994 Wheel Loader Power Train Caterpillar

Illustration 1	g00307844
Transmission Hydraulic Schematic (The Early Torque Converter is Shown.) (The Transmission is in Neutral and the Engine is not Running.) (1) Oil Filter from the Transmission Charging Section of the Pump. (2) Oil Filter from the Torque Converter Charging Section of the Pump. (3) Torque Converter. (4) Torque Converter Outlet Relief Valve. (5) Bypass Passage. (6) Lockup Clutch Valve. (7) Transmission Charging Section of the Pump. (8) Torque Converter Charging Section of the Pump. (9) Oil Cooler. (10) Line and Passage for the Lubrication System. (11) Magnetic Screens. (12) Port. (13) Speed Selection Spool. (14) Load Piston. (15) Modulation Relief Valve. (16) Bottom Manifold. (17) Pressure Differential Valve. (18) Direction Selection Spool. (19) Transmission Neutralizer Valve. (20) Ratio Valve. (21) Selector and Pressure Control Valve. (22) Orifice. (23) Orifice. (24) Flow Control Orifice. (A) Pressure Tap for Transmission Pump. The Tap is Located on the Filter. (B) Pressure Tap for the Outlet of the Torque Converter. (C) Pressure Tap for the Lockup Clutch. (D) Pressure Tap for the Lockup Clutch Valve. (E) Pressure Tap for Pilot Oil Pressure. (F) Pressure Tap for the Torque Converter Pump and Inlet to the Torque Converter. (G) Pressure Tap for the Speed Clutch ("P1" ). (H) Pressure Tap for the Direction Clutch ("P2" ).

Illustration 2	g00307845
Transmission Hydraulic Controls (12) Port. (13) Speed Selection Spool. (16) Bottom Manifold. (The Bottom Manifold Includes the Neutralizer Valve.) (18) Direction Selection Spool. (21) Selector and Pressure Control Valve. (25) Oil Tube (Inlet for the transmission oil). (26) Oil Tube (Inlet for the Torque Converter Oil). (27) Top Manifold. (28) Plate. (29) Opening to the No. 1 Clutch. (30) Opening to the No. 2 Clutch. (31) Opening to the No. 3 Clutch. (32) Opening to the No. 4 Clutch. (33) Opening to the No. 5 Clutch.

The hydraulic controls for the transmission are installed on the case of the transmission. The controls consist of top manifold (27) , selector and pressure control valve (21) , plate (28) , and bottom manifold (16) . The bottom manifold includes the neutralizer valve.

Oil from transmission pump (7) is used for operation of the hydraulic controls. Oil is pulled from the reservoir in the case for the output transfer gears. Before the oil leaves the case, the oil flows through magnetic screens (11) . Oil then flows through filter (1) and tube (25) to pressure control valve (21) .

Note: On machines 9YF28-UP, oil flows through filter (1) , and the oil then flows to the priority valve. Oil flows from the priority valve through tube (25) to pressure control valve (21) .

Torque converter pump (8) also pulls oil from the reservoir in the case of the output transfer gear. The oil flows through magnetic screens before the oil flows to the pump. This oil flows through filter (2) , to the torque converter (3) and through tube (26) to ratio valve (20) .

Brake system oil enters through port (12) in bottom manifold (16) . The oil moves neutralizer valve (19) which neutralizes the transmission when the left brake pedal is used to apply the brake.

Oil flows through openings (29) , (30) , (31) , (32) , and (33) which are in bottom manifold (16) to the clutches for the speed and direction that are selected.

Illustration 3	g00307847
Transmission Neutralizer Valve (12) Port. (16) Bottom Manifold. (34) Spring. (35) Bolt. (36) Spool.

The neutralizer valve is located in bottom manifold (16) of the hydraulic controls for the transmission.

The neutralizer valve is activated by the left brake pedal. When the left brake pedal is pushed, brake pressure oil moves spool (36) to the left. This opens a passage from the pressure control valve to the tank. Oil in the direction clutch flows to the tank and the oil releases the clutch.

Because a speed clutch and a direction clutch must be engaged for power to flow through the transmission, power does not flow through the transmission when the left brake pedal is pushed.

When the left brake pedal is released, the passage to the tank is closed. The direction clutch engages. Power flows through the transmission.

Illustration 4	g00307848
Selector and Pressure Control Valve (First Speed Forward and Neutral.) (13) Speed Selection Spool. (14) Load Piston. (15) Modulation Relief Valve. (17) Pressure Differential Valve. (18) Direction Selection Spool. (20) Ratio Valve. (21) Selector and Pressure Control Valve.

Selector and pressure control valve (21) is the second component of four components that make up the hydraulic controls for the transmission. The position of the cables for the speed and direction controls valve (21) . The valve directs the oil to the appropriate speed and direction clutches.

Speed selection spool (13) is connected to the speed control cable from the transmission control in the cab.

Direction selection spool (18) , which is connected through the transmission neutralizer, is connected to the direction control cable from the transmission control in the cab. The position of spool (18) opens the No. 2 transmission clutch to oil from the pump. The spool also opens the right end of pressure differential valve (17) to the reservoir. The No. 4 and No. 5 clutches are opened to the reservoir. The No. 4 and No. 5 clutches are clutches that are used for speed. The position of spool (18) also opens the No. 1 and No. 2 clutches to the reservoir. The No. 1 and No. 2 clutches are clutches that are used for direction.

Reference: For more information on the cables for the selector and pressure control valve, make reference to Service Manual, SENR4745, "994 Wheel Loader, Testing and Adjusting for the Power Shift Transmission".

Operation

Starting the Engine (Transmission in Neutral)

Illustration 5	g00307849
Transmission Hydraulic Controls (Neutral with the Engine Started.) (13) Speed Selection Spool. (14) Load Piston. (15) Modulation Relief Valve. (17) Pressure Differential Valve. (18) Direction Selection Spool. (19) Transmission Neutralizer Valve. (20) Ratio Valve. (21) Selector and Pressure Control Valve. (23) Orifice. (24) Flow Control Orifice.

When the engine is started, oil from pump (7) flows through flow control orifice (24) to the No. 3 clutch, ratio valve (20) , and pressure differential valve (17) .

Oil from the pump flows through an orifice in ratio valve (20) and the oil fills the slug chamber.

Oil from the pump also flows through an orifice in pressure differential valve (17) . Oil starts to fill the chamber at the left end of the valve spool. When the transmission control is in NEUTRAL, direction selection spool (18) blocks the passage between orifice (23) and the reservoir.

The pressure in the chamber which is at the left end of valve (17) increases and the pressure moves the spool to the right. Now, orifice (23) is not located over the passage to the reservoir. When valve (17) moves to the right, the valve also blocks the passage from the right end of load piston (14) to the reservoir. Now, pressure differential valve (17) is in a position for allowing a pressure increase in the system.

As the pressure in the chamber at the left end of pressure differential valve (17) increases, the valve spool moves to the right against the cover. This allows oil from the pump to flow to direction selection spool (18) . Until a direction is selected, spool (18) blocks oil flow to No. 1 and No. 2 clutches. The spool of valve (17) also blocks the oil passage from the right end of the valve spool to the reservoir. As a result, pressure increases to spool (18) and to the spring chamber of valve (17) .

When the pressure at spool (18) is at the maximum pressure, the combined force moves the spool of valve (17) to the left. The combined force is the force of the pressure in the spring chamber and the force of the springs. The valve moves to the left until the flow of oil from the pump to the spool (18) is blocked. At this time, the movement of the valve spool stops. The spool of valve (17) moves to the left and to the right in order to maintain a constant pressure to the direction selection spool (18) .

Oil from the transmission pump also flows to modulation relief valve (15) . Oil fills the chamber around valve (15) . Oil flows through an orifice in the valve spool and the oil moves a poppet valve to the left. Oil fills the chamber to the right of the slug in valve (15) .

Oil also flows through an orifice to the right end of load piston (14) . Because the right end of piston (14) is blocked to the reservoir by the position of valve (17) , oil pressure increases. An orifice in the passage to the right end of piston (14) delays the pressure increase.

When the No. 3 clutch is full of oil, pressure to valve (15) starts to increase. The increase is felt in the slug chamber. When the pressure is at the initial setting of valve (15) , the valve moves to the right. The moving of the valve allows some oil from the transmission and the torque converter to combine and the combined oil flows to the torque converter.

The pressure that is felt by modulation relief valve (15) is also felt on the right end of load piston (14) . The orifice in the supply passage to piston (14) causes a short delay in the pressure increase at the right end of the piston. The pressure at the right end of the load piston combines with the force of the springs.

As pressure in the No. 3 clutch increases, valve (15) moves to the right and piston (14) moves to the left. The orifice in the supply passage to the load piston allows oil to flow to the piston at a specific rate. As valve (15) moves to the right and piston (14) moves to the left, pressure in the No. 3 clutch increases gradually. This gradual increase in pressure is known as modulation.

Piston (14) moves to the left until the chamber at the right end is opened to a drain passage. At this time, modulation stops. As oil flows out of the drain passage, oil comes through the supply passage to the chamber at the right end of load piston (14) . This keeps piston (14) in position without any further movement. Modulation relief valve (15) moves to the right which allows extra oil to flow to the torque converter. The operation of the modulation relief valve (15) and load piston (14) keeps a constant pressure in the system.

Running Engine (Neutral to First Speed Forward)

Illustration 6	g00307850
Transmission Hydraulic Controls (First Speed Forward.) (13) Speed Selection Spool. (14) Load Piston. (15) Modulation Relief Valve. (17) Pressure Differential Valve. (18) Direction Selection Spool. (19) Transmission Neutralizer Valve. (20) Ratio Valve. (21) Selector and Pressure Control Valve. (23) Orifice. (24) Flow Control Orifice.

When the hydraulic control valve for the transmission is moved to FIRST SPEED FORWARD, speed selection spool (13) and direction selection spool (18) are in the positions that are shown in the schematic.

The position of direction selection spool (18) opens a passage to the No. 2 clutch. Passages from the No. 3 clutch and the No. 1 clutch are opened to the reservoir.

The position of speed selection spool (13) allows oil from the pump to flow to the No. 5 clutch. The No. 4 clutch is open to the reservoir.

When the shift to FIRST SPEED FORWARD is made, the No. 3 clutch drains to the reservoir. At the same time, pressure decreases for a moment to valve (15) and valve (17) . Springs move modulation relief valve (15) to the left. Springs move pressure differential valve (17) to the left until orifice (23) is blocked by the valve body.

As valve (17) moves to the left, the chamber at the right end of load piston (14) opens to the reservoir. The pressure differential valve is now in a position for allowing the pressure to increase in the speed clutch circuit.

Pump oil starts to fill the No. 5 clutch for FIRST SPEED. When the No. 5 clutch is full of oil, pressure in the speed circuit starts to increase. The increase is felt in the slug chamber of modulation relief valve (15) . The increase is also felt in the chamber at the left end of pressure differential valve (17) . In valve (17) , oil flows through orifice (22) . Oil starts to fill the chamber at the left end of the valve spool.

When pressure in the No. 5 clutch is approximately 380 kPa (55 psi), valve (17) moves to the right. This opens the No. 2 clutch to pump oil. At the same time, the pressure closes a passage from the right end of load piston (14) to the reservoir.

When the No. 2 clutch is full of oil, the pressure increases in the spring chamber of valve (17) . The pressure combines with the force of the springs in order to act against the left end of valve (17) . At this point, the pressure in the speed clutch and the direction clutch increases at the same rate.

Pressure differential valve (17) controls the rate of pressure increase in the direction clutch circuit. As the pressure in the speed clutch circuit increases, the pressure in the chamber at the left end of valve spool (17) moves the pressure differential valve to the right. This opens the No. 2 clutch to pump oil.

As the pressure increases in the No. 2 clutch circuit, the combined force of oil pressure in the spring chamber and the springs moves valve (17) to the left. This stops the flow of oil to the No. 2 clutch. The pressure increases and valve (17) moves to the right. As the pressure in the No. 2 clutch circuit increases again, this pressure plus the springs force valve (17) to the left. This cycle of operation gives a gradual increase in pressure to the No. 2 clutch. The cycle continues until the pressure is at the maximum pressure.

When the pressure in the No. 2 clutch is at the maximum pressure, the pressure in the spring chamber plus the force of the springs move the valve spool to the left. The valve moves to the left until the flow of oil to the No. 2 clutch is stopped. At this time, the valve spool moves to the left and right in order to maintain a constant pressure in the No. 2 clutch. The No. 2 clutch pressure is less than the pressure in the speed clutch by approximately 380 kPa (55 psi). This difference is determined by the springs of the pressure differential valve.

Pump oil also flows through flow control orifice (24) to ratio valve (20) of the torque converter. Pump oil flows through an orifice in valve (20) and the oil fills the slug chamber.

Pump oil also flows to modulation relief valve (15) . Oil fills the chamber around the valve and flows through an orifice in the valve spool. The oil opens a poppet valve and the oil fills the slug chamber.

Modulation relief valve (15) and load piston (14) control the rate of pressure increase in the speed clutch circuit. As the pressure in the No. 5 speed clutch increases, modulation relief valve (15) moves to the right and load piston (14) moves to the left. The orifice in the supply passage to piston (14) allows oil to flow to the load piston at a specific rate. As valve (15) moves to the right and piston (14) moves to the left, the pressure in the No. 5 clutch gradually increases. This gradual increase in pressure is known as modulation.

Piston (14) moves to the left until the chamber at the right end is opened to a drain passage. At this time, modulation stops. As oil flows out of the drain passage, oil comes through the supply passage to the load piston chamber. Valve (15) moves to the right which allows extra oil to flow to the torque converter. The operation of the modulation relief valve and the load piston keeps a constant pressure in the speed clutch.

After the pressures in the clutches are at the maximum, modulation relief valve (15) prevents the pressures from going too high. The modulation relief valve sends the extra oil to the torque converter.

Starting the Engine (Transmission in a Speed and Direction)

Illustration 7	g00307851
Transmission Hydraulic Controls (The Engine is Started while the gear selector is in First Speed Forward.) (13) Speed Selection Spool. (14) Load Piston. (15) Modulation Relief Valve. (17) Pressure Differential Valve. (18) Direction Selection Spool. (19) Transmission Neutralizer Valve. (20) Ratio Valve. (21) Selector and Pressure Control Valve. (23) Orifice. (24) Flow Control Orifice.

When the engine is started with the transmission control lever in either FORWARD or REVERSE, the operation of pressure differential valve (17) prevents movement of the machine.

When direction selection spool (18) is in either FORWARD or REVERSE, the position of the spool opens the chamber at the left end of pressure differential valve (17) to the reservoir. This connection is through orifice (23) .

As the pressure in the system increases, the pressure in the chamber at the left end of pressure differential valve (17) does not increase. The springs of valve (17) force the valve all the way to the left. Because valve (17) does not move, supply oil is blocked to a direction clutch.

The transmission selection lever must be moved to NEUTRAL before pressure will increase in the chamber at the left end of pressure differential valve (17) . When the control lever is moved to NEUTRAL, direction selection spool (18) blocks the passage between the chamber and the reservoir. The chamber is at the left end of valve (17) . Pressure will then move pressure differential valve (17) to the right. The movement of the valve spool blocks orifice (23) . Valve (17) is now in a position for allowing the pressure increase in the direction clutch circuit.

Neutralizer Valve

When the left brake pedal is pushed, brake pressure oil moves down neutralizer valve (19) . This opens a passage to the oil reservoir in the chamber at the left end of pressure differential valve (17) . The combined force moves the pressure differential valve to the left. The combined force is the force of the springs of valve (17) and the oil pressure from the No. 1 or No. 2 direction clutch. As a result, the engaged direction clutch can drain to the reservoir through the right end of the valve spool. As the valve spool moves to the left, the spool blocks the flow of oil to the direction clutch. Now, the pressure differential valve is in a position for allowing the pressure to increase in the direction clutch when the left brake pedal is released.

Ratio Valve for the Inlet of the Torque Converter

Ratio valve (20) controls the maximum pressure to the torque converter. The main function of the ratio valve is the prevention of damage to the components of the torque converter when the engine is started with cold oil.

Inlet pressure at the torque converter, which is caused by restrictions to flow, is felt at the left end of valve (20) . The pressure forces the large diameter of the valve. Pressure from the "P1" clutch circuit acts against the slug in the chamber of valve (20) . The pressure only applies a force against the diameter of the slug. As a result, the pressure that is needed for moving the valve to the right is less than the pressure that is needed for moving the valve to the left.

When the inlet pressure to the torque converter increases to the maximum, the valve spool moves to the right. This allows the extra oil to flow to the reservoir. When the pressures are again in balance, the valve moves back to the right.

Oil which is not used by the clutches flows to the ratio valve for the inlet oil of the torque converter. Clutch oil moves the modulation relief valve to the right and combines with inlet oil for the torque converter at the ratio valve.