil starts to fill the Number 5 clutch.

When the Number 5 clutch is full of oil, the pressure in the speed clutch circuit starts to increase. The increase is felt in the slug chamber of modulating relief valve (13) and in the chamber at the top of pressure differential valve (19) .

The oil to pressure differential valve (19) starts to fill the chamber at the top of the valve spool through the small orifice.

When the pressure in the Number 5 clutch is approximately 380 kPa (55 psi) pressure differential valve (19) starts to move down. The movement of the valve spool opens the Number 2 clutch to pump oil. The movement of the valve spool also closes a passage from the chamber behind load piston (20) to the reservoir.

When the Number 2 clutch is full of oil, the pressure increases in the direction clutch circuit. The increase is felt in the spring chamber of pressure differential valve (19). The combination of the pressure in the spring chamber and the force of the springs moves the valve spool upward against the speed clutch pressure at the top of the valve spool.

As the pressure increases in the speed clutch circuit, the pressure increases in the chamber at the top of pressure differential valve (19) .

The increase in pressure moves the valve spool downward against the force of the springs. This opens the Number 2 clutch to the flow of pump oil.

As the pressure increases in the Number 2 clutch, the pressure increases in the spring chamber of pressure differential valve (19). The combination of the increased pressure in the spring chamber and the force of the springs moves the valve spool upward. This stops the flow of oil to the Number 2 clutch. This function continues until the pressure in the Number 2 clutch is at the maximum. At this time, the combination of the pressure in the spring chamber and the force of the springs moves the spool upward until the flow of oil to the clutch is stopped. Now, the valve spool moves down and the valve spool moves up. This keeps a constant pressure in the Number 2 clutch. This pressure is less than the speed clutch pressure by approximately 380 kPa (55 psi). This difference is caused by the force of the springs of the pressure differential valve.

Oil from the pump also flows to modulating relief valve (13). This oil fills the chamber around the valve spool. The oil flows through an orifice in the valve spool. The oil then opens the poppet valve at the top end of the valve spool. This allows the oil to fill the slug chamber at the top of the valve spool.

The rate of the pressure increase in the speed clutch circuit is controlled by modulating relief valve (13) and by load piston (20) .

As the pressure increases in the Number 5 clutch, modulating relief valve (13) moves downward and load piston (20) moves upward. The orifice in the supply passage to the load piston causes the oil to flow at a specific rate. The oil flows to the area behind the load piston. As the modulating relief valve moves downward and the load piston moves upward, the pressure gradually increases in the Number 5 clutch. This gradual increase in pressure is known as modulation.

The load piston (20) moves more toward the top against the force of the springs. The load piston moves until the area behind the load piston is open to a drain passage. At this time, modulation stops. As the oil flows out of the drain passage, oil continues to fill the area behind the load piston. This keeps the load piston in the same position. After the pressures in the clutches are at the maximum, modulating relief valve (13) allows the extra oil to flow to the torque converter.

The operation of the load piston (20) and of the modulating relief valve (13) keeps the system pressure at a constant rate.

Pump oil also flows through flow control orifice (7) to converter inlet ratio valve (11). The pump oil flows through an orifice in the valve spool. This oil then fills the slug chamber. This pressure presses against the diameter of the slug.

The oil pressure to the torque converter is felt against the top of valve spool (11). The pressure presses against the entire diameter of the valve spool.

The pressure on the top of the valve spool (11) that is necessary to move the valve spool downward is less than the pressure in the slug chamber that is necessary to move the valve spool upward.

When the inlet pressure to the torque converter increases to the maximum, the valve spool (11) moves down. This allows the extra oil to flow to the reservoir. When the pressures are balanced again, the valve moves upward.

All of the oil that is not used by the clutches flows to the ratio valve for the torque converter .

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