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| Illustration 1 | g00282558 |
Lift Control Valve (HOLD Position) (1) Port to the head end of the lift cylinders. (2) Port to the rod end of the lift cylinders. (3) Spring. (4) Spring. (5) Stem. (6) Passage to the next control valve. (7) Passage from the implement pump. (8) Passage to the next control valve. (9) Passage to the hydraulic oil tank. (10) Spring. (11) Spring. (12) Port from the pilot valve. (13) Port from the pilot valve. (14) Passage to the hydraulic oil tank. (15) Passage. (16) Passage. (17) Passage. (18) Makeup valve for the rod end of the lift cylinder. (19) Check valve. | |
Lift Control Valve (HOLD Position)
The lift control valve is the last control valve in the series of valves that make up the main control valve. When the engine is running, oil flows from the implement pump and into the pump inlet. Then, the oil flows through the auxiliary control valve, through the tilt control valve, and into passage (7) .
When the lift control valve is in the HOLD position, the oil flows around stem (5) and into passages (6) and (8). The oil then flows through the lift control valve and to the hydraulic oil tank.
When the lift control valve is in the HOLD position, stem (5) blocks the flow of oil in ports (1) and (2). The oil in the circuits to the lift cylinders is stopped. The lift cylinders can not move until the pilot control lever and stem (5) are moved to the RAISE position, to the LOWER position, or to the FLOAT position.
Springs (3) and (4) are located on the left end of stem (5). Springs (10) and (11) are located on the right end of valve spool (5). The springs keep stem (5) centered when no pilot oil is being routed to the spool.
Lift Control Valve (RAISE Position)
When the lift control lever is moved to the RAISE position, pilot oil flows from the pilot valve and into port (12). Pilot oil combines with the force of springs (3) and (4) on the left end of stem (5). This combined force overcomes the force of springs (10) and (11) on the right end of stem (5). This causes the spool to move to the right.
When stem (5) moves to the right and into the RAISE position, pump oil flow from passage (7) is blocked from entering passages (6) and (8). The oil pressure in passage (7) increases until check valve (19) opens. When the check valve opens, the pump oil in passage (7) flows through the check valve, into passage (16), and out of port (1). Then, the oil flows to the head end of the lift cylinders.
The pump oil that is flowing into the head ends of the lift cylinders extends the cylinder rods and the lift arms raise. Return oil from the rod ends of the lift cylinders flows through port (2), through passage (9), through the hydraulic oil filter, and to the hydraulic oil tank.
An electromagnetic detent assembly in the pilot valve will hold the lift control lever in the RAISE position. The lift control lever is held in this position just until the lift arms are fully extended.
The main relief valve in the main control valve serves as the relief valve for the head ends of the lift circuit.
Lift Control Valve (LOWER Position)
When the lift control lever is moved to the LOWER position, pilot oil flows from the pilot valve and into port (13). Pilot oil combines with the force of springs (10) and (11) on the right end of valve spool (5). This combined force overcomes the force of springs (3) and (4) on the left end of stem (5). This causes the spool to move to the left.
When stem (5) moves to the left and into the LOWER position, pump oil flow from passage (7) is blocked from entering passages (6) and (8). The oil pressure in passage (7) increases until check valve (19) opens. When the check valve opens, the pump oil in passage (7) flows through the check valve, into passage (16), and out of port (2). The oil then flows to the rod ends of the lift cylinders.
The pump oil that is flowing into the rod ends of the lift cylinders retracts the cylinder rods and the lift arms lower. Return oil from the head ends of the lift cylinders flows through port (1), through passage (14), through the hydraulic oil filter, and to the hydraulic oil tank.
The main relief valve in the main control valve serves as the relief valve for the rod ends of the lift circuit.
Lift Control Valve (FLOAT Position)
When the lift control lever is moved to the FLOAT position, pilot oil flows from the pilot valve and into port (13). Pilot oil combines with the force of springs (10) and (11) on the right end of stem (5). This combined force overcomes the force of springs (3) and (4) on the left end of stem (5). This causes the spool to move to the left.
When stem (5) moves to the left and into the FLOAT position, pump oil flow from passage (7) is blocked from entering passage (16). The oil in passage (7) flows into passage (6) and to the hydraulic oil tank.
The position of stem (5) allows oil from the rod ends to flow through port (2) and into passage (16). The oil in passage (16) closes check valve (19). Then, the oil flows into passage (15) and out of port (1). When the oil pressure in both ends of the lift cylinders is equal, the weight of the lift arms and of the work tool causes the work tool to lower to the ground.
While the lift control lever and the valve spool are in the FLOAT position, the work tool will move along the contour of the ground during machine travel.
Makeup Valve
Makeup valve (18) is mounted in passage (9) to the rod ends of the lift cylinders. The makeup valve functions when the control valve spool is in the FLOAT position or in the LOWER position.
Spring force combines with the oil pressure in the spring chamber in order to hold makeup valve (18) closed. The makeup valve opens only when the pump can not supply enough oil to the lift cylinders. This prevents cavitation in the lift cylinders.
When not enough oil is being supplied to the rod ends of the lift cylinders, the oil pressure in passage (9) is low. The higher pressure of the return oil that is in the passage to the hydraulic oil tank opens the makeup valve. This allows more oil to flow to the rod ends of the lift cylinders.
Tilt Control Valve
Note: All 938G Wheel Loaders are equipped with one tilt cylinder. All IT38G Integrated Toolcarriers are equipped with two tilt cylinders.
938G Wheel Loaders
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| Illustration 2 | g00282559 |
Tilt Control Valve (HOLD Position) (1) Port to the rod end of the tilt cylinder. (2) Port to the head end of the tilt cylinder. (3) Spring. (4) Spring. (5) Stem. (6) Passage to the next control valve. (7) Passage from the implement pump. (8) Passage to the next control valve. (9) Passage to the hydraulic oil tank. (10) Spring. (11) Spring. (12) Port from the pilot valve. (13) Port from the pilot valve. (14) Relief valve for the rod end of the tilt cylinder. (15) Passage to the hydraulic oil tank. (16) Passage. (17) Passage. (18) Passage. (19) Relief valve for the head end of the tilt cylinder. (20) Check valve. | |
IT38G Integrated Toolcarriers
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| Illustration 3 | g00282560 |
Tilt Control Valve (HOLD Position) (1) Port to the head end of the tilt cylinders. (2) Port to the rod end of the tilt cylinders. (3) Spring. (4) Spring. (5) Stem. (6) Passage to the next control valve. (7) Passage from the implement pump. (8) Passage to the next control valve. (9) Passage to the hydraulic oil tank. (10) Spring. (11) Spring. (12) Port from the pilot valve. (13) Port from the pilot valve. (14) Relief valve for the head end of the tilt cylinder. (15) Passage to the hydraulic oil tank. (16) Passage. (17) Passage. (18) Passage. (19) Relief valve for the rod end of the tilt cylinder. (20) Check valve. | |
Tilt Control Valve (HOLD Position)
The tilt control valve is the next control valve in the series of valves that make up the main control valve. When the engine is running, oil flows from the implement pump and into the pump inlet. The oil then flows through the coupler/auxiliary control valve and into passage (7) .
When the tilt control valve is in the HOLD position, the oil flows around stem (5), into passages (6) and (8), and to the lift control valve. If the lift control valve is also in the HOLD position, the oil flows through the lift control valve and to the hydraulic oil tank.
When the tilt control valve is in the HOLD position, valve spool (5) blocks the flow of oil in ports (1) and (2). The oil in the circuits to the tilt cylinder(s) is stopped. The tilt cylinder(s) can not move until the tilt control lever and stem (5) are moved to the TILT BACK position or to the DUMP position.
Springs (3) and (4) are located on the left end of valve spool (5). Springs (10) and (11) are located on the right end of stem (5). The springs keep the spool centered when no pilot oil is being routed to the spool.
Tilt Control Valve (TILT BACK Position)
When the tilt control lever is moved to the TILT BACK position, pilot oil flows from the tilt pilot valve and into port (13). Pilot oil combines with the force of springs (10) and (11) on the right end of stem (5). This combined force overcomes the force of springs (3) and (4) on the left end of stem (5). This causes the stem (5) to move to the left.
When stem (5) moves to the left and into the TILT BACK position, pump oil flow from passage (7) is blocked from entering passages (6) and (8). The oil pressure in passage (7) increases until check valve (20) opens. When the check valve opens, the pump oil in passage (7) flows through the check valve, into passage (17), and out of port (2). Then, the oil flows to the head end of the tilt cylinder(s).
The pump oil that is flowing into the head end of the tilt cylinder for the 938G Wheel Loader extends the cylinder rod and the work tool tilts back. The pump oil that is flowing into the rod end of the tilt cylinders for the IT38G Wheel Loader retracts the cylinder rod and the work tool tilts back. Return oil from the tilt cylinder(s) flows through port (1), through passage (15), through the hydraulic oil filter, and to the hydraulic oil tank.
Some of the oil in passage (7) does flow to the lift control valve. This allows some lifting. As the need for oil in the tilt cylinders is lessened, more oil is available to the lift control valve.
Tilt Control Valve (DUMP Position)
When the tilt control lever is moved to the DUMP position, pilot oil flows from the pilot valve and into port (12). Pilot oil combines with the force of springs (3) and (4) on the left end of stem (5). This combined force overcomes the force of springs (10) and (11) on the right end of stem (5). This causes the spool to move to the right.
When stem (5) moves to the right and into the DUMP position, pump oil flow from passage (7) is blocked from entering passages (6) and (8). The oil pressure in passage (7) increases until check valve (20) opens. When the check valve opens, the pump oil in passage (7) flows through the check valve, into passage (17), and out of port (1). Then, the oil flows to the rod end of the tilt cylinders for the 938G Wheel Loader. Then, the oil flows to the rod end of the tilt cylinders for the IT38G Integrated Toolcarrier.
The pump oil that is flowing into the rod end of the tilt cylinder for the 938G Wheel Loader retracts the cylinder rod and the work tool dumps. The pump oil that is flowing into the head end of the tilt cylinders for the IT38G Integrated Toolcarrier extends the cylinder rod and the work tool dumps. Return oil from the tilt cylinder(s) flows through port (2), through passage (9), through the hydraulic oil filter, and to the hydraulic oil tank.
Some of the oil in passage (7) does flow to the lift control valve. This allows some lifting. As the need for oil in the tilt cylinder is lessened, more oil is available to the lift control valve.
After the bucket is dumped and the tilt control lever is placed in the TILT BACK position, an electromagnetic detent assembly in the tilt pilot valve will hold the tilt control lever in the TILT BACK position. The tilt control lever will be held until the bucket returns to the desired operating angle.
Makeup And Relief Valves
Two relief valves are mounted in the tilt control valve. The two makeup and relief valves operate in the same manner as the two makeup and relief valves in the auxiliary control valve.
Reference: For additional information on the pressure setting of the makeup and relief valves, refer to the Service Manual module Specifications, "Relief Valve (Line)" for the machine that is being serviced.
Reference: For additional information about the operation of the makeup and relief valves, refer to the Service Manual module System Operation, "Coupler/Auxiliary Control Valve" for the machine that is being serviced.
Coupler/Auxiliary Control Valve
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| Illustration 4 | g00282561 |
Coupler/Auxiliary Control Valve (HOLD Position) (1) Port to the head ends of the auxiliary cylinder. (2) Port to the rod ends of the auxiliary cylinder. (3) Spring. (4) Spring. (5) Stem. (6) Passage to the next control valve. (7) Passage from the implement pump. (8) Passage to the next control valve. (9) Passage to the hydraulic oil tank. (10) Spring. (11) Spring. (12) Port from the pilot valve. (13) Port from the pilot valve. (14) Relief valve on the rod end of the auxiliary cylinder. (15) Passage to the hydraulic oil tank. (16) Passage. (17) Passage. (18) Passage. (19) Relief valve on the head end of the auxiliary cylinder. (20) Check valve. | |
Note: The optional auxiliary control valve for 938G Wheel Loaders and the coupler/auxiliary control valve for IT38G Integrated Toolcarriers operate in the same manner. Only the auxiliary control valve is described here.
There are two separate control valves in the main control valve manifold. One control valve controls the tilt cylinders and the other control valve controls the lift cylinders.
When an auxiliary cylinder is added, a third control valve is also added. The three control valves are assembled so that the auxiliary control valve is on the side with the pump inlet.
Oil flow from the implement pump enters the auxiliary control valve first. The oil then flows to the tilt control valve, and then to the lift control valve. The oil flows from the lift control valve, through the return hydraulic oil filters, and then to the hydraulic oil tank.
This arrangement provides priority flow for the pressure oil that is flowing to the work tool. This means that there is a reduced flow of oil to the tilt control valve and to the lift control valve when the auxiliary control valve is actuated. Also, there is a reduced flow of oil to the lift control valve when the tilt control valve is actuated.
Coupler/Auxiliary Control Valve (HOLD Position)
When the engine is running, oil flows from the implement pump and into the pump inlet. Then, the oil flows into passage (7). If the auxiliary control valve is in the HOLD position, the oil flows around stem (5) and into passages (6) and (8) .
The oil flows through passages (6) and (8) and to the tilt control valve. If the tilt control valve and the lift control valve are also in the HOLD position, the oil flows through the tilt control valve and into the lift control valve. Then, the oil flows through the lift control valve and to the hydraulic oil tank.
When the auxiliary control valve is in the HOLD position, stem (5) blocks the flow of oil in ports (1) and (2). The oil in the circuits to the auxiliary cylinder is stopped. The auxiliary cylinder can not move until the control lever and stem (5) are moved to the OPEN position or to the CLOSED position.
Springs (3) and (4) are located in the left end of stem (5). Springs (10) and (11) are located in the right end of the stem (5). The springs keep the spool centered when no pilot oil is being routed to the spool.
Coupler/Auxiliary Control Valve (OPEN Position)
When the auxiliary control lever is moved to the OPEN position, pilot oil flows from the auxiliary pilot valve and into port (12). Pilot oil combines with the force of springs (3) and (4) on the left end of stem (5). This combined force overcomes the force of springs (10) and (11) on the right end of stem (5). This causes the spool to move to the right.
When stem (5) moves to the right and into the OPEN position, pump oil flow from passage (7) is blocked from entering passages (6) and (8). The oil pressure in passage (7) increases until check valve (20) opens. When the check valve opens, the pump oil in passage (7) flows through the check valve, into passage (17), and out of port (1). Then, the oil flows to the head ends of the auxiliary cylinders.
The pump oil that is flowing into the head ends of the auxiliary cylinders extends the cylinder rods and the work tool closes. Return oil from the rod ends of the auxiliary cylinders flows through port (2), through passage (9), through the hydraulic oil filter, and to the hydraulic oil tank.
Some of the oil in passage (7) does flow to the tilt control valve and to the lift control valve. This allows some tilting and some lifting. As the need for oil in the auxiliary cylinders is lessened, more oil is available to the tilt control valve or to the lift control valve.
Coupler/Auxiliary Control Valve (CLOSED Position)
When the auxiliary control lever is moved to the CLOSED position, pilot oil flows from the pilot valve and into port (13). Pilot oil combines with the force of springs (10) and (11) on the right end of stem (5). This combined force overcomes the force of springs (3) and (4) on the left end of stem (5). This causes the spool to move to the left.
When stem (5) moves to the left and into the CLOSED position, pump oil flow from passage (7) is blocked from entering passages (6) and (8). The oil pressure in passage (7) increases until check valve (20) opens. When the check valve opens, the pump oil in passage (7) flows through the check valve, into passage (18), and out of port (2). Then, the oil flows to the rod ends of the auxiliary cylinders.
The pump oil that is flowing into the rod ends of the auxiliary cylinders retracts the cylinder rods and the work tool closes. Return oil from the head ends of the auxiliary cylinders flows through port (1), through passage (15), through the hydraulic oil filter, and to the hydraulic oil tank.
Some of the oil in passage (7) does flow to the tilt control valve and to the lift control valve. This allows some tilting and some lifting. As the need for oil in the auxiliary cylinders is lessened, more oil is available to the tilt control valve or to the lift control valve.
Makeup and Relief Valves
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| Illustration 5 | g00282563 |
Makeup and Relief Valve in the Rod End of the Auxiliary Circuit (1) Port to the rod end. (15) Passage to the hydraulic oil tank. (16) Passage. (22) Spring. (23) Pilot valve. (24) Valve. (25) Spring. (26) Valve. (27) Ports. (28) Passage. | |
The two pilot operated auxiliary relief valves are mounted in the auxiliary control valve.
Pump oil in passage (16) flows through passage (28) and into the spring chamber of spring (25). When the pressure of the oil in the spring chamber becomes greater than the relief valve pressure setting, the pressure overcomes the force of spring (22). Then, the oil causes pilot valve (23) to move to the left and off the seat.
When the pilot valve is off the seat, oil in the spring chamber is released. Oil in passage (16) acts against valve (26) in order to move the valve to the left.
When valve (26) moves to the left, port (27) opens. Some of the pump oil from port (1) flows through passage (16) and into passage (28). Then, the oil flows out of ports (27), into passage (15), and to the hydraulic oil tank.
The relief valve also provides makeup oil to the return side in order to prevent cavitation. Oil in the return circuit acts against valve (26). The valve moves to the left and off the seat. Then, the oil flows from passage (15), into passage (16), and to the rod end of the cylinder.
Note: The makeup and relief valve in the head end port operates in the same manner.
Reference: For additional information on the pressure setting of the makeup and relief valves, refer to the Service Manual module Specifications, "Relief Valve (Line)" for the machine that is being serviced.