973 Track-Type Loader Power Train Caterpillar

Schematics with Oil Flow

Designations for Oil Flow

AA - Supply oil from the charge pump

BB - Supply oil from servo valves

CC - Oil from the throat of the venturi

EE - Charge oil to the drive loops

GG - Lubrication oil

HH - Blocked oil

LL - Return oil and charge pump suction

MM - High pressure drive oil

NN - Low pressure drive oil

Systems Operation

The following schematics explain these seven system conditions:

Engine Off

Park

Brakes Off

Maximum FORWARD

Partially Depressed Center Pedal

Fully Depressed Center Pedal

Control Lever in FORWARD With a Starting Engine

Illustration 1	g00523261
Engine OFF System Components (1) Pilot valve (2) Operate/brake valve spool (3) Pressure tap for brake pressure (top of the valve body) (4) Brake line (5) Charge pressure check valve (6) Oil line from servo supply relief valve to the low pressure side of the drive loop (7) Start/vent spool (8) Speed/brake spool (9) Pressure tap for charge pressure (right side of the valve body) (10) Servo supply relief valve (11) Main control valve (12) Pressure tap for servo supply pressure (front of the valve body) (13) Pressure tap for charge pump (filter housing) (14) Pressure tap for venturi upstream pressure (front of the valve body) (15) Pressure tap for venturi throat pressure (front of the valve body) (16) Underspeed cut-in adjustment screw (17) Filter relief valve with 170 kPa (25 psi) (18) Pressure tap for underspeed throat pressure (top of the valve body) (19) Venturi (20) Oil filter (21) Quick response valve (22) Charge pump (23) Pressure tap for underspeed upstream pressure (top of the valve body) (24) Suction screen (25) Reservoir (tank) (26) Overspeed valve (27) Underspeed valve (28) Underspeed/override valve (A) FORWARD (B) Brakes OFF (C) Brakes ON or PARK (D) Brakes OFF (E) REVERSE (LL) Return oil and charge pump suction

The schematic of the hydrostatic drive system shows that the engine is OFF and the transmission control lever is in PARK.

These major components are required to change the following operator input signals to hydraulic signals: speed, direction and steer

Illustration 2	g00529385
Engine OFF (continued schematic ) System components (continued components) (29) Piston pumps and servo cylinders (30) Charge and main relief valve (31) Hydraulic lines to brakes (32) Drive motor (left and right) (33) Shuttle spool (34) Towing plug (35) Access plug (towing spool) (36) REVERSE drive lines (XT-6 hoses) (37) Charge pressure relief valve (38) FORWARD drive lines (XT-6 hoses) (39) Pressure tap for left drive pressure (FORWARD and REVERSE) (40) Main pressure relief valve (41) Oil cooler (42) Synchronization cutoff valve (part of left side charge valve) (43) Balance line (44) Pressure tap for right drive pressure (FORWARD and REVERSE) (45) Oil cooler bypass valve (46) Synchronization valve (part of right side charge valve) (47) Servo levers (F) Left (G) Right (LL) Return oil and charge pump suction

These major components are required to change the following operator input signals to hydraulic signals: speed, direction and steer

Illustration 3	g00529437
Control Lever in PARK with Operating Engine System components (1) Pilot valve (2) Operate/brake valve spool (4) Brake line (5) Charge pressure check valve (7) Start/vent spool (8) Speed/brake control spool (10) Servo supply relief valve (16) Underspeed cut-in adjustment screw (17) Filter relief valve 170 kPa (25 psi) (19) Venturi (21) Quick response valve (26) Overspeed valve (28) Underspeed/override valve (48) Orifice (49) External line (50) Passage (51) Orifice (52) Passage (53) Passage (54) Overspeed valve spool (A) FORWARD (B) Brakes OFF (C) PARK (D) Brakes OFF (E) REVERSE (H) FORWARD (J) PARK (K) REVERSE (AA) Supply oil from charge pump (BB) Supply oil for servo valves (EE) Charge oil to the drive loops (LL) Return oil and charge pump suction

Shading is used to indicate the pressures in the system during different operating conditions.

AA is used to indicate supply oil from the charge pump. The flow from the pump and the pressure of the oil varies with engine speed.

BB is used to represent servo supply oil. All flow through venturi (19) is directed past servo supply relief valve (10) to the servo valves (not shown).

During a PARK condition, the servo valves are in a NEUTRAL position. The servo valves do not require flow from the venturi in order to change the positions of the swashplates.

Therefore, the pressure increases in the servo supply passage and the servo relief valve opens in order to bypass the oil.

The relief valve opens when the pressure at the servo valves is approximately 2450 kPa (355 psi). During normal operation, the charge pump provides more flow to the servo valves in order to move the servo cylinders.

The servo relief valve remains open in order to maintain a constant pressure at the servo valves and a constant flow to the drive loops (EE).

Underspeed/override valve (28) is in a position in order to block the oil at the top of the underspeed valve from going to tank. Also, the underspeed/override valve indicates HIGH IDLE position of the governor control lever.

When the governor control lever is in HIGH IDLE, the underspeed/override valve is held in the CLOSED position by the governor linkage.

As the governor lever is moved toward LOW IDLE, the governor linkage opens the override valve in order to lower the pressure at the top of the underspeed valve. This keeps the valve in the UP position.

LL represents the oil which opens to the tank through passages in the valve body.

When the engine is started and the transmission control lever is in PARK, oil LL is drawn from the reservoir at the bottom of the transmission case. Oil LL is drawn through the screen to the inlet port of the charge pump.

The single-section gear pump sends oil AA through an external line to the transmission oil filter housing. The inlet passage in the filter housing connects with the chamber at the top of filter relief valve (17) .

The spring chamber of the filter relief valve connects with the outlet passage from the filter. All flow from the charge pump must pass through the filter element before the oil can enter the remainder of the system.

Restriction in the filter element causes a pressure increase at the inlet side of the filter. If the pressure at the inlet side becomes 170 kPa (25 psi) higher than the pressure at the outlet side of the filter, the relief valve opens. This directs pump flow back to the reservoir.

If the pressure differential across the filter is less than 170 kPa (25 psi), the relief valve remains closed. Also, all of the flow is sent to the supply passage of the venturi in the main control valve.

As the oil enters the supply chamber, the flow divides. Most of oil (BB) goes through the venturi to the servo valves (not shown). Part of the flow is sent through two internal passages in the supply chamber. Part of the flow is sent through the passage in the venturi throat.

The first passage in the supply chamber sends the venturi upstream oil (Pu) to underspeed cut-in valve (16). The second passage sends the upstream oil (Pu) to the slug chamber in overspeed valve (26). Then, the upstream oil is sent through an orifice in quick response valve (21) .

The third passage which is located in the throat of the venturi also contains orifice (51). Orifice (51) prevents the underspeed valve from rising rapidly as the track speed slowly accelerates. The underspeed valve rises slowly when track speed accelerates. This prevents excessive lugging of the engine. This condition exists when the speed/direction control lever is moved rapidly to maximum speed FORWARD or REVERSE.

The venturi causes the upstream pressure to be higher than the pressure in the throat oil (Pt).

The difference between upstream pressure and the throat is determined by the flow rate through the venturi.

The cut-in adjustment valve can reduce the flow through the venturi by allowing part of the upstream oil to bypass the venturi. A passage in the cut-in valve allows supply oil (AA) to combine with the charge oil (EE) .

The speed of the engine and the position of the cut-in adjustment valve determines the flow through the venturi. The rate of flow determines the differential pressure (delta P) between upstream oil (Pu) and throat oil (Pt).

The differential pressure (delta P) controls the positioning of the underspeed valve at specified engine speed. The underspeed cut-in adjustment valve controls the differential pressure at a given engine speed.

The orifice in the quick response valve and the upstream oil in the spring chamber keep the quick response valve in the closed position. The flow out of the quick response valve divides and the flow goes through two internal passages.

Passage (52) directs the flow back to the main control valve while passage (53) sends upstream oil in order to fill a chamber around overspeed valve spool (54) .

The chamber in the overspeed valve connects with the bottom of the underspeed valve through a metal tube. The oil at the bottom of the underspeed valve is used to move the valve UP when the transmission lever is moved from PARK.

Passage (52) from the quick response valve to the control valve directs the flow of upstream oil to speed/brake control spool (8) and to operate/brake valve spool (2) .

The oil at the speed/brake spool fills a drilled passage in the center of the stem. The drilled passage is plugged at one end (left). Also, the passage has a cross-drilled hole at the other end.

When the center pedal is UP (shown), the cross-drilled hole is blocked by the position of the spool in the valve body. Upstream oil cannot flow through the center of the speed/brake spool until the center pedal is depressed.

The oil at the operate/brake valve spool fills a chamber around the spool. When the drive system is in PARK, the valve spring keeps the operate/brake valve spool in a position in order to allow upstream oil to flow past the spool to another internal passage.

The oil enters a passage that connects with the following areas: speed/brake control spool (8), the passage from throat of the venturi, the quick response valve and the spring chamber of the overspeed valve.

The oil at the speed/brake spool does NOT connect with the drilled passage in the center due to the position of the valve spool when the center pedal is UP.

At the venturi throat, the higher pressure upstream oil is separated from the lower pressure oil in the valve spring in order to keep the overspeed valve closed.

The upstream oil at the quick response valve is blocked by the position of the valve spool.

The upstream oil in the spring chamber of overspeed valve (26) works in conjunction with the valve spring in order to keep the overspeed valve closed.

From the spring chamber of the overspeed valve, the upstream oil is sent through a metal tube to the top of the underspeed valve and underspeed/override valve (28) .

The pressure at the top of the underspeed valve adds to the force of the valve spring. This keeps the underspeed valve in the DOWN position.

While the control lever is in PARK, the bottom of the underspeed valve is connected to the top of the underspeed valve. This is due to the position of operate/brake valve spool (2) during PARK.

The underspeed valve remains in a DOWN position,until the operate/brake valve spool moves from this position. Then, the upstream oil is replaced by the lower pressure oil from the venturi throat.

The operate/brake valve spool moves when the transmission control lever moves to BRAKES OFF. Also, the operate/brake valve spool moves when the transmission control lever moves past BRAKES OFF.

The oil at the underspeed/override valve is blocked by the position of the valve stem. (The blockage is shown.) The position of the valve stem indicates that the governor control lever is in the HIGH IDLE position.

The underspeed/override lever is mechanically connected to the governor control linkage. When the governor control is in HIGH IDLE position, the underspeed/override valve is closed.

When the system is in PARK and the governor control is set at HIGH IDLE, the following pressure taps are approximately the same readings: underspeed throat pressure and underspeed upstream pressure.

Due to the position of the operate/brake valve spool, these pressure taps are approximately the same reading: underspeed throat and underspeed upstream pressure.

The pressure taps for the underspeed valve are located at the top of the control valve. These pressure taps sense the pressure at the top and at the bottom of the underspeed valve.

The pressure tap readings at the venturi throat and the venturi upstream pressure show a pressure differential (delta P) of approximately 1240 kPa (180 psi) during PARK.

The pressure taps for the venturi are located on the front of the control valve. The pressure taps sense the pressure in the supply passage to the venturi (upstream oil). Also, the pressure taps sense the pressure in the venturi throat (throat oil).

Servo Supply and Charge Oil

Most of the flow from the charge pump goes through the venturi to the servo valves (not shown).

The charge pump provides excess flow to the servo valves in order to control the operation of the servo cylinders.

The maximum pressure of the servo supply oil pressure is limited to approximately 2450 kPa (355 psi) by servo supply relief valve (10) .

When the pressure exceeds the relief setting, the valve opens. This directs oil flow (EE) to the following components: main control valve, pilot valve, charge valves and charge relief valve.

Line (49), which is located directly above the servo relief valve, is filled with oil (EE). Line (49) is connected with the charge valves.

Line (4), which is located at the top of the schematic, goes to the brakes. (Line (4) is filled with LL oil.)

The oil to the main control valve is sent through charge pressure check valve (5). Also, the oil to the pilot valve is sent through the charge pressure check valve (5). The pressure in the charge circuit moves the check valve against the force of the spring. This allows oil to flow past operate/brake valve spool (2) and start/vent spool (7) to pilot valve (1) .

The charge pressure check valve prevents sudden pressure drops in the main control valve during swashplate movement.

When the servo valves are moved, the servo cylinders fill. The filling of the servo cylinders causes the pressure in the charge circuit to decrease momentarily.

The momentary decrease in pressure causes the check valve to close. This isolates the main control valve in order to briefly maintain the pressure inside the valve body.

The check valve has an orifice in order to slow the reverse flow of oil while the check valve maintains the pressure in the control valve. The check valve maintains the pressure in the control valve for approximately two seconds.

When the transmission control lever is in PARK, the pilot spool is in a position in order to direct the oil from the charge check valve. The oil passes through passage (50) into a chamber at the end of start/vent spool.

The pressure at the end of the start/vent spool moves the spool (left) against the force of the spring. The spool moves until the spool makes contact with the cover at the end of the valve body. Approximately 730 kPa (106 psi) of pressure is required in order to keep the spool in this position. The same amount of pressure is necessary in order to move the spool into this position.

When the start/vent spool is shifted, the flow of charge oil from the charge check valve flows past the operate/brake valve spool to the start/vent spool. The start/vent spool then directs the flow which fills a chamber around the operate/brake valve spool.

The oil is then sent through orifice (48) in an internal passage to the chamber at the end of operate/brake valve spool (2) .

The oil in the chamber at the end of operate/brake valve spool is also directed back to the pilot valve.

While the system is in PARK, the position of the pilot valve spool directs the flow to the tank. While the flow is open to the tank, the pressure DOES NOT increase in the chamber at the end of the operate/brake valve spool.

Without an increase in pressure, the spring keeps the operate/brake valve spool in the position shown. Orifice (48) in the supply passage to the chamber prevents a pressure loss in the remainder of the charge circuit.

With the operate/brake valve spool in this position, charge oil cannot enter a passage that connects with brake line (4). The brakes remain engaged.

The operate/brake valve spool does not change positions in order to direct charge oil through the brake release line until the pilot spool moves. Then, the pilot spool blocks the passage to the tank. See the illustration 5 (Control Lever That is Moved to Brakes OFF) in this manual.

Note: If the pressure of the charge oil is less than 730 kPa (106 psi), the start/vent spool does NOT move. Also, if the pilot spool is not correctly positioned during PARK, charge oil cannot be directed to the chamber at the end of the start/vent spool.

Without pressure oil in the chamber, the start/vent spool does not move. See the illustration 16 (Starting the Engine with the Control Lever in FORWARD) in this manual.

Illustration 4	g00529657
Control Lever in PARK with Operating Engine (Continued Schematic) System components (continued components) (31) Hydraulic line to brakes (32) Drive motor (left and right) (34) Towing plug (35) Access plug (37) Charge pressure relief valve (41) Oil cooler (45) Oil cooler bypass valve (55) Common passage (56) Line (F) Left (G) Right (H) FORWARD (J) PARK (K) REVERSE (BB) Supply oil from servo valves (EE) Charge oil to the drive loops (GG) Lubrication oil (HH) Blocked oil (LL) Return oil and charge pump supply

When the hydraulic drive system is in the PARK condition, the lines and passages in the drive loops are filled with charge pressure oil. Charge pressure relief valve (37) is shown in the approximate center of the schematic.

The relief valve limits the pressure in the charge circuit to approximately 1380 kPa (200 psi). The charge oil in both charge valves is sent to the charge pressure relief valve through common passage (55) in the transmission head assembly.

The oil that is sent through the charge relief valve is directed to oil cooler (41). The oil cooler is at the rear of the machine and the oil cooler is adjacent to the radiator.

The pressure in the cooler is limited to approximately 480 kPa (70 psi) by oil cooler bypass valve (45). The oil cooler bypass valve and the charge pressure relief valve are installed at the bottom of the transmission head assembly.

For identification, brake line (31) at the top of the schematic connects the operate/brake valve spool. The automatic brake purge valve is located on the left side of the machine. The brake purge valve directs the oil to the brakes when the operate/brake valve spool is shifted.

Line (56) provides the flow in order to replace the oil loss in the drive loops due to normal leakage. The flow of replenishing oil enters the charge valve on the left side (top). Then, the flow of replenishing oil enters a tube to the charge valve which is on the right side (bottom).

The supply oil in the servo valves is shown as BB. The following components are shown in the PARK position: servo valves, mechanical linkage and servo cylinders.

HH shading in the servo cylinders indicates that the oil is blocked by the position of the servo valves.

During PARK, the rotating groups are turning at engine speed, but the swashplates are at ZERO angle. As a result, the pumps do not provide flow through the drive loops.

When the control lever is in PARK, each charge and main relief valve operates the same way.

The charge oil that enters the left end of the charge valve fills the supply chamber. Then, the charge oil goes to either the forward or reverse drive line. Part of the oil in the supply chamber is sent through the tube that connects the charge valves.

The path of oil to either the forward or reverse drive line is dependent upon the position of the shuttle spool. The positioning of the shuttle spool is controlled by pressure.

When the engine first starts, the spool is in the original position. This was the last position of the spool when the machine operated the last time. This illustration shows the shuttle spool in the REVERSE position. With the shuttle spool in the REVERSE position, all flow is directed to the forward drive line.

As the oil fills the forward drive circuit, part of the oil flows through the drilled passages at the left end of the spool. The flow from the passage fills the chamber at the right end. This causes the spool to move to the left.

The spool moves from the "RIGHT" position (REVERSE drive position) to a "CENTERED" position in the valve bore. When the spool is centered, both drive circuits (forward and reverse) are open a small amount to the supply passage in the charge valve. This allows all the passages and chambers inside the valve body to fill. Charge pressure oil fills the passages. Charge pressure oil fills the chambers.

The drive motors are connected to the tank through separate case drain lines. The return line from left drive motor (32) is routed directly to the tank. However, the return line from right drive motor (32) connects with the return line from cooler (41) .

The drive motor case drain pressure should be 0 kPa (0 psi) during operation. However, the case drain pressure on the right side could reach 14 kPa (2 psi) due to the routing on the right side drive motor. Before checking case drain pressures, always remove the plug to the transmission inlet for the cut-in tool in the top cover.

The case drain pressure taps are located on the drive motors. The drive motors also have a lubrication valve.

Illustration 5	g00529740
Control Lever That is Moved to Brakes Off System components (1) Pilot valve (2) Operate/brake valve spool (4) Brake line (5) Charge pressure check valve (8) Speed/brake control spool (10) Servo supply relief valve (27) Underspeed valve (48) Orifice (49) External line (51) Orifice (57) Pilot spool (58) Levers (59) Adjustment screw (60) Groove (61) Speed link (A) FORWARD (B) Brakes OFF (C) PARK (D) Brakes OFF (E) REVERSE (AA) Supply oil from the charge pump (BB) Supply oil for servo valves (CC) Oil from the throat of the venturi (EE) Charge oil to the drive loops (HH) Blocked oil (LL) Return oil and charge pump suction

The first half of the hydraulic schematic shows the position of the components when the brakes are in BRAKES OFF. Notice that pilot spool (57) has moved. The operate/brake valve spool (2) has shifted. Also, the underspeed valve has moved to the FULL UP position.

The positions on the underspeed valve of directional speed link (61) and levers (58) moved very slightly. This indicates that the mechanical linkage arrangement moved very slightly. As a result, the machine remains stationary during this condition.

The flow from the charge pump to the venturi is the same way that is described for PARK. Charge pump oil (AA) at the upstream side of the venturi flows through the two internal passages.

The first passage goes to the underspeed cut-in adjustment valve. The second passage sends the flow to the slug chamber of the overspeed valve and to the quick response valve.

When the oil passes through the orifice in the quick response valve, the flow divides in two directions.

First, one passage goes to the overspeed valve. Then, the passage goes to the bottom of underspeed valve (27) .

The second passage sends the upstream oil back to the main control valve. Then, the oil fills the passage in the center of speed/brake control spool (8). The oil also fills a chamber around operate/brake valve spool (2) .

When operate/brake valve spool (2) is in this position, the flow of upstream oil is blocked. (The operate/brake valve spool is shifted.)

The oil flows from the servo relief valve through external line (49) in order to replenish the drive circuits. The oil is also sent to the pilot valve by the following paths: through charge pressure check valve (5), operate/brake valve spool and start/vent spool.

When pilot spool (57) was in PARK, charge pressure oil was directed to the start/brake spool in order to fill the chamber at the right end. The pressure in the chamber caused the start/brake spool to move to the left. (In PARK, the start/brake spool is shown in this position.)

With the start/vent spool in this position, the supply of charge pressure oil from the charge check valve is now sent back to the operate/brake valve spool.

The oil flows through orifice (48) to the chamber at the left end of operate/brake valve spool (2) and to a chamber in pilot valve (1) .

When the pilot spool was in PARK, the chamber in the pilot valve was open to the tank. This prevented the pressure from increasing in the chamber at the left end of the operate/brake valve spool.

When the pilot spool is moved to BRAKES OFF (in either FORWARD or REVERSE), the tank passage in the pilot valve is blocked. The pressure in the chamber at the left end of the operate/brake valve spool NOW increases. This causes the operate/brake valve spool to move to the right.

The spool then moves until the spool makes contact with the end cover on the valve body (shown position).

The spool is now in a position in order to send pressure oil through the brake line. Also, the spool blocks the flow of upstream oil from the quick response valve. The flow is blocked to the top of the underspeed valve. Oil flow is also blocked to the following components:chamber around the quick response spool, underspeed/override valve and spring chamber of the overspeed valve.

The upstream oil to these areas is replaced by the lower pressure oil (CC) from the venturi throat.

A decrease in throat oil Pt occurs at the top of the underspeed valve. This causes higher pressure of upstream oil Pu at the bottom of the valve in order to move the valve piston and roller UP. The piston moves up until contact is made with the plug at the top of the underspeed valve body.

The overspeed valve does not affect the operation of the drive system during BRAKES OFF. The valve functions only when the speed of the engine exceeds a specified rpm.

When the drive system is in PARK, upstream oil from the venturi fills the chamber at both ends of the valve spool.

When the transmission control lever is moved from PARK to BRAKES OFF, the upstream oil in the spring chamber is replaced with the lower pressure oil from the venturi throat (CC) .

The spring chamber connects through a metal tube to the top of the underspeed valve. Any pressure change in the spring chamber is felt at the top of the underspeed valve.

The slug chamber (far right) directly receives the upstream oil from the upstream passage of the venturi. The chamber in the center receives underspeed upstream oil (AA) after the oil has passed through the orifice in the quick response valve.

The chamber is connected to the bottom of the underspeed valve through a metal tube. A change in underspeed upstream pressure is felt in this chamber and at the bottom of the underspeed valve.

The overspeed valve spool has groove (60) around the outside. The upstream oil from the bottom of the underspeed valve fills the groove through two drilled passages in the valve spool.

During normal mode operation, the pressure differential of the underspeed valve does not move the spool against the force of the spring. The pressure differential (delta P) is between the upstream oil (bottom) and the throat oil (top) at the underspeed valve.

However, if the overspeed valve is incorrectly adjusted, the pressure of the upstream oil could cause the valve spool to move to the left. This allows upstream oil to enter the spring chamber. The upstream oil goes to the top of the underspeed valve. As a result, the underspeed valve would move down.

Adjustments to the overspeed valve are made by turning adjustment screw (59) at the left. This changes the force of the spring. Turn IN the plunger in order to compress the spring. The force against the valve spool increases.

If the plunger is turned OUT, spring force is reduced. The correct adjustment of the overspeed valve is critical to the operation of the overspeed valve.

Leakage by the overspeed valve can prevent full movement of the underspeed valve. Also, leakage may limit movement of the underspeed valve. During testing and/or troubleshooting procedures, check the condition of the overspeed valve.

This is done by comparing the differential pressure (delta P) at the venturi with the differential pressure (delta P) at the underspeed valve. When the control lever is in BRAKES OFF, a large variance in differential pressures can indicate upstream oil in the spring chamber of the valve.

At the underspeed/override valve, the oil from the venturi is blocked by the position of the valve stem. This indicates that the governor control is at HIGH IDLE.

Illustration 6	g00530023
Underspeed/override valve (H) Pin (J) Plunger

Section F-F shows that the underspeed/override valve contains an overtravel for plunger (J). When the governor control is at HIGH IDLE, pin (H) in the valve stem makes contacts with plunger (J) .

At this point, throat oil of the venturi (Pt) is blocked by the position of the stem (section G-G).

When the governor control lever is moved toward LOW IDLE, the valve stem rotates. This connects the throat oil to the drain. As a result, the pressure at the top of the underspeed valve is reduced. Less pressure is required at the bottom of the valve in order to keep the valve in the UP position.

The schematic is showing the drive portion during BRAKES OFF. Notice that the mechanical linkage and servo cylinders have not moved from the PARK position.

Until the transmission control lever is moved beyond BRAKES OFF, the mechanical linkage does not move the servo valves. Also, the swashplates would not move from the ZERO angle .

When comparing signals in the system to other components during PARK and BRAKES OFF, the only difference is the pressure in the brake release line.

Illustration 7	g00530030
Brake valve (4) Brake line

The oil in brake line (4) is first sent to the automatic brake purge valve on the left side of the machine.

The valve is accessible from ground level. Any air which is trapped in the brake is sent back to the valve. The air is sent through the line that connects at the top.

To remove air, move the transmission control lever from PARK to BRAKES OFF several times. Any air in the brake is sent through the air bleed line. The air is purged through the main control valve.

Illustration 8	g00530034
Brake valve (62) Bleed fitting (63) Ball (64) Stem

The brake valve has ball (63) that is installed in the air bleed passage from the brakes. Also, stem (64) is installed in the air bleed passage from the brakes.

When the brakes are released, pressure oil is sent through the valve body in order to fill the chambers at the brake pistons. The pressure in the chambers causes the pistons to move. Then, the brakes are released.

The brake bleed line also connects to the chambers for the brake pistons. When the supply oil is sent through the valve body in order to release the brakes, part of the oil enters the bleed passage. This fills the chamber that is above ball (63) .

As the brake chambers fill, the pressure of the supply oil keeps ball (63) down. The ball is also kept against bleed fitting (62) in the valve body.

Ball (63) prevents the supply oil from entering the brake bleed line. When the brakes are fully released, the pressure on both sides of ball (63 ) is equal.

When the brake oil is vented to the tank in PARK, the pressure in the brake chambers decreases. As a result, the Belleville Springs that are part of the brake move back the pistons. Then, the brakes engage.

Then, any oil and air at the top of the brake chambers is forced through the brake bleed line to the purge valve. This causes ball (63) to move up. The flow then goes through the brake supply line to the control valve.

Any air in the brake chambers returns to the tank with the oil. Therefore, the air is removed as the brakes are engaged. The transmission control lever should be moved from PARK to BRAKES OFF several times in order to remove all the air from the brake system.

Illustration 9	g00530142
Fitting location

During transmission testing, brake engagement may be necessary in order to prevent machine movement.

In order to prevent flow from the brake line to the brakes, the brake line (arrow) must be disconnected and plugged at the brake valve. When the brake line is removed, install a cap loosely on the open fitting and plug the hose.

When the machine is towed with an inoperable engine, releasing the brakes is necessary. FT1845 External Pump is required in order to provide the pressure that is needed in order to release the brakes.

FT1845 External Pump is installed on the fitting (arrow) at the inlet side of the purge valve. For more information on towing the machine, refer to Operation and Maintenance Manual, SENR6915, "Towing Information".

Illustration 10	g00530152
Control Lever That is Moved to Maximum FORWARD (1) Pilot valve (2) Operate/brake valve spool (4) Brake line (7) Start/vent spool (57) Pilot spool (61) Speed link (A) FORWARD (B) Brakes OFF (C) PARK (D) Brakes OFF (E) REVERSE (H) Maximum FORWARD (AA) Supply oil from the charge pump (BB) Supply oil for servo valves (CC) Oil from the throat of the venturi (EE) Charge oil to the drive loops (HH) Blocked oil (LL) Return oil and charge pump suction

Illustration 11	g00530180
Control lever in maximum Forward position

When the transmission shifts at HIGH IDLE rpm from PARK to MAXIMUM REVERSE, the control linkage moves pilot spool (57). (The shift may also be made to MAXIMUM FORWARD.) The pilot spool moves past BRAKES OFF to REVERSE (shown position).

The pilot spool is now in a position in order to block the drain passage in pilot valve (1). This allows the pressure at the left end of operate/brake valve spool (2) to increase. The pilot spool also blocks the flow of charge pressure oil back to the chamber at the right end of start/vent spool (7) .

A drilled passage in start/vent spool (7) prevents the spool from shifting by allowing charge pressure oil to enter the chamber at the right.

Blocked oil (HH) fills the chamber to the pilot valve. Also, blocked oil (HH) fills the passage to the pilot valve. (Blocked oil (HH) is shown.) The HHshading indicates the blocked passage at the pilot valve due to the position of the pilot spool. The pressure in the chamber is necessary to prevent the start/brake spool from moving to the right and venting the drive loops.

As the pressures increase in the chamber at the left end of operate/brake valve spool (2), the spool moves to the right. This directs charge pressure oil through brake line (4). Also, the flow of underspeed upstream oil (AA) is blocked to the top of the underspeed valve.

The position of the underspeed/override valve blocks the flow at that point. The reduced pressure at the top of the underspeed valve causes the higher pressure at the bottom to move the valve piston and roller UP. The position of the roller can now convert movement of the control lever to mechanical linkage movement through speed link (61).

Illustration 12	g00530203
Center pedal

The center pedal is used in order to slow the movement of the machine. The center pedal also stops the movement of the machine. The amount of pedal movement determines the position of the speed/brake spool inside the main control valve.

The position of the operate/brake valve spool prevents upstream oil (AA) from entering the passage that is filled with underspeed throat oil (CC) .

The system directs the throat oil to the top of the underspeed valve during the following conditions: BRAKES OFF, MAXIMUM FORWARD and MAXIMUM REVERSE. This allows the underspeed valve to move UP.

Illustration 13	g00530239
Control Lever That is Moved to MAXIMUM FORWARD (Continued Schematic) System components (continued components) (37) Charge pressure relief valve (42) Synchronizarion cutoff valve (43) Balance line (45) Cooler bypass valve (46) Synchronization valve (47) Servo levers (F) Left (G) Right (H) FORWARD (J) PARK (K) REVERSE (BB) Supply oil from servo valves (EE) Charge oil to the drive loops (GG) Lubrication oil (LL) Return oil and supply to charge pump (MM) High pressure drive oil (NN) Low pressure drive oil

The mechanical linkage moves servo lever (47) and servo valves in order to direct servo supply oil (BB) to the servo cylinders. As the cylinders fill and the cylinder rods move, the swashplates move from the ZERO angle to the MAXIMUM FORWARD angle.

In MAXIMUM FORWARD, the schematic shows the positions of the following components: mechanical linkage, swashplates and drive circuit.

The arrows at the rotating groups indicate the direction of flow through the pumps. The shading of (NN) oil represents the oil in the low pressure side of the drive loops (reverse drive lines).

The shading of (MM) oil represents the oil in the high pressure side of the drive loops (forward drive lines).

Charge pressure oil (EE) is shown at both ends of the charge valves. The brake line is filled with charge pressure oil (EE) during MAXIMUM FORWARD and MAXIMUM REVERSE.

Brake pressure is less than the charge pressure by0 to 70 kPa (0 to 10 psi). Charge pressure should not drop below 1100 kPa (160 psi) during a MAXIMUM FORWARD or MAXIMUM REVERSE condition. A minimum charge pressure of approximately 860 kPa (125 psi) is required to keep the operate/brake valve spool shifted.

If charge pressure is between 860 kPa (125 psi) and 1025 kPa (150 psi), pump leakage and/or motor leakage is an indicated occurrence.

During MAXIMUM FORWARD or MAXIMUM REVERSE condition, each drive loop operates the same way. Each piston pump sends high pressure oil through a charge valve and a drive line to the track motor. The return oil from the motor is sent back through the charge valve. Then, the return oil is sent to the inlet side of the piston pump.

Inside the charge valve, the main relief valve senses the pressure in the high pressure side of the drive loop. The oil in the low pressure side is limited to a maximum pressure by charge pressure relief valve (37) in the transmission head assembly.

The shading of (EE) oil and the shading of (NN) oil are both limited to a maximum pressure of 1380 + 280 - 100 kPa (200 + 40 - 15 psi) by the charge pressure relief valve. The shading of (NN) oil is used to indicate the oil in the drive loop.

Oil Flow in MAXIMUM FORWARD for Left Drive Circuit

Note: Oil flow in MAXIMUM FORWARD is the same operation for right drive circuit.

The main control valve directs charge pressure oil to both ends of the charge valve during PARK. The main control valve also directs charge pressure oil to both ends of the charge valve during BRAKES OFF. Then, the charge pressure oil goes through the brake line in order to release the brakes.

The charge pressure oil at the left end of the charge valve enters the supply passage when the flow divides. Part of the oil is sent through a steel tube to the right side charge valve. The remainder of the flow is directed past the shuttle spool to the low pressure side of the left drive circuit.

The position of the shuttle spool is determined by the high pressure oil from the piston pump. When the swashplate is at a forward angle, the pump provides flow through the forward drive line. (This position is shown.)

Supply oil for the pump is then provided through the reverse drive line. The oil (MM) in the forward drive line fills two chambers around the shuttle spool. One of the chambers connects with a drilled passage at the right end of the spool. The drive oil flows through the passage in the spool in order to fill a chamber at the right.

The pressure in the chamber causes the shuttle spool to move all the way to the left (position that is shown). The shuttle spool is now in a position in order to connect the low pressure side of drive loop (NN) with the supply passage for charge pressure (left).

The shuttle spool also directs high pressure oil from the forward drive line to the main relief valve. Then, high pressure oil goes through synchronization cutoff valve (42) to synchronization valve (46) .

The main relief valve limits the pressure in the forward drive line to approximately 38000 kPa (5500 psi). If the pressure in the drive line exceeds the pressure setting of the relief valve, the valve opens. The open valve directs the high pressure oil to the low pressure side of the loop.

Part of the oil from the high pressure side of each drive loop is sent to synchronization valve (46). The synchronization valve is fastened to the right side charge valve.

The high pressure oil from the left side charge valve is first sent through synchronization cutoff valve (42) and balance line (43) before entering the synchronization valve. The synchronization valve contains a valve spool and a ball.

If the pressures for both drive loops are not equal, the ball moves in order to block the passage from the drive circuit which is providing the least amount of pressure.

The drive oil with higher pressure is then directed to the end of the synchronization valve spool. If the pressure at the end of the valve spool is less than 8300 kPa (1200 psi), the valve spring keeps the spool in the position shown. This directs the higher pressure oil from one drive circuit to the other drive circuit.

The illustration indicates the right drive loop (bottom) with higher pressure than the left drive loop. The illustration also indicates the pressure in the forward drive line by the position of the valve spool. The pressure in the forward line on the right side is less than 8300 kPa (1200 psi).

If the pressure in the drive line on the right side was higher than 8300 kPa (1200 psi), the valve spool would move to the left. The valve spool moves against the force of the spring. This blocks the flow to the left side circuit.

The synchronization valve helps the machine to keep tracking straight during conditions when the drive pressures are less than 8300 kPa (1200 psi). The synchronization cutoff valve separates the drive loops during testing procedures. Always return the cutoff valve to the open position after system testing is completed. (This position is shown.)

The illustration also shows that both of the drive lines with low pressure oil (NN) are connected to charge pressure relief valve (37) due to the position of the shuttle spools. The pressure in the cooler is limited to 480 kPa (70 psi) by cooler bypass valve (45).

Illustration 14	g00530337
Partially Depressed Center Pedal Plunger at Top System components (2) Operate/brake valve spool (4) Brake line (5) Charge pressure check valve (7) Start/vent spool (8) Speed/brake control spool (48) Orifice (A) FORWARD (B) Brakes OFF (C) PARK (D) Brakes OFF (E) REVERSE (M) Passage to underspeed valve (N) Pu oil (P) Oil from valve spool (Q) Passage to the drain (AA) Supply oil from the charge pump (BB) Supply oil for servo valves (EE) Charge oil to the drive loops (HH) Blocked oil (LL) Return oil and charge pump supply

When the operator partially depresses the center pedal, the position of the components indicates pedal movement. The pedal moves until the pedal just makes contact with the top of the spring loaded plunger.

The plunger is installed in the base of the pedal. The plunger serves as a stop. When the operator depresses the pedal to this position (plunger at top), the underspeed valve moves to FULL DOWN.

A spacer is installed at the bottom of the underspeed valve. The spacer controls the position of the underspeed roller when the underspeed valve is fully down. If the transmission control lever is in a FULL directional position, the spacer does not allow the underspeed valve to move down far enough. (Place the control lever in FULL FORWARD or FULL REVERSE.) The mechanical linkage moves the servo valves to a centered position or PARK position.

As a result, a small amount of machine movement still occurs. When the servo valves are near PARK position, the swashplates are near ZERO angle. This reduces the flow through the drive loops. Then, the machine slows. (This is called dynamic braking.)

The chamber at the left end of operate/brake valve spool (2) is connected through an internal passage in the valve body to a second passage in the speed/brake control spool.

When the center pedal is NOT depressed, the oil in the chamber at the left end of the operate/brake valve spool does NOT connect to the drain passage in the valve body. When the center pedal is partially depressed, the same condition occurs.

Due to the position of the speed/brake spool, the operate/brake valve spool remains shifted. This continues to provide oil to the brakes. If the operate/brake valve spool moved left, the flow to the brakes would stop. The brakes would engage.

Illustration 15	g00530642
Fully Depressed Center Pedal Fully Depressed Plunger System components (2) Operate/brake valve spool (4) Brake line (5) Charge pressure check valve (7) Start/vent spool (8) speed/brake control spool (48) Orifice (A) FORWARD (B) Brakes OFF (C) PARK (D) Brakes OFF (E) REVERSE (M) Underspeed valve (N) Pu oil (P) Oil from valve spool (Q) Passage to the drain (AA) supply oil from charge pump (BB) Supply oil for servo valves (EE) Charge oil to the drive loops (HH) Blocked oil (LL) Return oil and charge pump suction

After the center pedal is fully depressed against the stop, operate/brake valve spool (2) moves to the left in order to stop the flow of oil through the brake line (LL shaded line).

When the pedal is fully depressed, speed/brake control spool (8) rotates farther inside the valve bore. This connects the chamber at the left end of the operate/brake valve spool to the drain passage in the valve body.

Upstream oil (AA) flows through the passage in the center of the speed/brake control spool. Upstream oil (AA) flows to the top of the underspeed valve. (See the schematic section with M and N.)The operate/brake valve spool is also in a position in order to send upstream oil to the top of the underspeed valve.

When the center pedal is fully depressed, the underspeed valve moves down against a spacer. Then, the mechanical linkage and the servo cylinders are in PARK or near PARK (shown position). The brake line is filled with drain oil.

The spacer in the underspeed valve does NOT allow the servo valves to move all the way to the PARK position. During this condition, an increasing noise from the transmission becomes louder.

When the center pedal is released the following components return the original position: the speed/brake spool, operate/brake valve spool and the underspeed valve.

The speed/brake spool blocks the flow of upstream oil to the top of the underspeed valve. This allows the lower pressure oil from the venturi throat to fill the chamber at the top of the valve piston.

The oil is now called underspeed throat. This allows the underspeed valve to move UP. This returns the swashplates to the original angles through the mechanical linkage arrangement.

The speed/brake spool blocks the drain passage. This increases the pressure at the left end of the operate/brake valve spool. The pressure at the left end causes the operate/brake valve spool to move to the right. This opens a passage for pressure oil. The pressure oil which is sent releases the brakes.

Illustration 16	g00530654
Control Lever in FORWARD With a Starting Engine System components (1) Pilot valve (2) Operate/brake valve spool (5) Charge pressure check valve (7) Start/vent spool (57) Pilot valve spool (A) FORWARD (B) Brakes OFF (C) PARK (D) Brakes OFF (E) REVERSE (AA) Supply oil from the charge pump (BB) Supply oil for servo valves (CC) Oil from the throat of the venturi (EE) Charge oil to the drive loops (LL) Return oil and charge pump suction

When the engine starts with the FORWARD/PARK/REVERSE lever in FORWARD, this schematic shows the position of the pilot spool.

Charge pressure oil is sent through charge pressure check valve (5), past operate/brake valve spool (2) and start/vent spool (7) to pilot valve (1) .

The position of the pilot spool blocks the flow of charge pressure oil. This prevents the pressure oil from entering the chamber at the right end of the start/vent spool.

Without pressure in the chamber, the start/vent spool remains in this position. This blocks the flow of pressure oil back to the operate/brake valve spool. Also, this DOES NOT release the brakes.

The drive system remains in this condition until the transmission lever is returned to PARK. At that point, the start/brake spool moves and the charge pressure oil is available to move operate/brake valve spool (2) to the right. Pilot valve spool (57) is again moved from the PARK position.