349E Excavator Hydraulic System Pilot Hydraulic System Caterpillar


Pilot Hydraulic System
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1.1. Hydraulic Schematic with Heavy Lift
2.1. Pilot Oil Circuit
3.1. Power Shift Pressure System
4.1. Pilot Control Valve Circuits
5.1. Pressure Switch Circuits
6.1. Pressure Sensors
7.1. Straight Travel Valve Circuit
8.1. Swing Parking Brake
9.1. Boom Priority
10.1. Swing Priority
11.1. Automatic Travel Speed Change Valve
12.1. Heavy lift



Illustration 1g03583644

(1) Displacement change valve (left travel motor)

(2) Displacement change valve (right travel motor)

(3) Line (pilot oil from swing brake solenoid valve)

(4) Swing parking brake (left)

(5) Swing parking brake (right)

(6) Pilot line (pilot pressure to displacement change valves)

(7) Pilot line (pilot oil to travel pilot control valve)

(8) Travel pilot control valve

(9) Pilot line (stick drift reduction valve)

(10) Stick drift reduction valve

(11) Swing control valve

(12) Left travel control valve

(13) Straight travel control valve

(14) Solenoid valve (straight travel)

(15) Right travel control valve

(16) Main control valve

(17) Pilot line (BOOM RAISE)

(18) Boom I control valve

(19) Pilot line (boom drift reduction valve)

(20) Boom drift reduction valve

(21) Stick II control valve

(22) Pilot control valve for stick and swing

(23) Travel pressure switch (left travel)

(24) Pilot control valve for boom and bucket

(25) Travel pressure switch (right travel)

(26) Pilot line (pilot oil to pilot control valve for the stick and swing)

(27) Pilot line (pilot oil to pilot control valve for the boom and bucket)

(28) Pilot line (pilot oil to pilot control valves)

(29) Pilot line (STICK IN)

(30) Solenoid valve for swing priority

(31) Idler pump pressure sensor

(32) Drive pump pressure sensor

(33) Pilot line (BOOM LOWER)

(34) Pressure sensor (boom down control)

(35) Implement pressure switch

(36) Pilot line (pilot pressure to implement pressure switch)

(37) Pilot line (pilot pressure to the pressure sensor for swing control)

(38) Pressure sensor (swing control)

(39) Pilot line (pilot oil to the pressure reducing valve for boom priority)

(40) Pressure reducing valve for boom priority

(41) Travel speed solenoid valve

(42) Passage

(43) Swing brake solenoid valve

(44) Valve

(45) Hydraulic activation solenoid valve

(46) Pilot manifold

(47) Passage

(48) Pilot line (pilot oil flow to pilot oil manifold)

(49) Passage

(50) Passage

(51) PRV for boom regeneration valve

(52) Pilot relief valve

(53) Pilot filter

(54) Passage

(55) Pilot pump

(56) Drive pump

(57) Pilot line (pilot oil flow from pilot pump to pilot oil filter)

(58) Idler pump

(59) Proportional reducing valve (power shift pressure)

(60) Pilot line (pilot oil flow to pump regulators)

(61) Negative flow control valve

Hydraulic Schematic with Heavy Lift




Illustration 2g02707456

Heavy lift (partial schematic)

(16) Main control valve

(46) Pilot oil manifold

(51) PRV for boom regeneration valve

(52) Pilot relief valve

(55) Pilot pump

(62) Main relief valve

(63) Heavy lift solenoid valve

Pilot Oil Circuit




Illustration 3g02141897

Ports and solenoids at the pilot manifold

(41) Travel speed solenoid valve

(43) Swing brake solenoid valve

(44) Valve (hydraulic activation)

(45) Hydraulic activation solenoid valve

The pilot circuit pressure is limited by pilot relief valve (52) .

The oil delivery from pilot pump (55) performs the following main functions.

  • Create pilot oil pressure in order to control the output flows of the main pumps.

  • Provide pilot oil pressure to the pilot control valves for implements, swing, and travel in order to perform machine operations.

  • Create pilot oil pressure in order to operate the control devices automatically.

The pilot circuit is classified into the following circuits and each circuit performs one of the above functions.

  • Power shift pressure system

  • Pilot control valve circuit

  • Pressure switch circuits

  • Pressure sensors

  • Straight travel valve circuit

  • Swing parking brake

  • Boom priority

  • Swing priority

  • Automatic travel speed change

  • Heavy lift

Power Shift Pressure System




Illustration 4g02430380

(16) Main control valve

(31) Idler pump pressure sensor

(32) Drive pump pressure sensor

(55) Pilot pump

(56) Drive pump

(58) Idler pump

(59) Proportional reducing valve (PS pressure)

(64) Engine speed sensor (flywheel housing)

(65) Machine ECM

(66) Monitor

(67) Engine speed dial

(76) Engine ECM

(77) Switch panel ECM

During machine operation, machine ECM (65) receives input signals from the following components:

  • Engine speed dial (67)

  • Engine speed sensor (64) that is located on the flywheel housing

  • Idler pump pressure sensor (31)

  • Drive pump pressure sensor (32)

  • Monitor in the cab (66)

The machine ECM (65) continually monitors all of the input signals. The input signals are processed by the machine ECM and an output signal is sent to proportional reducing valve (59) at the pump regulator. The proportional reducing valve assists in controlling the output flow of idler pump (58) and drive pump (56) .

The oil delivery from pilot pump (55) flows through the pilot filter to proportional reducing valve (59) at the pump regulator. The electrical signal that is sent from machine ECM (65) causes proportional reducing valve (59) to regulate the pilot pressure to a reduced pressure. This reduced pressure is called power shift pressure (PS). The proportional reducing valve sends the reduced pilot oil pressure through the idler pump regulator and through the drive pump regulator. The output flow of idler pump (58) and drive pump (56) is controlled in accordance with the power shift pressure. The power shift pressure is used to regulate the maximum allowable hydraulic pump output.

The output signal that is sent from the machine ECM to the proportional reducing valve will change when the machine ECM detects a change in any of the input signals. The power shift pressure that is sent to the regulators at the idler pump and the drive pump will change in order to regulate the maximum allowable hydraulic pump output. The desired engine speed is maintained.

A decrease in engine speed increases the power shift pressure. An increase in power shift pressure causes a destroke condition of the idler pump and the drive pump. The maximum allowable hydraulic power output is decreased.

An increase in engine speed decreases the power shift pressure. A decrease in power shift pressure causes an upstroke position of the idler pump and the drive pump. The maximum allowable hydraulic power output is increased.

Note: For more information concerning the operation of the machine ECM, refer to Systems Operation/Testing and Adjusting, "Machine Electronic Control System".

Pilot Control Valve Circuits

Oil from pilot pump (55) flows through pilot line (57) , pilot filter (53) , and pilot line (48) to pilot manifold (46) . When the hydraulic activation control lever is shifted to the UNLOCKED position, the machine ECM energizes the hydraulic activation solenoid valve (45) . The pilot oil then shifts valve (44) . The pilot oil now flows through valve (44) and pilot line (28) . The pilot oil now flows to pilot control valves (8) , (22) and (24) for implements, swing, and travel in order to perform machine operations. When the joysticks and/or travel levers/pedals are moved, the pilot oil flows to main control valve (16) in order to control the machine functions.




Illustration 5g02139259

Pilot lines at the main control valve (top view)

When joystick (22) and/or joystick (24) are operated, the pilot control valves send pilot pump oil through the pilot lines to pilot ports at the main control valve in order to shift the spools in the main control valve. Refer to Illustration 5 and Table 1 for the location of the pilot lines and machine operations.

Table 1
Pilot line     Control Valve     Machine Operation    
(68)     Stick II control valve     STICK OUT    
(69)     Boom I control valve     BOOM RAISE    
(70)     Bucket control valve     BUCKET CLOSE    
(71)     Right travel control valve     REVERSE RIGHT TRAVEL    
(72)     Left travel control valve     REVERSE LEFT TRAVEL    
(73)     Swing control valve     SWING LEFT    
(74)     Stick I control valve     STICK OUT    
(75)     Boom II control valve     BOOM RAISE    

Pilot oil from the pilot control valves flows through pilot lines to the ports on the bottom of the main control valve in order to perform the opposite operation.

The following example is given for the BOOM LOWER operation and the BOOM RAISE operation. Machine operations for a stick operation, bucket operation, travel operation, and swing operation are accomplished in the same manner as the boom operation.

When the joystick for the boom is moved to the BOOM RAISE position, pilot oil from pilot control valve (24) flows through pilot line (17) to boom I control valve (18) . The pilot pressure shifts the boom I control valve. The oil delivery from the drive pump flows to the head end of the boom cylinders in order to perform the BOOM RAISE operation.

When the joystick for the boom is moved to the BOOM LOWER position, pilot oil from pilot control valve (24) flows through pilot line (33) to boom I control valve (18) . The pilot pressure shifts the boom I control valve. The pilot oil also flows through pilot line (19) in order to open boom drift reduction valve (20) . The return oil from the head end of the boom cylinders flows through the boom drift reduction valve and the boom I control valve to the hydraulic tank. The BOOM LOWER operation is now performed.

Pressure Switch Circuits

Pressure switches (23) and (25) are connected to travel pilot control valve (8) . Pressure switch (35) is connected to pilot control valve (22) and pilot control valve (24) . When all of the joysticks and/or travel levers/pedals are in the NEUTRAL position, the pilot oil pressure to the pressure switches is low. Pressure switches (23) , (25) , and (35) are OFF. The machine ECM recognizes the OFF condition of all of the pressure switches. The AEC system is activated in order to lower the engine rpm.

If any of the joysticks and/or travel levers/pedals are moved from the NEUTRAL position, the increased pilot oil pressure is sent to the pressure switches. If pressure switch (23) , (25) , and/or (35) is ON, the machine ECM activates the AEC system in order to increase the engine rpm.

If both travel levers/pedals and either one of the joysticks are moved from the NEUTRAL position simultaneously, pilot oil pressure will cause pressure switches (23) , (25) , and (35) to be in the ON position. An electrical signal is sent to the machine ECM. The machine ECM energizes straight travel solenoid (14) . Pilot pressure now activates straight travel control valve (13) .

The straight travel control valve provides hydraulic oil flow to travel control valves (12) and (15) from the drive pump. The idler pump supplies all oil delivery to the remaining valves within the main control valve.

If the machine is equipped with a straight travel pedal, there will be a separate pressure switch that sends a signal to the machine ECM. A signal is sent to the machine ECM in order to energize straight travel solenoid (14) when the straight travel pedal is moved from the NEUTRAL position.

For more information concerning the operation of the straight travel control valve, refer to Systems Operation, "Control Valve (Straight Travel)".

Pressure Sensors

Pressure sensor (34) is connected to pilot oil for boom down control. Pressure sensor (38) is connected to pilot oil for a swing RIGHT operation and swing LEFT operation. These two pressure sensors send an electrical signal to the machine ECM.

When a boom DOWN function is performed, pressure sensor (34) sends an electrical signal to the machine ECM. The machine ECM then sends a signal to the following components:

  • PRV for boom regeneration valve (51) limits pilot oil to the boom regeneration valve.

  • The AEC system is activated in order to lower the engine rpm.

  • Swing brake solenoid valve (43) is activated in order to release the swing parking brake.

  • Negative flow control valve (61) regulates the pressure that will be supplied to the pump regulators. The negative flow control pressure forces the swashplate in the pump to the correct angle to limit the flow and pressure from the main pumps.

When a swing operation is performed, pressure sensor (38) sends an electrical signal to the machine ECM. The machine ECM then sends a signal to the following components:

  • The solenoid valve for swing priority (30)

  • Swing brake solenoid valve (43) is activated in order to release the swing parking brake.

  • Negative flow control valve (61) regulates the pressure that will be supplied to the pump regulators. The negative flow control pressure forces the swashplate in the pump to the correct angle to limit the flow and pressure from the main pumps.

Drive pump pressure sensor (32) and idler pump pressure sensor (31) are located on main control valve (16) .

Straight Travel Valve Circuit

When a swing operation and/or implement operation is performed during a travel operation, the increase of pilot pressure in pilot line (36) and/or pilot line (37) activates implement pressure switch (35) and/or swing pressure sensor (38) . The implement pressure switch and/or the swing pressure sensor sends an electrical signal to the machine ECM. The machine ECM activates the solenoid valve for straight travel (14) . Straight travel control valve (13) shifts. The straight travel control valve maintains straight travel even though there is a swing operation or an implement operation during travel. For more information concerning the operation of the straight travel control valve, refer to Systems Operation, "Control Valve (Straight Travel)".

Swing Parking Brake

When the hydraulic activation control lever is placed in the UNLOCKED position, pilot oil in passage (49) flows through valve (44) and passage (50) in order to provide oil to the swing brake solenoid valve (43) . When any of the joysticks are moved from the NEUTRAL position, the increase of pilot pressure activates one of the following pressure sensors or pressure switches: implement pressure switch (35) , pressure sensor (34) for boom down control and pressure sensor (38) for swing control. The pressure switch and/or the pressure sensor sends an electrical signal to the machine ECM. An electrical signal from the machine ECM energizes the swing brake solenoid valve (43) once the pilot oil pressure exceeds 500 kPa (73 psi). Pilot oil in line (3) flows to swing parking brake (4) and swing parking brake (5) . This oil releases the swing brake.

For more information concerning the operation of the swing brake, refer to Systems Operation, "Pilot Valve (Swing Parking Brake)".

Boom Priority

During combined operations of BOOM RAISE and STICK IN, the pilot oil pressure in pilot line (39) activates the pressure reducing valve for boom priority (40) . The pressure reducing valve for boom priority allows priority flow to the head end of the boom cylinders during these combined hydraulic operations by disabling stick II control valve (21) . For more information concerning the pressure reducing valve for boom priority, refer to Systems Operation, "Boom Hydraulic System".

Swing Priority

During a swing operation, pilot oil flows from pilot control valve (22) to swing control valve (11) . Pressure sensor (38) for swing control sends an electrical signal to the machine ECM. The machine ECM sends an electrical signal to solenoid valve (30) in order to activate swing priority once the pilot oil pressure in line (37) exceeds 2750 kPa (400 psi). Most of the flow from the idler pump now goes to the swing motor.

Note: When the solenoid valve for swing priority is OFF, the swing priority is enabled.

For more information concerning the pressure reducing valve for swing priority, refer to Systems Operation, "Swing Hydraulic System".

Automatic Travel Speed Change Valve

Pilot oil in passage (42) flows to travel speed solenoid valve (41) . When the travel speed switch on the right console is set at the HIGH SPEED position, the travel speed solenoid valve opens. This action allows pilot oil to flow through travel speed solenoid valve (41) and through line (6) . The oil then flows to the displacement change valve for the left travel motor (1) and the displacement change valve for the right travel motor (2) . As the displacement change valve operates, the travel speed is maintained at the HIGH SPEED position.

When the travel speed switch on the right console is set at the HIGH SPEED position, the pressure sensors for the pump delivery pressure control the travel speed in accordance with the travel load. For example, low speed during a high load condition and high speed during a low load condition.

For more information concerning the operation of the displacement change valves, refer to Systems Operation, "Displacement Change Valve".

Heavy lift

Machines that are equipped with heavy lift have a high-pressure setting and a low pressure setting for the main relief valve. A switch on the console activates a solenoid valve. The solenoid valve directs pilot oil pressure to the pilot oil port on the main relief valve. The pilot oil pressure is ported through the valve in order to move a piston that creates a higher spring force on the main relief valve. The increase of spring force on the main relief valve causes the relief pressure in the main hydraulic system to rise.

For more information concerning the operation of the heavy lift system, refer to Systems Operation, "Relief Valve (Main) (Heavy Lift)".

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