TRANSMISSI HYDROLIC SYSTEM
TORQUE CONVERTER HYDRAULIC SYSTEM
The 785 Truck is equipped with a split hydraulic system similar to that which was introduced on the 769C, 773B, and 777 Off-Highway Trucks.In thissystem, the transmission hydraulic circuit is separated from the torque converter, brake, and hoist circuits. Trucks equipped with this system are easily identified by looking at the hydraulic tank.For the 785 Truck, the tank is located on the right side of the machine.The tank is divided into two separate compartments.Each compartment has its own fill tube, cover, and sight gauge.
The front (smaller) compartment supplies oil for operation of the transmission circuit which uses 30W oil for most operating conditions.The rear (larger) compart:ment supplies oil for operation of the torque converter, parking and seeondary brakes, hoist, and brake eooling circuit. This eircuit uses lOW oil for most operating eonditions.
When compared to hydraulic systems from earlier trucks, the split hydrau lic system has three main advantages:
1. The split hydraulie system prevents cross-contamination between the transmission eircuit and the converter, brake release, and hoist circuit. If contaminants enter the system, only one circuit will have to be cleaned.This increases major component life because contaminants cannot spread throughout the entire system.
2. Because the transmission circuit is separate from the remainder of the system, the transmission has its own oil eooler. This reduces heat buildup and permits the transmission to operate cooler during long downhill hauls.
3. The use of 30W oil in the transmission eircuit helps to improve transmission life due to the better lubrication and cooling properties of the heavier oil.The remainder of the system can continue to use lOW oil.
The inlet relief valve is used to protect the converter when the oil is cold and thick by
limit the maximum pressure of the oil that is sent to the converter. The outlet relief valve limits the maximum pressure of the oil in the converter.Because these valves operate identically to those used in earlier machines, a detailed explanation of their operation will
not be provided in this presentation.
NOTE TO THE INSTRUCTOR: FOR AN EXPLANATION OF CONVERTER INLET AND OUTLET RELIEF VALVES, SEE STMG 486 "TORQUE CONVERTER HYDRAULIC SYSTEM FOR 769C, 773R AND 777 OFF-HIGHWAY TRUCKS"
Lockup clutch engagement and release a controlled
In converter drive, the lockup solenoid is not energized.Flow from the transmission charging pump is blocked at the solenoid and at the transmission relay valve. In this condition, a small amount of oil (brown) from the transmission lube circuit flows through the orifice at the upper end of the lockup clutch relay valve.The lube oil flows through the line from the lockup clutch relay valve to the drain passage in the transmission relay valve.This keeps the line connecting the two relay valves filled with warm oil to provide quicker response time during the transition from converter drive to direct drive.The lube oil pressure is never high enough to move the spool in the lockup clutch relay valve.
In converter drive, supply oil from the parking brake release pump enters the lockup clutch control valve at the inlet below the pressure reducing valve spool.Some of this oil (red) is sent to the modulating reducing valve where it is blocked by the valve spool.The remainder of the oil (red dots) flows past the pressure reducing valve spool to the lockup clutch relay valve where it is blocked by the relay valve spool. At the same time, the pilot oil (red dots) flows past the ball check valve and fills the slug chamber in the pressure reducing valve spool. As the pressure of the oil in the slug
chamber increases to the pilot pressure setting, the spool moves to the left. During all operating conditions, the pressure reducing valve maintains the pressure of the pilot oil to the lockup clutch relay valve at a constant 350 psi (2415 kPa).
It is important to note, however, that the pressure reducing valve has no effect on the pressure setting of the supply oil (red). The supply oil pressure is limited by the relief valve portion of the parking and secondary brake valve.
This opens the valve in the base of the solenoid and permits pressure oil to fill the chamber at the right end of the transmission relay valve.The relay valve spool then moves to the left and permits signal oil to flow to the chamber at the upper end of the lockup clutch relay valve. Pressure in the upper chamber of the lockup clutch relay valve moves the spool down and holds it in that position during the entire time that the solenoid is energized. Notice that some of the signal oil from the upper chamber flows through the orifice (on the left) to the transmission lube circuit.
The downward movement of the lockup clutch relay valve spool permits pilot oil (red dots) to flow through the check valve at the left of the modulating reducing valve spool to the chamber at the upper end of the selector piston. Pilot pressure at the end of the selector piston can then move the piston down as shown.Movement of t:he selector piston accomplishes two purposes: (l) The drain passage at the right of the selector piston is blocked and (2) the load piston spring is compressed.
Compressing the load piston spring moves the modulating reducing valve spool down against the force of the spring (at the lower end of the spool).This opens the chamber at the center of the spool and permits pressure oil (red and white stripes) to flow to the lockup clutch.As the clutch fills, pressure oil opens the ball check valve and fills the slug chamber at the lower end of the spool.
At the same time, oil flows through the load piston orifice and fills the chamber between the end of the load piston and the selector piston.The load piston orifice provides a pressure drop and time delay in the flow of oil to the load piston chamber. This helps control the rate of modulation.Filling the load piston chamber is made possible when the selector piston covers the drain passage at the right side of the piston
The lockup clutch pressure and the pressure in the slug chamber increase at the same rate.Just after the clutch is filled, the pressure in the slug chamber moves the reducing valve spool up against the force of the load piston spring.This movement restricts the flow of pressure oil to the lockup clutch and briefly limits the increase of clutch pressure.The pressure in the load piston chamber then moves the load piston further down.
This increases the load piston spring force and reopens the supply passage permitting the clutch pressure to again increase.This cycle continues until the load piston has moved completely down (against the stop). The lockup clutch pressure is then at its maximum setting.During modulation, the reducing valve spool moves up and down (open and closed) while the load piston moves down smoothly.
At the end of the modulation cycle, the pressure in the slug chamber moves the reducing valve spool up a small distance to restrict the flow of supply oil to the lockup clutch.This is the "metering position" of the reducing valve spool.In this position, the valve maintains precise control of the pressure during the entire time that the lockup clutch is engaged.
Energizing the lockup solenoid also causes oil to flow to valve station D in the transmission pressure control group.Later it will be shown how valve station D helps to lower the maximum pressure in the transmission hydraulic system while the machine is in direct drive
To check the lockup clutch pressure, it is first necessary to remove the wiring harness connector from the lockup solenoid and the large connector from the end of the transmission control box.Connect a test lead between socket number 1 and socket number 5 on the large connector.With the transmission in NEUTRAL, start and run the engine at low IDLE. The initial gauge reading must be zero.Then, touch the harness connector to the lockup solenoid and read the maximum pressure on the gauge.After recording the pressure, remove the solenoid connector and verify that the pressure decreases to zero.If the lockup clutch pressure is within specifications, additional tests will not be necessary.
If the lockup clutch pressure is not correct, the parking brake release and lockup clutch pilot pressures should be checked before adjusting the lockup clutch pressure.These two pressure taps are on the control valve next to the lockup clutch pressure tap.The parking brake release pressure is checked with the transmission in NEUTRAL, the parking brakes engaged, and the engine running at HIGH IDLE. The lockup clutch pilot pressure is checked with the transmission in NEUTRAL, the parking brakes engaged, and the engine running at LOW IDLE.
TRANSMISSION HYDRAULIC SYSTEM
The major component in the transmission hydraulic system is the selector and pressure control valve group.In the machine, the control valve group is mounted on top of the planetaries.This illustration shows the four main components that make up the control group: the pressure control valve group (number 1), the selector valve group (number 2), the distribution manifold (number 3), and the rotary actuator (number 4)
The pressure control valve group has seven modulating reducing valves which are referred to as "valve stations."Each valve station is identified by a letter (A, B, C, D, E, F, and G).The H station is not used in this application.Six of the seven valve stations individually control the engagement and maximum pressure for each of the six clutches in the transmission.The seventh station (D) helps to lower the maximum pressure in the system during operation in direct drive.
In the valve stack, the selector valve group is directly below the pressure control valve group.The selector valve group has four functions:
1. A dual stage relief valve limits the maximum system pressure in converter drive and lowers the maximum pressure setting in direct drive.
2. A rotary selector spool (which is connected to the rotary actuator) sends pilot oil to the pressure control valve group to permit engagement of the correct clutches for each gear range.
3. A reset spool permits pilot oil to flow to the rotary selector spool when the engine is started with the rotary selector spool in the NEUTRAL (Nl) position.
A priority reducing valve controls the pilot pressure and makes sure that pilot oil is available at the reset spool before oil can flow to the remainder of the system
To initiate a shift, pressure oil from either the upshift or downshift solenoid is sent to the rotary actuator. Inside the actuator housing is a rotating vane which divides the actuator into two chambers.Pressure oil from the upshift solenoid causes the vane to rotate in one direction while pressure oil from the downshift solenoid causes the vane to rotate in the o osite direction. The vane is connected to and causes rotation of the rotary selector spool inside the selector valve group
The transmission hydraulic system is From left to right, the three sections nre: lubrication, and scavenge.From the flows through a filter and is sent transmission relay valve, and the
flow is blocked at the lockup solenoid, the solenoid.Because the downshift solenoid
continuously energized in NEUTRAL (Nl), the valve in the base of the oil to flow to the rotary actuator. rotating vane in the rotary actuator spool in the Nl position until a shift is made.
Most of the flow from the charging section of the pump is sent to the selector valve group.This oil is first sent to the priority reducing valve (upper left corner of selector valve body).From the priority reducing valve , the oil flows to the reset spool , the rotary selector spool and the dual stage relief valve.Oil is also sent to the pressure control valve group to fill and engage the number 3 clutch.
The lubrication section of the pump sends oil through an oil filter and an oil cooler to lthe transmission lube circuit . The oil cooler is equipped with a bypass valve to protect the cooler when the oil is cold and thick and during high flow condition . A lube relief valve limits the maximum pressure in the lube circuit.Oil from the bottom of the transmission case is returned to the tank by the scavenge section of the pump.
Pressure oil from the solenoid valves causes the rotating vane and hollow shaft to turn in either the clockwise or counter clockwise direction
There are two oil ports on outside of the actuator housing. port (number 3) is for pressure oil from the downshift solenoid port (number ) is for pressureil from the upshift solenoid. is installed in each of the two oil passages
Notice that in this view, the rotating vane has moved as far as possible in the counterclockwise direction .This is the NEUTRAL (Nl) position of the rotary actuator.
To shift from NEUTRAL ( N1) to any other gear, the rotating vane must turn in the clockwise direction to t:he selected gear position.When the shift is indicated, pressure oil from the upshift solenoid is sent to the lower inlet port (number 4).
The presure oil moves the check valve toward the center of the actuator housing until the check valve covers a drain passage located near the inner end of the inlet passage.The pressure oil then flows through the check valve and fill the small space between the two vanes.As the pressure increases, the moves in the clockwise direction to the appropriate gear positi that was in the chamber on the nonpressurized (downshift the vane is forced out of the chamber by the movement of the vane. the oil flows out of the chamber, it moves the upper check valve away from the; center of the actuator housing.
This movement opens a drain passage located near the inner end of the upper check valve passage and permit the oil to flow out of the center chamber. This sequence is just the opposite for downshifts (when the rotating vane moves in the counterclockwise direction)
Remember, the clockwise sequence for upshifts at the rotary actuator is
Nl-N2-R-l-2-3-4-5-6.If the operator moves the transmission selector lever from NEUTRAL to FIRST, the rotating vane moves from the Nl position and momentarily stops in N2 to permit engagement of the number 1 clutch.
The rotating vane then "skips" past the REVERSE position and moves directly to the FIRST gear position. The Nl-N2-R-l configuration permits the downshift side of the rotating vane to be continuously pressurized in the Nl position (see preceding slide). This locks the transmission in NEUTRAL and prevents a shift to any other gear until the operator moves the selector lever.
gear.This view better illustrates how the rotating and stationary vanes divide the center chamber into two chambers.Except for in NEUTRAL (Nl), the chambers are not pressurized until an upshift or downshift is called for by the electronic controls.When a sh.ift is indicated, the appropriate solenoid valve permit pressure oil to flow to the chamber. The pressure oil moves the rotating vane to the indicated gear position.
Movement of the rotating vane and its hollow shaft cause the rotary selector spool to turn. The rotary selector turns the flexible coupling which is connected to the transmission switch.The transmission switch sends an electrical signal to the transmission control.The signal indicates the gear in which the transmission is now operating.The transmission control then deactivates the solenoid.This blocks the flow of pressure oil t:o the rotary actuator
It is important to realize that, while the shift cycle takes some time to explain , the complete sequence of events happens very rapidly. sequence of very rapidly
Visible in this view are the detent cam (number l) and the detent spring (number 2).
The detent cam is installed in the end of the rotary selector spool and and turns with spool. A bolt and flat washer ( number 3) help to retain the rotary the valve body.The detent spring are bolted to a support block (number 4) which is part of the access cover for the dual stage relief valve .Pins in the the support block and flat retainers help to align the detent spring and hold them in position
For any gear except NEUTRAL (Nl and N2),two of the outlet ports from the upper chamber are aligned with drilled passages in the selector valve body. For Nl and N2, only one outlet port permits pilot oil to flow to the pressure control valve group.(In Nl the number 3 clutch is engaged and in N2 the number 1 clutch is engaged.)
The lower chamber in the rotary selector spool is always open to drain (as indicated by the green arrow). For each gear position except Nl and N2, four drain ports are aligned with drilled passages in the valve body to prevent engagement of the clutches that are not used in that particular gear.Five passages are drained in each of the two NEUTRAL positions.
The priority reducing valve is installed in the bore on the left ifde of the valve body. It consists of a spool, poppet valve, and spring.The priority reducing valve has two functions:It controls the pressure of the pilot oil (red dots) that is used to initiate clutch engagement and it makes sure that pilot pressure is available at the reset spool before pressure oil (red) is sent to the remainder of the system
To the right of the priority reducing valve is the reset spool which contains a spring and a ball check valve.Notice that a small orifice is machined in the check valve seat just to the right of the check valve orifice.The reset spool permits pilot oil to flow to the rotary selector spool when the engine is started with the rotary selector spool in the Nl position.If the rotary selector spool is not in the Nl position during engine startup, the reset spool will block the flow of pilot oil to the rotary selector spool.This condition will occur only if there is a malfunction in the system.
Directly below the reset spool is the dual stage relief valve which consists of a spool, slug, and ball check valve.The relief valve also has two purposes:It limits the maximum system pressure during operation in converter drive and provides a reduced system pressure for direct drive operation.The reduced pressure in direct drive is controlled by valve station D in the pressure control valve group.
Just above and to the right of the reset spool is a check valve.When the lockup solenoid is energized,' oil from the solenoid to valve station D must flow through this check valve.
The rotary selector spool is installed in the bore on the right side of the valve body.
Notice that the plug and screen assembly divides the center cavity of the spool into two separate chambers.The upper chamber is filled with pilot oil and the lower chamber is always open to drain.
During operation in Nl, supply oil from the pump fills the chamber around the priority reducing valve spool and flows to the reset spool where the oil is momentarily blocked.At the same time, the oil flows through the poppet
valve in the priority reducing valve to the upper end of the reducing valve spool. As the pressure at the end of the reducing valve spool increases to the pilot pressure setting, the spool moves down and opens an outlet passage (just below the supply passage). When the pilot pressure reaches its maximum setting, the reducing valve spool moves down to the position shown to limit the pilot pressure.Notice, however, that the outlet passage remains open.
From the outlet passage at the priority reducing valve, the oil is sent in two directions.A small amount of the oil is sent through a passage (number 1) to the upper end of the rotary selector spool.
In the Nl position, a drilled passage in the rotary selector spool permits the oil to fill the chamber (number 2) near the center of the reset spool.Pressurizing this chamber is also permitted because its drain port (number 3) near the bottom of the rotary selector spool is blocked in Nl.
Pressure oil can then fill the drilled chamber in the reset spool, flow through the small orifice in the check valve seat (at the right of the check valve orifice), and fill the chamber at the upper end of the reset spool.When the reset spool moves down to the position shown, pilot oil from the priority reducing valve can fill the upper chamber in the rotary selector spool.(The pilot oil also holds
the reset spool in the position shown during normal operation in gear ranges other than Nl).
In the Nl position, the outlet port (number 4) in the rotary selector spool for valve station C is open and the drain port (number 5) for valve station C is blocked. This permits pilot oil to flow to the pressure control valve group.Since the remaining outlet ports for the upper chamber are blocked
and the rema n ng drain ports for the lower chamber are open, only valve station C is supplied with pilot oil.
The remainder of the oil from the outlet passage at the priority reducing valve goes to the dual stage relief valve and the pressure control valve group.The oil flow to the pressure control group fills the chambers around the modulating reducing valves and remains available for clutch engagement. In converter drive, the dual stage relief valve functions as a simple relief valve. As the pressure in the slug chamber increases to the maximum relief valve setting, the spool moves up and permits oil to flow to the oil cooler and lube circuit.
Valve Station A Number 2 Clutch
Valve Station B Number 1 Clutch
Valve Station c Number 3 Clutch
Valve Station D Dual Stage
Relief Valve Valve Station E Number 4 Clutch
Valve Station F Number 5 Clutch
Valve Station G Number 6 Clutch
Notice that pressure oil is available at all of the modulating reducing valves, but only the number 3 clutch is engaged.Modulation of the clutch pressure began when pilot oil from the rotary selector spool was sent to the outer end of the selector piston in valve station C.
In this view, the number 3 clutch is shown fully engaged. Both the selector piston and the load piston are moved completely to the right against the stop.The selector piston was moved by pilot oil pressure (red dots) and the load piston was moved during
modulation by clutch pressure (red and white stripes).Initial movement of the selector piston must take place before modulation can begin
A close look at an individual valve station will better illustrate how modulation and clutch pressures are controlled.
Components that are identical for all seven stations include: the reducing valve spool, ball check valve, inner spring (at the left end of the spool), slug, selector piston, and load piston.
The six stations that control the clutches contain load piston orifices; valve station D does not.The retaining springs for the load piston orifices are identical but the orifices vary in thickness from one station to another. Five of the stations are equipped with decay orifices; stations D and G are left open.The load piston springs are different from one station to
another.
In this schematic, the engine has been started, but the clutch for this station has not been engaged.While the engine is running, pump (or system) pressure is always available
at the reducing valve spool; but, until pilot oil from the rotary selector spool is sent to the right (outer) end of the selector piston, there can be no valve movement and the
clutch cannot be engaged.
The drain passage at the decay orifice is blocked, and (2) the load piston springs are compressed
Compressing the load piston springs moves the reducing valve spool to the left against the force of the inner spring.This opens the supply passage (from the pump) and permits pressure oil to flow to the clutch. As the clutch fills, pressure oil opens the ball check valve and fills the slug chamber at the left end of the
reducing valve spool.
At the same time, oil flows through the load piston orifice and fills the chamber between the end of the load piston and the selector piston.The load piston orifice provides a pressure drop and time delay in the flow of oil to the load piston chamber.This helps control the rate of modulation.
Filling the load piston chamber is made possible when the selector piston covers the drain passage at the decay orifice.
The clutch pressure and the pressure in the slug chamber increase at the same rate.Just after the clutch is filled, the pressure in the slug chamber
moves the reducing valve spool to the right.This movement restricts the flow of pressure oil to the clutch and briefly limits the increase of clutch
pressure. The pressure in the load piston chamber then moves the load piston further to the left.
This increases the spring force and reopens the supply passage permitting the clutch pressure to again increase.This cycle continues until the load piston has moved completely to the left (against the stop).The clutch pressure is then at its maximum setting.During modulation, the reducing valve spool moves left and right while the load piston moves smoothly to the left.
a modulating reducing valve, the maximum pressure setting of the clutch is lower than the system pressure.At the end of the modulation cycle, the pressure in the slug chamber moves the reducing valve spool a small distance to the right to restrict the flow of supply oil to the clutch.This is the "metering position" of the reducing valve spool.In this position, the valve maintains precise control of the clutch pressure.
During operation, an engaged clutch is designed to leak a relatively small
but steady volume of oil.This leakage helps prevent high oil temperatures and provides additional lubrication for the planetary gears and bearings.As clutch leakage occurs, the clutch pressure and the pressure of the oil in the slug chamber will start to decrease.At this point, the load piston springs move the reducing valve spool a small distance to the left to open the supply passage.
Pressure oil from the pump again enters the clutch circuit and replaces the leakage.
When the clutch pressure is again at its maximum value, the pressure in the slug chamber moves the spool back to the right thereby restricting the flow of supply oil to the clutch.
This metering action continues during the entire time that the clutch is engaged
Near the end of the modulation cycle, the pin contacts the reducing valve spool, moves the spool completely to the left, and opens the clutch passage to supply oil.At this point, modulation ends and the clutch pressure immediately increases to maximum.
During the time that the clutch is engaged, the reducing valve spool remains completely to the left (as shown) and the clutch pressure is equal to the system pressure.
Replacement of clutch leakage for valve stations A, B, and F requires no additional valve movement because the pin holds the reducing valve spool in the open position.
As a result, supply oil is always available at the clutch
Restricting the rate of clutch pressure decay helps to maintain a positive torque at the transmission output shaft. This minimizes the effects of tire and axle "unwinding" and permits smoother shifts.An immediate drop in clutch pressure would permit a rapid deceleration of the power train components that remain connected to
the differential during the shift.
This could result in rough shifts and accelerated wear of the power train
components.
When a clutch is disengaged, the chamber at the right (outer) end of the selector piston is opened to drain through the lower chamber in the rotary selector spool.
This permits the selector piston and load piston to move to the right as shown.
Clutch pressure starts to decrease, but cannot drop to zero until the chamber
between the load piston and the selector piston is drained.The only way that oil can flow out of this chamber is through the decay orifice which was uncovered when the selector piston moved to the right.As the load piston springs force the oil from the load piston chamber, the clutch pressure gradually decreases.When
the load piston has moved completely to the right, the clutch pressure is zero.
NOTE: VALVE STATIONS D AND G DO NOT HAVE DECAY ORIFICES. INSTEAD, THE DRAIN PORTS ARE LEFT OPEN. THIS PERMITS THE PRESSURES CONTROLLED BY THESE STATIONS TO DECREASE VERY RAPIDLY. VALVE STATION D CONTROLS THE DUAL STAGE RELIEF VALVE AND VALVE STATION G CONTROLS THE NUMBER 6 CLUTCH WHICH IS USED ONLY IN REVERSE.
The colors designate the hole size of the orifice--yellow is .055 inches (1.40 mm) in diameter, green is .062 inches (1.57 mm) , and red is .078 inches (1. 98 mm).
The green and red orifices are made of steel while the yellow ori.fices are plastic.
The plastic orifices are equipped with a small screen to help prevent restriction of their relatively small hole diameters.
NOTE:SOME OTHER MACHINE MODELS ARE ALSO EQUIPPED WITH BLUE PLASTIC ORIFICES WITH A HOLE SIZE OF .047 INCHES (l.19 mm)
1. The lockup solenoid is energized.
This opens the transmission relay valve and directs oil to the lockup clutch relay valve to initiate engagement of the torque converter lockup clutch.
2. Oil from the lockup solenoid is also sent through the check valve at the right of the reset spool to valve station D.
3. Valve station D sends reduced pressure oil through an orifice at the right of the dual stage relief valve to the chamber at the bottom of the relief valve spool.
Flow from the lockup solenoid enters the valve body at the check valve to the right of the reset spool. This moves the check valve down to cover the small drain hole at the right side of the check valve.
Pressure oil can then flow to valve station D and fill the chamber at the end of the selector piston. When the machine shifts from direct drive to converter drive, the selector piston chamber for station D is drained at the check valve
Just below and to the right of the dual stage relief valve are two orifices. The upper (supply) orifice has a larger hole size than the lower (drain) orifice.In direct drive, oil from valve station D flows through the upper orifice and fills the chamber at the lower end of the dual stage relief valve spool.
The pressure of this oil works against the end of the spool thereby increasing the effective reaction area of the relief valve. With a larger effective area, a lower system pressure is required to overcome the spring force and open the relief valve.Therefore, the system pressure in direct drive is lower than the system pressure in converter drive.When changing from direct to converter drive, the oil in the chamber at the bottom of the spool is drained through the lower orifice.
For any gear range except Nl, the chamber (number 1) at the center of the reset spool is open to drain.This occurs because the pressure oil passage (number 2) at the top of the rotary selector spool is blocked and the drain port (number 3) at the lower chamber is open.It is important to note, however, that the reset spool does not move up to block the flow of pilot oil when the center chamber is drained.This is due to the fact that the small cross-drilled holes near the bottom of the reset spool are covered by the valve body.
The overall design of the pressure control valve group permits a relatively easy check of the clutch pressures.Since each clutch has its own modulating reducing valve, a gauge connected at each valve station will show the clutch pressure for each gear range as the transmission is manually upshifted and downshifted.All of the clutch pressures are checked with the engine operating at its maximum governor setting.At the minimum governor setting, only a check of the clutches engaged in REVERSE, NEUTRAL, and FIRST is necessary.
Instead, the correct procedure is to first check the system pressure in converter drive.
If this pressure is not within specifications, add or remove shims at the dual relief valve.
Then, check the system pressure in direct drive. If the direct drive pressure
is not correct, add or remove shims at the load ton for valve station D.
NOTE: IN THIS VIEW, THE TAP FOR CHECKING SYSTEM PRESSURE IS LOCATED ON THE SIDE OF THE SUPPLY MANIFOLD.ON NEWER MACHINES, THIS PRESSURE TAP IS LOCATED ON THE FRONT OF THE SUPPLY MANIFOLD.
The lube relief valve (arrow) is not part of the selector and pressure control valve group.Instead, it: is threaded into the inside of the transmission case at t:he right of the control group.
Then, remove the top cover from the transmission case as shown.
The clutch pressures for the converter drive gears (R-Nl-N2-l) are checked at both the MINIMUM and MAXIMUM GOVERNOR SETTING.The direct drive clutch pressures are checked at the MAXIMUM GOVERNOR SETTING only. The pilot pressure must remain within specifications for all test conditions.
Notice : that pressure gauge is installed at valve station D.If desired , this gauge can be used to verify that valve station D is operating in the direct drive gear.
pressure readings.
If adjustment of the clutch pressures and/or primary pressures is necessary, adding shims to a modulating reducing valve will increase the pressure and removing shims will decrease the pressure.If the pilot pressure is not within specifications, shims must be added or removed at the priority reducing valve.
Correct adjustment of the primary pressures is very important because they control clutch phasing and fill times.Remember, the rates of clutch pressure rise and clutch pressure decay are individually controlled by the valve stations.During a normal shift, a clutch can start to fill before a released clutch is completely drained. When the primary pressure settings of the clutches are correct, the pressure rise of an engaged clutch does not begin until the pressure at the released clutch is low enough to permit a small amount of slippage.
In this condition, the clutches are phased correctly.Incorrect primary pressure settings, however, can cause a clutch (or clutches) to engage before the pressure at the released clutch is low enough to permit the normal slippage that occurs during a shift.This incorrect phasing can result in extreme damage to the clutches and planetaries.High
primary pressure settings will cause fast, rough shifts that can result in damage to the power train and discomfort for the operator.Low primary pressure settings will cause slow or delayed shifts. The slow shifts will often be smooth, but they can cause decreased clutch life due to excess ve clutch slippage.
CONCLUSION
This presentation has discussed the operation of the electronic and hydraulic control systems for the 785 Truck transmission and torque converter.The electronic controls provide a more rapid response for maximum power train efficiency.The main advantage of the split hydraulic system is its ability to supply cleaner, cooler oil for a longer component life than was present in some earlier machines.
Individually controlled clutch pressure rise and pressure decay will permit smoother shifts and a comfortable ride which will also enhance component life.A thorough understanding of these systems should make troubleshooting easier and minimize repair time.
THANK YOU FOR YOUR VISITED
TRANSMISSI AND TORQUE CONVERTER PART 2 ON 785 OFF-HIGHWAY TRUCK
Reviewed by heri
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Reviewed by heri
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