Treinamento de Serviço Material do Aluno

Centro de treinamento de Itabira Janeiro 2012

POWER TRAIN

Instrutor:___________________________________
 Power Train

Aluno:_______________________________________

Sumário
POWER TRAIN.....................................................................................................................................3
Power Train Main Components and Theory of Operation...................................................................................................3
Torque...................................................................................................................................................................................4
INTRODUCTION TO TORQUE CONVERTERS............................................................................4
TORQUE MULTIPLICATION...........................................................................................................................................6
FEATURE........................................................................................................................................................................6
BENEFIT.........................................................................................................................................................................6
AUTOMATIC......................................................................................................................................................................6
FEATURE........................................................................................................................................................................7
Components of Torque Converter........................................................................................................................................8
Lockup Clutch....................................................................................................................................................................11
Impeller Clutch...................................................................................................................................................................12
One-Way Clutch.................................................................................................................................................................13
Torque Converter Inlet Relief Valve..............................................................................................................................14
Torque Converter Outlet Relief Valve...............................................................................................................................15
PLANETARY POWERSHIFT TRANSMISSION............................................................................................................16
PLANETARY GEAR SET COMBINATIONS.................................................................................................................17
PLANETARY POWERSHIFT TRANSMISSION ASSEMBLY......................................................................................18
ONE PLANETARY GEAR FOR EACH DIRECTION AND SPEED.............................................................................22
PLANETARIES.................................................................................................................................................................27
CLUTCHES........................................................................................................................................................................28
COUNTER-SHAFT POWERSHIFT TRANSMISSION..................................................................29
CLUTCHES........................................................................................................................................................................30
TRANSMISSION CONTROL ELECTRONIC (TCE) HYDRAULIC VALVE............................34
INDIVIDUAL CLUTCH MODULATION (ICM) TRANSMISSION HYDRAULIC CONTROL
VALVE..................................................................................................................................................42
Rotary actuator.................................................................................................................................46
785 - 793 PRESSURE CONTROL GROUP THIRD SPEED FORWARD................................53
Electronic Clutch Pressure Control (ECPC).................................................................................70

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Power Train

Power Train Main Components and Theory of Operation

A power train is a group of components that work together to transfer power from the source where it
is produced to a point where it is used to perform work. This definition might be compared to a
"freight train." A freight train is a group of components, a locomotive and cars, that transfers freight
from where it is produced to where it is needed.

The term power train is not new. It has been used since the earliest times to describe the components
that transfer power from one place to another. For example, in early water-powered mills (Figure
1.1.1) used in colonial times, the term power train was used to describe the machinery that carried
power from the water wheel to perform work such as milling flour, weaving cloth, or sawing lumber.

In a typical modern industrial machine, the power train transfers power from the rotating flywheel of
an engine to the road wheels or tracks that do the work of propelling the machine. But it does more
than just transfer power. If an engine were coupled directly to the drive wheels of a vehicle, the
vehicle would run constantly at engine speed.

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Torque

Torque is a twisting effort applied to an object that tends to make the object turn about its axis of
rotation. The amount of torque is equal to the magnitude of the applied force multiplied by the
distance between the object's axis of rotation and the point where the force is applied. Just as a force
applied to an object tends to change the linear rate of motion of the object, a torque applied to an
object tends to change the object's rate of rotational motion.

The amount of torque available from a source of power is proportional to the distance from the center
at which it is applied. In Figure 1.1.5 the lever has more torque as the fulcrum gets closer to the object
of power application (right diagram). But the lever must also be rotated farther to get this torque.

The power trains used in most of today’s construction machinery can be classified into three basic
types:

 Mechanical
 Hydrostatic
 Electric

Introduction to Torque Converters

This topic describes torque converters and discusses their features, benefits and applications in
Caterpillar machines.

This segment describes the basic operation of the torque converter and how it is used in Caterpillar
products.

The hydraulic coupling transmits power from an engine to a driven unit ( in this case, a transmission is
the driven unit). There are two types of hydraulic mechanisms used to transmit power - the fluid
coupling and the torque converter. Both use the energy of a fluid in motion to transmit power.
However, because fluid couplings operate on much the same principles as torque converters, an
understanding of their operation will make the study of torque converters easier.

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The hydraulic coupling transmits power from an engine to a driven unit ( in this case, a transmission is
the driven unit).

There are two types of hydraulic mechanisms used to transmit power - the fluid coupling and the
torque converter. Both use the energy of a fluid in motion to transmit power. However, because fluid
couplings operate on much the same principles as torque converters, an understanding of their
operation will make the study of torque converters easier.

A fluid coupling consists of an impeller and turbine with internal vanes that face each other. The
impeller, sometimes called the pump, is attached to the engine flywheel and the turbine is attached to
the transmission input shaft. The impeller is the driving member, and the turbine is the driven
member. When the engine is started, the impeller starts turning and forces oil from its center toward
the outside edge.

Centrifugal force causes the oil to strike the turbine blades. The force and energy of the oil starts the
turbine turning, coupling the engine to the transmission and transmitting the power required to move
the machine.

A torque converter is a fluid coupling with the addition of a stator. Like the fluid coupling, the torque
converter couples the engine to the transmission, and transmits the power required to move the
machine. Torque converter basic components are an impeller, turbine, stator and output shaft.

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Unlike the fluid coupling, the torque converter can also multiply torque from the engine which
increases torque to the transmission. The torque converter uses a stator which redirects fluid back into
the impeller in the direction of rotation. The force of oil from the stator increases the amount of torque
transferred from the cresting torque multiplication.

TORQUE MULTIPLICATION

FEATURE

A feature of the torque converter is that it multiplies torque from the engine to the drivetrain.

BENEFIT

The benefit that this provides increased output when working against a load.

AUTOMATIC
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FEATURE

A feature of the torque converter is that it automatically couples the engine to the transmission.

Torque converts are the coupling devices used in most Caterpillar products.

The torque converter is standard with all powershift transmission in track - type tractors. Some tractors
are equipped with a flywheel clutch instead of a torque converter.

Wheel type machines are equipped with torque converters with the exception on motor graders and
hydraulic drive machines.

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Components of Torque Converter

The torque converter consists of four components contained in a housing that is filled with fluid by the
transmission pump: impeller ( driving member), turbine (driven member), stator (reaction member)
and output shaft.

The impeller is the driving member of the torque converter. It is connected to the engine flywheel and
driven at engine speed. The impeller acts as a pump as it picks up the fluid in the torque converter and
directs it toward the turbine. The vanes used in the impeller are curved to accelerate the oil flow as it
leaves the impeller.

The turbine is the driven member of the torque converter whose vanes receive the oil flow from the
impeller The turbine rotates to turn the torque converter output shaft. The inlet sides of the turbine
vanes are curved toward the impeller to absorb as much energy or power as possible from the fluid
flow.

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The stator is the stationary reaction member of the torque converter whose vanes multiply force by
redirecting fluid flow from the turbine back to the impeller. The stator is fastened to the torque
converter housing and does not rotate. The purpose of the stator is to change the direction of the flow
of oil between the turbine and the impeller. This direction change increases the momentum of the
fluid, thereby increasing the output torque of the converter.

The output shaft is splined to the turbine and sends power to the input shaft of the transmission. The
output shaft is connected to the transmission through a yoke and driveshaft or directly to the
transmission input gear.

In this segment, torque converter power flow is described.

The flow of oil through the torque converter creates the power flow for the driver train. We will
examine the complete power flow process as it relates to creating torque for the transmission.

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Oil fills the torque converter through the inlet port. The oil goes through a passage in the hub to the
impeller.

The impeller turns with the housing at engine speed and forces the oil toward the outside of the
impeller, around the inside of the housing and against the blades of the turbine. As the oil hits the
turbine blades, the turbine begins to rotate causing the output shaft to turn. This sends power to the
transmission. At this point in time, the torque is not multiplied and the torque converter is operating
like a fluid coupling.

As the oil hits the turbine blades, the oil is forced toward the inside of the turbine. Oil leaving the
turbine, moves in a direction opposite the direction of the impeller rotation. The stator redirects the oil
back into the impeller in the direction of impeller rotation which causes torque multiplication. Oil
leaves the torque converter through the outlet passage.

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Lockup Clutch

Shown is a sectional view of the torque converter. The major components include the rotating housing,
the impeller, the turbine, the stator, the impeller clutch, and the lockup clutch.

The carrier (orange) and the stator (orange) are assembled together. This torque converter is not
equipped with a free wheel stator.

The turbine is splined to the output shaft. In TORQUE CONVERTER DRIVE, the force that rotates
the output drive shaft is developed by the oil pressure that is directed to the torque converter inlet oil
pressure port.

In DIRECT DRIVE, the lockup clutch connects the turbine (blue) to the housing (red). The lockup
clutch discs are splined to the turbine and the lockup clutch plates (yellow) are splined to the housing
(red). When the lockup clutch modulating valve is energized, the oil is directed through the output
drive shaft from the lockup clutch oil pressure port. The force that is developed by the oil pressure will
engage the lockup clutch. The housing, turbine, impeller, and the output drive shaft rotate as a unit at
engine rpm.

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Impeller Clutch

In the torque converter, the rotating housing (red) is splined to the engine flywheel (not shown) and is
driven by the flywheel. The impeller (pink) connects to the rotating housing through the impeller
clutch discs and plates (yellow). The clutch discs are splined to the impeller and the clutch plates are
splined to the rotating housing. When the impeller clutch modulating valve (not shown) is de-
energized, oil flows to the impeller clutch oil pressure port through the impeller clutch oil pressure
port. The force of the oil on the impeller clutch piston engages the discs and plates. When the impeller
clutch is engaged, the impeller rotates with the housing.

The turbine (blue) and the output drive shaft (blue) are fastened together.

When the impeller clutch modulating valve is energized, the oil pressure at the impeller clutch oil
pressure port is reduced to the dump pressure. The dump pressure will be maintained in order to
eliminate refilling the clutch. This improves the response time of the impeller clutch.

The force on the impeller clutch piston is at a minimum. The engagement between the impeller clutch
discs and plates will develop the minimum amount of torque. The engagement between the housing
and the impeller will slip.

As the current to the impeller clutch modulating valve decreases, the oil pressure at the impeller clutch
oil pressure port increases. As that pressure increases, force at the impeller clutch will increase and the
allowable slippage is reduced.

The carrier (orange) and the stator (orange) are assembled together. This torque converter is not
equipped with a free wheel stator.
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One-Way Clutch

The one-way clutch holds stator when the torque converter is used. The one-way clutch allows stator
to turn freely when the torque converter is not used.

Detail of One-Way Clutch

(6) Stator

(14) Cam

(15) Rollers

(16) Springs

(17) Openings in cam

(18) Carrier

Splines connect stator (6) to cam (14). Cam (14) is turned by the stator. Carrier (18) does not turn.

Rollers (15) are the mechanical connection between cam (14) and carrier (18). Rollers (15) are in
openings (17) of cam (14). Springs (16) are also in openings (17). The left side of openings (17) are
smaller than the right side of openings (17) because the opening has a taper. Normally, springs (16)
retain rollers (15) in the taper at the left side of openings (17) .

When the speed of impeller (5) and turbine (4) is slow, stator (6) is held stationary. Rollers (15) are
held in the taper of openings (17) by springs (16). There is a mechanical connection between cam (14)
and carrier (18). Since carrier (18) is held stationary, cam (14) is held stationary. Since cam (14) can
not turn, stator (6) does not turn. Stator (6) can deliver oil back to impeller (5) .

As the speed of impeller (5) and turbine (4) increases, stator (6) starts to turn in the same direction as
impeller (5) and turbine (4). When the stator (6) starts to turn, cam (14) starts to turn. The movement
of cam (14) causes rollers (15) to move from the tapers of openings (17). The mechanical connection
between cam (14) and carrier (18) is broken. Stator (6) and cam (14) turn freely. Stator (6) does not
deliver oil back to impeller (5) .
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Torque Converter Inlet Relief Valve

Torque converter inlet relief valve controls the maximum pressure of the oil that is flowing into the
torque converter. Supply oil comes from the torque converter charge pump section. The oil flows
through the torque converter hydraulic filter. Before this oil flows into the torque converter, the oil is
open to torque converter inlet relief valve at inlet passage. Spool will open when the pressure is above
the relief pressure. When spool is shifted, oil will flow through passage to the torque converter sump.
The relief pressure can be adjusted with shims.

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Torque Converter Outlet Relief Valve

Torque converter outlet relief valve controls the minimum pressure of the oil that is inside the torque
converter. Oil exits the torque converter through passage. If the oil is above the pressure setting of the
torque converter outlet relief valve, spool will be shifted. This allows the oil to exit through passage.
Then, the outlet oil goes through the torque converter screen to the remainder of the circuit. The relief
pressure can be adjusted with shims.

PLANETARY POWERSHIFT TRANSMISSION

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One type or transmission used on Caterpillar machines is the Planetary Powershift Transmission. This
topic describes the major planetary powershift transmission components, operation (including
power flow) and the performance testing and troubleshooting procedures.

Understanding planetary gear basics will help to understand how a planetary transmission functions.
This segment explains the basics behind planetary gear sets and how they are used to make a planetary
powershift transmission function.

Planetary gears are used in many ways in Caterpillar machines. Understanding planetary gear
principles will help in understanding the basics of the planetary powershift transmission of which
planetary gear sets are a large part.

Less space will be required in a transmission if planetary gear sets are used instead of gear sets are
used instead of external tooth gears, because all the gears can be inside the ring gear.

External tooth gears rotate in opposite directions, however, the directions of rotation is not changed
with a ring gear. The pinion gear and the ring gear turn in the same direction.

Another advantage of planetary gearing (internal tooth gears) is that they have twice the tooth contact
as external tooth gears. Planetary gears are stronger and have a longer wear life than the external tooth
gears.

To change rotation, a planet gear is put between the pinion and ring gear.

Planet gears turn freely on their own bearings, and the number of teeth does not affect the ratio of the
other two gears. With planetary gear sets there are three or four planet gears that turn on bearings.

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The planet gears (1) are attached to a base or carrier (2) that's called a Planet carrier.

The outer gear (3) is called the Ring Gear.

The pinion gear in the center (4) is called the Sun gear.

The planetary gear set components got their names because they act the same as our solar system. The
planet gears rotate around the sun gear just like the planets in our solar system rotate around our sun.

PLANETARY GEAR SET COMBINATIONS

Speed, direction and torque changes are accomplished by restraining or driving various components of
the planetary gear set. There are many different combinations possible, several examples will be
shown.

To transmit power through a planetary set one member is held, one member is driving, and one
member is driven. The member held is not always the ring gear. In this example the planet carrier is
held to get reverse rotation. If the sun gear is driving in a counterclockwise rotation and the planet
carrier is held, the ring is driven in the opposite directional of the sun gear.

If the sun gear is restrained and the ring gear is the drive gear, then the planet carrier will be driven.
The planet gears rotate about their own axis, driving the planet carrier at a slower speed than the ring
gear.

If the ring gear is restrained and the sun gear is the drive gear, then the planet carrier will be driven.
The planet gears rotate about their own axis, driving the planet carrier at a slower speed than the sun
gear.

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If no member of the gear se is restrained the gears will idle and no power will be transmitted.

If two members are restrained or locked, the results is direct drive. The output speed is the same as the
input speed.

If the planet carrier is the drive gear and the ring gear is restrained, the sun gear will be driven in high
gear.

PLANETARY POWERSHIFT TRANSMISSION ASSEMBLY

We have been study drawings of a very simple planetary powershift transmission to get a basic
understanding of the relationship of planetary gear relationship of planetary gear sets. This picture
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shows an assembled planetary powershift transmission.

This picture shows a two — piece shaft similar to those used in the transmission arrangement we have
been studying.

This (N°.l) shaft is the input shaft. The sun gears of the reverse and forward planetary gear sets are
mounted on the input shaft.

This (N°.2) shaft is the output shaft. The sun gears for the second speed and first speed planetaries are
mounted on the output shaft.

1
2

Let's add some planet gears to each sun gear to build up a basic planetary powershift transmission.
Planetary sets are usually referred to by numbers starting from the input (left) end; they are numbers 1,
2, 3 and 4.

1 2
3
4

This is a typical carrier. Note that the planet gears are supported by large round shafts mounted in the
carrier housing.

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Carriers appear in many shapes and sizes, but all have the same function - to support the planet gear
shafts.

The front carrier of the reverse planetary gear set has been added to this illustration. Half of the carrier
is omitted to show how it is mounted and how it holds the planet gears.

This is a typical center carrier.

The center carrier connects the (N°l) input shaft to the (blue) output shaft. It contains the planet gears
for forward speed and for second speed.
1

All three carriers are mounted in this illustration. From left to right, they are the front carrier, the
center carrier, and the rear carrier.

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In this illustration, the four planetary gear sets have been installed. From the input end (left) they are:
N° 1 (reverse), N° 2 (forward), N° 3 (second) and N° 4 (First).

To make a complete transmission, ring gears and clutches must be added and the entire assembly put
into a protective housing.

This is a picture of the transmission 994F

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ONE PLANETARY GEAR FOR EACH DIRECTION AND SPEED

In the planetary powershift transmission there is a planetary gear set for each transmission speed - a
set for forward and a set for reverse. This drawing shows, assembly into a compact group, four
planetary gear sets.

This drawing is a schematic representations of two — speed, two — direction planetary powershift
transmission. It is ai exploded view of the assembled transmission show in the previous picture.
Engine power is transmitted to the (N°l) input shaft through a torque converter or a torque divider. The
sun gears for both the forward and reverse directions are mounted on the input shaft, and always rotate
when the input shaft is driven. The (N°2) member in the center is the carrier for the second speed set.
The (N°3) output shaft and the sun gears for the second and first speeds are mounted on it.
The arrangement of the planetary gear sets from the engine to the output shaft (left or right) are;
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reverse, forward, second and first

This slide shows the planetary gear sets for forward and reverse - the directional half of the
transmission. Power is transmitted from the engine to the (N°l) input shaft. The ring gear of the
planetary gear set for forward is stopped. This portion of the transmission is now engaged in forward
gear.

The input shaft is driven and because the (N°l) sun gears are mounted on the input shaft, the sun gears
are also driven. The reverse sun gear (the one on the left) is rotating the planet gears. However, no
power is transmisted through the reverse planetary because no member of the planetary is held. The
sun gear of the forward planetary rotates with the input shaft. Therefore, the planet gears rotate in the
opposite direction. Because the ring gear is stopped the planet gears must revolve in the same direction
as sun gear rotation. This is the power flow for the forward direction.

This picture shows the power flow when the planet carrier for the reverse gear planetary is stopped.
The (N°l) input shaft drives the sun gear of the reverse planetary. The sun gear drives the (N°2) planet
gears. Because the planet carrier is stopped the planet gears must rotate in place and drive the ring
gear. Ring gear rotation is now opposite sun gear rotation. The ring gear of the reverse planetary is
fastened to the carrier of the planet gears of the forward planetary.

Therefore, the planet carrier of the forward planetary also rotates in a direction opposite to input gear
rotation.

This is the speed part of the transmission. The planet carrier on the left is part of the planet carrier of
the forward planetary and is driven either clockwise or counterclockwise, depending upon which
planetary gear set (forward or reverse) is transmitting power. In this picture the ring gear of the
planetary for second gear is stopped. Because the planet carrier is rotating and the ring gear is stopped,
the sun gear of the second gear planetary is driven. The sun gear and output shaft rotate in the same
direction as the planet carrier. No member of the first gear planetary is held. Therefore, all components
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are free to rotate and no power is transmitted through the first gear planetary.

For first gear operation, the ring gear of the second gear planetary is released and the ring of the first
gear planetary is stopped. The planet carrier at the left is still driven through the directional half of the
transmission. The load on the output shaft provides resistance to sun gear rotation.

Therefore, the ring gear of the second gear planetary must rotate. This ring gear is fastened to the
planet carrier of the first gear planetary. Because the ring gear of the first gear planetary is stopped the
sun gear is driven. Its rotation is in the same direction as the rotation of the carrier on the left. In
review, the center carrier is driven. It drives the second speed ring gear that is connected to the first
speed planet carrier. Because the first speed ring gear is stopped, the planet gears walk around the
inside of the ring gear and drive the first speed sun gear and the output shaft.

Let's review the basic two — speed planetary powershift transmission we have been studying. It has
two basic groups; a directional group (forward and reverse), and a speed group that determines how
fast the vehicle moves in either direction. Remember the arrangement of the planetary gear sets:
reverse is nearest the engine; then forward; the second gear; and then first gear.

Let's follow the power flow when the ring gear of the forward planetary is stopped. The input shaft
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driven counterclockwise. No member of the reverse planetary is held. Therefore, the reverse planetary
is rotating freely and is not transmitting power. The ring gear of the forward planetary is stopped. The
driven sun gear makes the planet gears rotate clockwise. Because the ring gear is stopped, rotation of
the planet gears causes the gears to revolve around the sun gear in a counterclockwise direction. The
carrier of the planet gear must also rotate counterclockwise. The carrier of the forward planetary is
connected to the carrier of the second speed planetary. There is no power flow through the speed
section of the transmission because no member of the two speed planetaries is stopped, so there is no
power delivered to the output shaft. The speed planetary gears are idling, because nothing is held.

In this picture the only member stopped is the ring gear of the first gear planetary. Trace the power
flow. The input shaft is the driving member of the directional group but no other member of the
directional group is held. Therefore, no member is driven and no power is transmitted through the
directional group. Consequently, stopping the ring gear of the first speed planetary does not cause any
power to be transmitted to the output shaft. The transmission is still in neutral. Both a direction ring
gear and a speed ring gear must be stopped before the transmission delivers power to the output shaft.

In this picture the ring gears of the forward and first speed planetaries are stopped. No power is
transmitted through the reverse planetary because no member is held. The ring gear of the forward
planetary is stopped, and the rotating sun gear causes the planet gears to revolve around the sun gear.
The forward planet gears are mounted on the center carrier, and the center carrier must rotate. The
rotating center carrier drives the ring gear of the second gear planetary. The sun gear of the second
gear planetary is the held member because its rotation is restricted by the load on the output shaft. The
ring gear of the second speed planetary is connected to the carrier of the first speed planetary. Because
the speed ring gear is held the planet gears drive the first speed sun gear and deliver power to the
output shaft. The vehicle moves forward in first speed.

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In this picture, the ring of the forward and second speed planetaries are stopped. Power from the input
shaft causes the forward sun gear to drive the forward planet gears. Because the forward ring gear is
stopped the planet gears revolve around the sun gear and cause the center carrier to rotate. The planet
gears of the second speed planetary are mounted on the center carrier and must revolve around the sun
gear of the second speed planetary. Because the ring gear of the second speed planetary is stopped the
rotation of the planet gears drives the sun gear. Power is transmitted to the output shaft.

The combination here is reverse and second speed. The carrier in the reverse planetary and the ring
gear of the second speed planetary are held. To transmit power through a planetary set one member is
held, one member is a driving member, and one member is driven. The member held is not always the
ring gear. As show here, the carrier of the reverse planetary is held to get reverse rotation. Any
member of one planetary set may be connected to any member of a second planetary set. For example,
locking up a ring gear of one set can lock up the carrier of a second planetary. In this picture input
shaft rotation is counterclockwise. Rotation of the input shaft and reverse sun gear make the planet
gears rotate clockwise. Because the planet carrier is stopped the planet gears cannot revolve around the
sun gear. Therefore, the ring gear must rotate in a clockwise direction. Because the center carrier is
connected to the ring gear of the reverse planetary it also has clockwise rotation opposite to the
rotation of the input shaft.

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For first speed reverse operation the carrier of the reverse planetary and the ring of the first gear
planetary are held. Remember that in reverse the carrier is held and the planet gears rotate and drive
the reverse ring gear. The reverse ring gear causes the center carrier to rotate and drive the second
speed ring gear. The second speed ring gear connects t the carrier of the first speed planetary set and
the first speed planetary gears must revolve around the first speed sun gear. Because the first speed
ring gear is held the planet gears revolve around the inside of the ring gear and drive the first speed
sun gear and the output shaft.

PLANETARIES

Planetary gear sets are the heart of the planetary powershift transmission. Planetaries are made up of a
member of different gears that rotate around or inside each other like a miniature solar system. The
main components of a planetary gear set are the ring gear, planet (idle) gears, planet carrier and sun
gear.

sun planet
carrier
ring
gear

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CLUTCHES

The transmission clutches are the disc — type and are housed in separate housings. Each clutch has a
number of discs and plates. The inside teeth of the discs are engaged with the outside teeth of the ring
gear. Notches on the outside diameter of the plates are engaged with pins in the clutch housing. The
pins keep the plates from rotating.

Springs are located between the clutch housing aid the piston. The springs keep the clutches
disengaged (not engaged) by keeping the clutch piston from pushing against the plates. The clutches
engage when oil is sent into the area behind the piston. When the pressure of the oil in the area behind
the piston increases, the piston moves to the right against the force of the spring and pushes the discs
and plates together. The clutch is now engaged and the ring gear is held stationary. When the pressure
of the oil holding the piston decreases, the spring forces the piston back into the housing releasing the
discs and plates. The ring gear is no longer held and rotates freely

The clutch plates are mounted inside the clutch housing. Notches on the outside diameter of the plates
are engaged with pins in the clutch housing that keep the plates from rotating.

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The clutch discs are fitted to the ring gear and rotate with the gear. The inside teeth of the discs are
engaged with the outside teeth of the ring gear. Caterpillar clutch discs are made of special friction
materials based on the requirements of the application.

Each clutch in the transmission has its own housing. The housing provides the clutch witch a place to
hold the piston that engages the clutch using hydraulic pressure. The housing also retains the clutch
plates and using pins prevents the plates from turning.

COUNTER-SHAFT POWERSHIFT TRANSMISSION

One type of transmission used on Caterpillar machines is the Counter-Shaft Powershift Transmission.
This topic describes the major counter-shift powershift transmission components, operation (including
power flow) and the performance testing and troubleshooting procedures.

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Counter-Shaft transmissions differ from planetary transmissions in that counter-shaft

transmissions use constant mesh spur gears. The transmission does not have sliding collars. Speed and
directional shifts are accomplished by hydraulically engaging various clutch packs

Advantages of this transmission include: fewer parts, less weigh and controlled failure mode
production. Electrically operate shift solenoids provide automatic shifting while eliminating the cable
control linkage from the operators station to the transmission control valve.

CLUTCHES

The clutches are engaged hydraulically and disengaged by spring force. The clutches are engaged in a
manner that provides the proper speed reduction and direction to the transmission output shaft.

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The clutch piston has an inner and outer seal. When the discs have worm one half oil groove depth, the
clutch piston travels far enough to unseat low) the outer seal. This prevents the discs and plates from
ever running metal to metal. Speed or directional clutch pressure fills the cavity behind the clutch
piston and moves the piston to the left against the piston spring and engages the clutch discs and
plates.
The clutch plates are mounted inside the clutch housing. Splines on the outside diameter of the plates
are engaged with splines in the clutch housing, Both the plates and the housing rotate together. The
clutch discs are stacked between the clutch plates. The inside teeth of the discs are engaged with the
outside teeth of the hub. The clutch discs have a friction material bonded to their surface so there is no
metal to metal contact between the clutch discs and the clutch plates.

The hub is the component in the clutch pack that the gear is splined to. When the clutch piston engages
the clutch plates and discs power is transferred through the hub to the gear.

The transmission shafts carry the gears inside the transmission. The number of shafts and gears is
determined by transmission and machine model.

Each of the transmission shafts have three internal oil passages. One passage is for carrying the oil for
the lubrication and cooling of the clutches, bearings and gears. The other two passages are for carrying
pressure oil for the engagement of the clutches on each shaft.

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14M Motor Grader

This illustration shows the power flow through the countershaft transmission. The countershaft
transmission provides eight forward speeds and six reverse speeds. The transmission contains eight
clutches which are engaged hydraulically and released by spring force. The input shaft is driven by the
flywheel of the engine. Also shown is the parking brake.

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In FOURTH GEAR FORWARD the forward high directional clutch engaged and the third gear
clutch is engaged. Power is transmitted from a gear on the input shaft to a gear on the forward
low/high shaft. The gear in the middle of the forward low/high shaft drives a gear on the
reverse/second shaft.

When the third gear clutch is engaged, the gear on the end of the third/first shaft is held. Power is
transferred from the gear on the second/reverse shaft to the held gear. The gear on the other end of the
third/first shaft transfers power to the gear on the output shaft.

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Transmission Control Electronic (TCE) Hydraulic Valve

The transmission hydraulic control valve group (1) is bolted to the top of the transmission planetary
clutch group. Shown are five clutch solenoids (2), the P3 pressure tap (3), the solenoid electrical
harness (4), the P1 pressure tap (5), the P2 pressure tap (6), and the plug (7) for the load piston.

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Also included in the transmission hydraulic controls are:

Modulation relief valve: Limits the maximum clutch pressure.

First and third speed selection spool: Directs oil flow to the No. 5 and No. 3 clutches.

Load piston: Works with the modulation relief valve to control the rate of pressure increase in the
clutches.

Second speed selector spool: Directs oil flow to the No. 4 clutch.

Pressure differential valve: Controls speed and directional clutch sequencing.

Directional selection spool: Directs oil to the FORWARD and REVERSE directional clutches.

Converter inlet ratio valve: Limits the pressure to the torque converter.

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Power Train Hydraulic Schematics

In this schematic, the engine is running and the transmission is in NEUTRAL.

When the operator moves the directional switch to the NEUTRAL position, the ECM energizes the
No. 3 clutch solenoid and the impeller clutch solenoid. The ECM also de-energizes the lockup clutch
solenoid.

Flow from the power train pump is sent through the transmission filter to the priority valve, the
impeller clutch solenoid valve, and the lockup clutch solenoid valve. The priority valve maintains a
minimum oil pressure to the impeller clutch solenoid valve and the lockup clutch solenoid valve
during transmission shifts.

When the power train pump supply pressure increases above the priority valve setting, the priority
valve opens and sends oil flow to the manifold for clutch solenoid valves No. 2 and 3, the manifold for
clutch solenoid valves No. 1, 5, and 4, and the inlet passage for the selector and pressure control
valves.

The oil at the clutch solenoid valve manifolds become the pilot oil for the transmission speed
and directional selector spools.

When the No. 3 clutch solenoid is ENERGIZED, the No. 3 clutch solenoid valve sends pilot oil to one
end of the selector spool for speed clutches No. 3 and 5. The pilot oil pressure overcomes the force of
the selector valve spring and moves the spool from its center position. Oil from the inlet passage flows
through the orifice, past the selector spool for speed clutches No. 3 and 5, and into the No. 3 speed
clutch.

When directional solenoids No. 1 and 2 are DE-ENERGIZED, pilot oil is blocked at the directional
solenoid valves. The directional clutch selector spool spring centers the valve. Oil flow from the
differential valve to the directional clutches is blocked.

When the oil requirements of the selector and pressure control valve have been satisfied, the remaining
power train pump oil flows to the torque converter.

Flow from the power train pump is sent to the torque converter filter. Oil flows from the filter and
joins with the oil from the selector and pressure control valve. The combined oil flows to the torque
converter. Flow continues through the torque converter to the torque converter outlet relief valve. The
torque converter outlet relief valve maintains the pressure in the torque converter. From the outlet
relief valve, flow continues through the cooler to the transmission lubrication circuit.

When the transmission is in NEUTRAL, the ECM de-energizes the optional lockup clutch solenoid.
When the lockup clutch solenoid is de-energized, the lockup clutch solenoid valve closes. The closed
valve blocks pump flow to the lockup clutch and allows the lockup clutch oil to flow to the tank. The
lockup clutch releases and disconnects the turbine from the rotating housing. No power is transmitted
through the turbine from the housing.

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FIRST SPEED FORWARD

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When the transmission is shifted to FIRST SPEED FORWARD, the ECM energizes clutch solenoids
No. 2 and 5. The ECM also de-energizes the impeller clutch solenoid and the lockup clutch solenoid.

When the ECM energizes the No. 2 clutch solenoid, the No. 2 clutch solenoid valve sends pilot oil to
one end of the directional clutch selector spool for directional clutches No. 1 and 2. Oil pressure
overcomes the force of the selector valve spring and moves the spool from its center position.
Directional clutch oil flows from the pressure differential valve, past the directional clutch selector
spool, and into the FORWARD directional clutch (No. 2).

When the ECM energizes the No. 5 solenoid, the No. 5 clutch solenoid valve sends pilot oil to one end
of the selector spool for speed clutches No. 3 and 5. The pilot oil pressure overcomes the force of the
selector valve spring and moves the spool from its center position. Oil from the inlet passage flows
through the orifice, past the selector spool for speed clutches No. 3 and 5, and into the No. 5 speed
clutch.

When the impeller clutch solenoid de-energizes, the impeller clutch solenoid valve fully opens. Power
train pump oil flows through the fully open valve and pressurizes the impeller clutch. The impeller
clutch locks the torque converter impeller to the torque converter rotating housing. The torque
converter housing is splined to the engine flywheel. The impeller and torque converter housing rotate
with the engine flywheel.

The torque converter outlet relief valve (1) is bolted to the left side of the torque converter (facing
the engine flywheel). The torque converter outlet relief valve limits the pressure in the torque
converter Torque converter outlet oil pressure can be checked at the pressure tap (2) on the outlet
valve.

The torque converter temperature sensor (3) sends a signal to the VIMS ECM indicating torque
converter oil temperature.

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The lockup clutch solenoid valve (1) and the impeller clutch solenoid valve (2) are mounted on the left
side of the torque converter housing. A cover plate is mounted on the torque converter housing in
place of the lockup clutch valve on machines not equipped with a lockup clutch.

The ECM energizes the lockup clutch solenoid to allow oil to flow through the lockup clutch valve to
the lockup clutch. The pressure increases in the lockup clutch, causing it to engage and the machine
operates in DIRECT DRIVE.

The lockup clutch solenoid is a proportional solenoid and is energized by a modulated signal from the
Power Train ECM. The Power Train ECM varies the amount of current to control the amount of oil
flow through the lockup clutch valve to the lockup clutch.

The Power Train ECM monitors the status of the impeller clutch solenoid and can determine certain
faults that may affect operation of the impeller clutch. These faults include: a short to +Battery, a short
to ground, or an open circuit.

When the Power Train ECM detects a fault in the impeller clutch solenoid circuit, a fault will be
displayed on the VIMS display panel.

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Individual Clutch Modulation (ICM) transmission hydraulic control valve

Rotary Actuator

Selector Valve Group

Output
Speed
Sensor

Input Transfer Planetary Group

Gears And Clutch

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Upshift Solenoid

Downshift Solenoid

Lockup Solenoid

Shown is the Individual Clutch Modulation (ICM) transmission hydraulic control valve. Transmission
clutch pressures are measured at the pressure taps (1).

The transmission hydraulic control valve contains a priority valve. The priority valve controls the
pressure that is directed to the selector pistons in each of the clutch stations. The transmission priority
valve pressure is adjusted to obtain a pump supply pressure of 2310 ± 70 kPa (335 ± 10 psi) at 1300
rpm while in DIRECT DRIVE. A pilot pressure between 2410 to 2755 kPa (350 to 400 psi) in
CONVERTER DRIVE will result from this adjustment. Pilot pressure is measured at plug (2).

The "D" Station (3) is used to control the dual stage relief valve setting for the clutch supply pressure.
In DIRECT DRIVE, clutch supply pressure is reduced to extend the life of the transmission clutch
seals. In DIRECT DRIVE, clutch supply pressure should be 1620 ± 70 kPa (235 ± 10 psi). The
corresponding transmission charge pressure is 2310 ± 70 kPa (335 ± 10 psi).

The transmission lube pressure relief valve (4) limits the maximum pressure in the transmission lube
circuit. The lubrication oil is used to cool and lubricate all of the gears, bearings, and clutches in the
transmission and transfer gears.

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Pressure Control Selector Valve Group

Valve Group

Distribution Manifold Rotary Actuator

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 (1),
selector valve group (2), distribution manifold (3), and the rotary actuator (4).

The pressure control 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, G, and H). In the larger trucks,
the H station is not used. The smaller trucks have seven speeds FORWARD and the larger trucks have
six speeds FORWARD. The D station in the larger trucks is used to lower the maximum pressure in
the system during operation in direct drive.

Valve stations individually control the engagement and maximum pressure for each of the clutches in
the transmission.

In the valve stack, the selector valve group is directly below the pressure control valve group. The
selector valve group has four functions:

1. In the small trucks, a single stage relief valve limits the maximum system pressure in torque
converter drive and direct drive. In the larger trucks, 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 neutralizer valve permits pilot oil to flow to the rotary selector spool when the engine is
started with the rotary selector spool in the NEUTRAL position.
4. A priority reducing valve controls the pilot pressure and makes sure that pilot oil is available
at the neutralizer valve 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
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the downshift solenoid causes the vane to rotate in the opposite direction. The vane is connected to and
causes rotation of the rotary selector spool inside the selector valve group.

In this illustration and those that follow, the colors used to identify the various pressures in the
systems are as follows:

 Red - Oil supply pressure

 Red and White Stripes - Clutch pressure
 Green - Drain or reservoir oil
 Brown - Lubrication or cooling pressure
 Orange - Torque converter pressure.

This view shows the selector valve group. The rotary actuator (1) is bolted to the top of the selector
group. The rotating vane in the rotary actuator is directly connected to the rotary selector spool. At the
opposite end of the rotary selector spool is a pair of detent springs (2). Rollers on the detent springs
engage with a detent cam which is pinned to the end of the rotary selector spool. Some of the oil
passages which connect the various chambers are machined in the top of the valve body.

The small cover (3) at the upper left corner of the valve body permits access to the priority reduction
valve. To the right of the small cover is a larger cover (4) which permits access to the neutralizer
valve.

Removal of the threaded cap (5) from the lower left corner of the valve body permits access to the
lube relief valve.

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Rotary actuator

Shown is the rotary actuator after the removal from the selector valve body. Inside the actuator
housing are a stationary vane (1) and a rotating vane (2). The stationary vane is bolted to the
actuator housing and the rotating vane is pinned to the rotary selector spool in the center of the
housing. Pressure oil from the solenoid valves causes the rotating vane and rotary selector spool to
turn in either the clockwise or counterclockwise direction.

Two oil ports are located on the outside of the actuator housing. The upper port (3) is for pressure oil
from the downshift solenoid and the lower port (4) is for pressure oil from the upshift solenoid. In
the respective oil passages to the rotary vane are a downshift (5) and upshift (6) valve (check valves)
that cover and uncover a drain passage.

Note the position of the plugs that limit the travel of the two valves: The downshift valve plug (7) has
a short rod on the plug and the upshift valve has a standard plug (8).

This view of the rotary actuator vane shows the vane in the NEUTRAL position (fully
counterclockwise). To shift from NEUTRAL to any other gear, the rotating vane must turn in the
clockwise direction to the selected gear position. When the shift is indicated, pressure oil from the
upshift solenoid is sent to the lower inlet port (4). The pressure 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 fills the small space
between the two vanes.

As the pressure increases, the rotating vane moves in the clockwise direction to the appropriate gear
position. Any oil that was in the chamber on the nonpressurized (downshift) side of the vane is forced
out of the chamber by the movement of the vane.

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As 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 permits 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).

The clockwise sequence for upshifts at the rotary actuator in the 769C and 773B trucks is N - R - 1 - 2
- 3 - 4 - 5 - 6 - 7. The upshift sequence in the 777C to 793B trucks is N1 - N2 - R - 1 - 2 - 3 - 4 - 5 - 6 -
7 (785B and 793B have only six speeds FORWARD).

In the 777C, as the rotary actuator moves the rotary spool from N1 to N2 past REVERSE to FIRST,
the N1 clutch remains engaged in the N2 position and then changes to another clutch in FIRST.

In the 785B and 793B trucks, the N1 clutch drops out and in N2 another clutch is engaged
momentarily to slow the rotation of the internal mass of the planetaries. When the rotary spool gets to
FIRST, the N2 clutch stays engaged and another clutch is engaged to complete the power flow path to
the wheels.

This close view shows the end of the selector valve group opposite the rotary actuator. Visible in this
view are the detent cam (1) and the detent springs (2). The detent cam is installed in the end of the
rotary selector spool and turns with the spool. A bolt and flat washer (3) help to retain the rotary
selector spool in the valve body.

The detent springs are bolted to a support block (4) which is part of the access cover for the main
relief valve. Pins in the support block and flat retainers help to align the detent springs and hold them
in position.

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NOTE: The support block (4) is notched where the bolts mount the block to the housing to allow
adjustment of the detent springs.

Rotary selector spool

The rotary selector spool is actually a hollow rotating shaft. A plug and screen assembly inside the
spool divides the center cavity into two separate oil chambers.

During operation, pilot oil from the upper chamber (indicated by the red arrow) is directed to the
pressure control valve group to initiate clutch engagement. For any gear except NEUTRAL (N1 and
N2), two of the outlet ports from the upper chamber are aligned with drilled passages in the selector
valve body. For N1 and N2, only one outlet port permits pilot oil to flow to the pressure control valve
group.

The lower chamber in the rotary selector spool is always open to drain (as indicated by the green
arrow). For each gear position except N1 and N2, all but two of the drain ports are open to drain.
Whenever a clutch station is engaged, the lower half of the spool blocks the drain passage to that
station.

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Removal of the pin and dent cam from the end of the rotary selector spool permits access to the plug
and screen assembly. The screen prevents plugging or restriction of the outlet ports from the upper
chamber by removing particles of dirt or foreign material from the pilot oil. The screen should be
inspected and cleaned whenever the selector valve group is disassembled.

In this view, the rotary selector spool has been cut in half to show how the plug and screen assembly
divides the center cavity into two separate oil chambers. Also, notice how the screen fits inside the
upper chamber (on the right). Pilot oil enters the upper chamber through the passage at the end of the
screen. The oil then flows into the open end of the screen and through the screen mesh to the outlet
ports.

This schematic shows the 769 - 777 selector valve group during operation in NEUTRAL. The priority
reducing valve is installed in the bore on the left side of the valve body. This valve has two functions:
It controls the pressure of the pilot oil (orange) that is used to initiate clutch engagement, and it makes
sure that pilot pressure is available at the neutralizer valve before pressure oil (red) is sent to the
remainder of the system.

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The neutralizer valve moves only when the rotary selector spool is in the NEUTRAL position. When
the rotary selector spool is in the NEUTRAL position and the engine is started, pump oil flows
through a passage in the center of the neutralizer valve, flows up around the check ball, pressurizes the
top of the valve, and then moves down. In this position, the neutralizer valve directs pilot oil to the
center of the rotary selector spool.

If the rotary selector spool is not in the NEUTRAL position during engine start-up, the neutralizer
valve will block the flow of pilot oil to the rotary selector spool.

Directly below the neutralizer valve is the main relief valve. This valve limits the maximum system
pressure during operation. Excess pump oil is directed to the lubrication circuit and the pressure is
maintained by the lube relief valve.

The rotary selector spool is installed in the bore on the right side of the valve body.

In NEUTRAL, the rotary selector spool directs pilot oil to one station for the engagement of only one
clutch. Check the appropriate service manual for which stations control specific clutches.

The 785 - 793 trucks have a slightly different selector valve group. The main relief valve is now a dual
stage relief valve and has two functions: It limits the maximum system pressure during converter drive
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and provides a reduced system pressure for direct drive. The reduced pressure in direct drive is
controlled by valve station D in the pressure control valve group.

The lube relief valve is replaced by a plug.

Just above and to the right of the neutralizer valve is a shuttle valve. When the lockup solenoid is
energized, oil from the solenoid to valve station D must flow through the shuttle valve. Each time the
lockup solenoid is de-energized, pump oil pressure (red) stops and the oil in station D can quickly
flow to drain through the uncovered drain passage opened by the shuttle valve.

The pressure control valve group contains seven modulation reduction valves. The identification
letters for the valve stations are stamped on the top cover plate and on the ends of the seven outer
covers (stations).

This view shows a typical pressure control valve group. The stations are as follows:

1. A
2. B
3. C
4. D (Plugged on the small trucks. On the larger trucks, station D is the dual stage relief valve.)
5. E
6. F
7. G
8. H

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769C - 777C PRESSURE CONTROL GROUP NEUTRAL

This schematic shows the pressure control group in NEUTRAL. Notice that pressure oil is available at
all of the modulation reducing valves, but only the station C 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 station C. Both the selector piston and the load piston were moved
by pilot oil pressure 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.
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785 - 793 PRESSURE CONTROL GROUP THIRD SPEED FORWARD

This schematic shows the components and the oil flow in the pressure control valve during operation
in THIRD SPEED FORWARD in the large trucks. Stations C and E control the operation of their
respective clutches. Valve movement in stations C and E is initiated by pilot oil from the rotary
selector spool, while valve movement in station D is initiated by pressure oil from the lockup solenoid.
Because station D does not contain a load piston orifice and a decay orifice, the pressure rise and
pressure drop controlled by this station occur very rapidly.

The overall design of the pressure control valve group permits a relatively easy check of the clutch
pressures. Since each clutch has its own modulation reduction valve, a pressure gauge connected at
each station will show the clutch pressure for each gear range as the transmission is manually
upshifted and downshifted.

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This schematic shows the conditions in the system with the ENGINE STARTED and thetransmission
in NEUTRAL. The priority reduction valve has three functions: First, It controls the pressure of the
pilot oil (orange) that is used to initiate clutch engagement. Second, it makes sure that pilot pressure is
available at the neutralizer valve before pressure oil (red) is sent to the remainder of the system. Third,
it is adjusted to obtain a pump supply pressure of 2310 ± 70 kPa (335 ± 10 psi) in DIRECT DRIVE. A
pilot pressure between 2410 to 2755 kPa (350 to 400 psi) in CONVERTER DRIVE will result from
this adjustment.

The neutralizer valve moves only when the rotary selector spool is in the NEUTRAL position. When
the rotary selector spool is in the NEUTRAL position and the engine is started, pump oil flows
through a passage in the center of the neutralizer valve, flows up around the check ball, pressurizes the
top of the valve, and then moves down. In this position, the neutralizer valve directs pilot oil to the
center of the rotary selector spool. If the rotary selector spool is not in the NEUTRAL position during
engine start-up, the neutralizer valve will block the flow of pilot oil to the rotary selector spool.

Directly below the neutralizer valve is the main relief valve. This valve limits the maximum system
pressure. The main relief valve is adjusted to obtain the following pressures in CONVERTER DRIVE
only:

Low Idle: > 2515 kPa (365 psi) High Idle: < 3065 kPa (445 psi)

The lube supply pressure is limited by the lube relief valve. The lubrication oil is used to cool and
lubricate all of the gears, bearings, and clutches in the transmission and transfer gears. 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 opposite direction. The vane is connected to and causes
rotation of the rotary selector spool inside the selector valve group.

Oil flows from the charging pump, through the charging filter, and is sent directly to the three
solenoids and the selector valve group. Pump flow is blocked at the upshift and lockup solenoid and,
because the downshift solenoid is continuously energized in NEUTRAL, the valve in the solenoid is
open. This condition permits oil to flow to the rotary actuator. Pressure on the downshift side of the
rotating vane in the rotary actuator keeps the vane and the rotary selector spool in the NEUTRAL
position until a shift is made.

The rotary selector spool is actually a hollow rotating shaft. A plug and screen assembly inside the
spool divides the center cavity into two separate oil chambers.

During operation, pilot oil from the upper chamber is directed to the pressure control valve group to
initiate clutch engagement. For any gear except NEUTRAL, two of the outlet ports from the upper
chamber are aligned with drilled passages in the selector valve body. For NEUTRAL, only one outlet
port permits pilot oil to flow to the pressure control valve group. The lower chamber in the rotary
selector spool is always open to drain. For each gear position except NEUTRAL, all but two of the
drain ports are open to drain. Whenever a clutch station is engaged, the lower half of the spool blocks
the drain passage to that station.

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This schematic shows the components and the oil flow in the system during operation in FIRST
GEAR DIRECT DRIVE. The upshift solenoid is energized and directs pump oil to the rotary actuator.
The rotary actuator moves the rotary selector spool to the desired gear position and the upshift
solenoid is de-energized. The rotary spool selects two stations (B and F) which modulate the two
clutches.

To shift from NEUTRAL to any other gear, the rotating vane must turn in the clockwise direction to
the selected gear position. When the shift is indicated, pressure oil from the upshift solenoid is sent to
the lower inlet port. The pressure 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 fills the small space between the two vanes.

As the pressure increases, the rotating vane moves in the clockwise direction to the appropriate gear
position. Any oil that was in the chamber on the nonpressurized (downshift) side of the vane is forced
out of the chamber by the movement of the vane.

As 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 permits the oil to flow out of the center chamber. The check valve closes and
prevents oil from flowing to the other solenoid.

This sequence is just the opposite for downshifts (when the rotating vane moves in the
counterclockwise direction).

The transmission control group uses a dual stage relief valve for clutch supply pressure. The "D"
Station is used to control the dual stage relief valve setting for the clutch supply pressure. In DIRECT
DRIVE, clutch supply pressure is reduced to extend the life of the transmission clutch seals.

The rotary selector spool is in a position that engages two clutches. Pump supply oil from the lockup
solenoid flows through a check valve to the selector piston in station "D." Station "D" reduces the
clutch supply pressure, and the reduced pressure flows to the lower end of the relief valve. Providing
oil pressure to the lower end of the relief valve reduces the clutch supply pressure. Station "D" should
be adjusted to obtain a DIRECT DRIVE clutch supply pressure of 1620 ± 70 kPa (235 ± 10 psi) when
engine speed is 1300 rpm.

NOTE: To engage the lockup clutch and put the torque converter in DIRECT DRIVE, use the
following procedure:

1. Label and disconnect the harness connectors from the upshift, downshift, and lockup solenoids.

2. Put a gauge on the pressure tap for station "C" (No. 3 clutch).

3. Make sure the wheels are blocked, the parking brake is ENGAGED, and the transmission is in
NEUTRAL. Start the engine.

4. In NEUTRAL, the downshift solenoid receives +Battery voltage from the

Transmission/Chassis ECM. Connect the downshift solenoid harness to the lockup solenoid
and the lockup clutch will ENGAGE.

5. Increase the engine speed to 1300 rpm and read the pressure on the gauge.
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The 793D truck transmission control and torque converter lockup pressure settings require that
pressures are set in the correct sequence. Use the recommended pressure adjustment sequence below:

1. CONVERTER DRIVE Pump Pressure: Adjust the main relief valve to obtain the following
pressures in CONVERTER DRIVE only. Low Idle: > 2515 kPa (365 psi) High Idle: < 3065
kPa (445 psi). Measure CONVERTER DRIVE Pump Pressure at the pressure tap on the
solenoid manifold (See Visual No. 121).

2. Clutch Supply Rail Pressure: Adjust Station "D" to obtain a DIRECT DRIVE clutch supply
pressure of 1620 ± 70 kPa (235 ± 10 psi) at 1300 rpm. Measure Clutch Supply Rail Pressure at
Clutch No. 3 (station C) while in NEUTRAL and DIRECT DRIVE.

3. DIRECT DRIVE Pump Pressure: Adjust the Priority Reducing Valve to obtain a DIRECT
DRIVE Pump Pressure of 2310 ± 70 kPa (335 ± 10 psi). Measure DIRECT DRIVE Pump
Pressure at the pressure tap on the solenoid manifold (See Visual No. 121). A pilot pressure
between 2410 to 2755 kPa (350 to 400 psi) in CONVERTER DRIVE will result from this
adjustment.

4. Lockup Clutch Pilot (RV) Pressure: Adjust the Lockup Clutch Pilot Pressure to obtain 1725 ±
70 kPa (250 ± 10 psi). Measure the pressure at the plug labeled "RV" on the torque converter
lockup valve.

5. Lockup Clutch Primary Pressure: Adjust the Lockup Clutch Primary Pressure to obtain 1030 ±
35 kPa (150 ± 5 psi). Measure the pressure at the pressure tap on the torque converter lockup
valve (See Visual No. 119). A Lockup Clutch Pressure of 2150 to 2350 kPa (310 to 340 psi) at
1300 rpm should result from this adjustment.

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Since the six valve stations that directly control clutches contain the same basic components, an
explanation of the operation of one station can be applied to the operation of the remaining five
stations. Station "D" is different.

The six stations that control the clutches contain load piston orifices (sometimes called "cascade"
orifices). The load piston orifices control the clutch modulation. The thicker the orifice, the slower the
modulation. The retaining springs for the load piston orifices are identical, but the orifices vary in
thickness from one station to another. Many of the stations are equipped with decay orifices. Check
the parts book for proper component placement.

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 modulation
reduction 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.

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This schematic shows the relative positions of the valve station components during clutch fill
(movement of the clutch piston to make contact with the discs and plates). Valve movement is
initiated when pilot oil from the rotary selector spool moves the selector piston to the left as shown.
Movement of the selector piston accomplishes two purposes:

1. The drain passage at the decay orifice is blocked.

2. The load piston springs are compressed.

Compressing the load piston springs moves the reduction valve spool to the left against the forceof the
return spring. This movement opens the supply passage 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 reduction 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. While the clutch is filling, the
pressure in the chamber between the end of the load piston and the selector piston is not high enough
to move the load piston inside the selector piston.

During clutch modulation, clutch pressure increases. After the clutch fills (the clutch piston has moved
against the discs and plates), the pressure in the clutch, in the slug chamber, and in the passage to the
load piston orifice starts to increase. When the pressure in the chamber reaches primary pressure, the
load piston starts to move inside the selector piston.

The load piston orifice controls the flow of oil to the load piston chamber. This condition 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.

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 Power Train

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 reduction valve 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 farther to the left. This movement
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 reduction valve spool moves
left and right while the load piston moves smoothly to the left.

The load piston has now moved completely to the left against the stop. The modulation cycle is
completed and the clutch pressure is at its maximum setting. The position of the two-stage relief valve
affects clutch maximum pressure. If the two-stage relief valve is at high relief (CONVERTER
DRIVE), the clutch supply pressure is high.

At the end of the modulation cycle, the modulation reduction valve controls clutch pressure, which
will be lower than the clutch supply pressure. The pressure in the slug chamber moves the reduction
valve a small distance to the right to restrict the flow of supply oil to the clutch. This is the "metering
position" of the reduction valve spool. In this position, the modulation reduction valve maintains
precise control of the clutch pressure.

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 Power Train

If the two-stage relief valve is at low relief (DIRECT DRIVE), the clutch supply pressure is lower
than the pressure which the modulation reduction valve is trying to maintain. The supply oil
connection to the clutch is not restricted, and the clutch pressure is the same as clutch supply pressure.

During operation, an engaged clutch is designed to leak a relatively small but steady volume of oil. 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 reduction 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. Then, the clutch 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.

During a shift, the pressure of the clutch (or clutches) being released does not immediately drop to
zero. Instead, the clutch pressure decreases at a controlled rate. Restricting the rate of clutch pressure
decay helps to maintain a positive torque at the transmission output shaft. This feature 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 a shift.

When a clutch is released, 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 condition 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.

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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.

The transmission charging pump supplies oil to the torque converter lockup clutch valve through the
inlet port (1). When the lockup clutch solenoid (located on the transmission housing) is energized by
the transmission control, signal oil flows though hose (2) and begins the sequence to ENGAGE the
lockup clutch in the torque converter.

Torque converter lockup clutch pressure can be measured at the tap (3). Torque converter lockup
clutch pressure should be 2150 to 2350 kPa (310 to 340 psi) at 1300 rpm. To test the lockup clutch
pressure, use the following procedure:

1. Label and disconnect the harness connectors from the upshift, downshift, and lockup
solenoids.
2. Make sure the wheels are blocked, the parking brake is ENGAGED, and the transmission
is in NEUTRAL. Start the engine.
3. In NEUTRAL, the downshift solenoid receives +Battery voltage from the
Transmission/Chassis ECM. Connect the downshift solenoid harness to the lockup solenoid
and the lockup clutch will ENGAGE.
4. Increase the engine speed to 1300 rpm and read the pressure on the gauge.

Do not adjust the lockup clutch maximum pressure. If the lockup clutch maximum pressure is not
correct, verify that the lockup clutch primary pressure is correct. If the lockup clutch primary pressure
is correct, check for loose or sticking components or debris in the valve. If these components are not
the problem, change the load piston springs. If the load piston springs are replaced, be sure to reset the
lockup clutch primary pressure.

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The Converter Output Speed (COS) sensor (4) sends an input signal to the Transmission/Chassis
ECM. The Transmission/Chassis ECM memory also contains the engine speed and the Transmission
Output Speed (TOS). The Transmission/Chassis ECM uses engine speed and COS to calculate lockup
clutch shift times. It uses COS, TOS, and the ratio for the gear being engaged to calculate transmission
shift times. The Transmission/ Chassis ECM provides the shift time information to VIMS.

Shown is a sectional view of the torque converter lockup clutch valve in DIRECT DRIVE. Supply oil
from the transmission charging pump is used to provide pilot pressure, signal pressure, primary
pressure, and lockup clutch pressure.

First, supply pressure is reduced to provide pilot (RV) pressure. Supply oil to the pilot Reducing Valve
(RV) flows through cross-drilled orifices in the spool, past a check valve, and enters the slug chamber.
The check valve dampens spool movement and reduces the possibility of valve chatter and pressure
fluctuation. Oil pressure moves the slug in the right end of the spool to the right and the spool moves
to the left against the spring force. The spring force and the force due to the pressure in the slug cavity
balance, and oil is metered into the pilot oil pressure passage.

The spring force can be adjusted with shims to control pilot (RV) pressure. Pilot (RV) pressure is 1725
± 70 kPa (250 ± 10 psi).

The lockup solenoid is energized and directs pump supply (signal) pressure to the relay valve. The
signal oil pressure moves the spool in the relay valve and flows to the inlet port of the transmission
lube pump. Since the signal oil flow is restricted, the signal pressure measured at the relay valve will
be less than pump pressure. When the relay valve spool is moved by the signal oil pressure, pilot oil
flows to a shuttle valve. Pilot oil moves the shuttle valve to the right which closes the drain and opens
the check valve. Pilot oil then flows to the selector piston. Moving the selector piston blocks a drain
passage and compresses the load piston springs.
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After the clutch fills, but the load piston is still at the top against the selector piston, lockup clutch
pressure is at its lowest controlled value. This value is called "primary pressure." Primary pressure is
1030 ± 35 kPa (150 ± 5 psi). Primary pressure is adjusted with shims in the load piston after the load
piston plug is removed.

When the selector piston moves down, the load piston also moves down and compresses the load
piston springs and moves the modulation reduction valve spool down against the force of the return
spring. This initial movement opens the supply passage (from the transmission charge 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 bottom of the reduction 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. While the clutch is filling, the pressure in the chamber is not high enough to move the
load piston inside the selector piston. After the clutch fills, the load piston orifice helps control the rate
of modulation.

At the end of modulation, the load piston has moved completely down against the stop and the clutch
pressure is at its maximum setting. Because this is a modulation reduction valve, the maximum
pressure setting of the clutch is lower than the transmission charge pressure. At the end of the
modulation cycle, the pressure in the slug chamber moves the reduction valve a small distance up to
restrict the flow of supply oil to the clutch. This is the "metering position" of the reduction valve
spool. In this position, the valve maintains precise control of the clutch pressure. The lockup clutch
pressure is 2150 to 2350 kPa (310 to 340 psi) at 1300 rpm.

Do not adjust lockup clutch final pressure. If the primary pressure is correct and final lockup clutch
pressure is incorrect, check for loose or sticking components or debris in the valve. If these
components are not the problem, change the load piston springs. If the load piston springs are
replaced, be sure to reset the lockup clutch primary pressure.

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The transmission charging pump supplies oil to the transmission hydraulic control valve and the shift
solenoids through the inlet port (1). Transmission charging oil not used to fill the clutches flows to the
torque converter inlet relief valve through the outlet hose (2).

The torque converter lockup clutch solenoid (3) is energized by the Transmission/Chassis ECM when
DIRECT DRIVE (lockup clutch ENGAGED) is required. Transmission charge pump supply (signal)
oil flows through the small hose (4) to the lockup clutch relay valve. The lockup clutch control valve
then engages the lockup clutch.

The transmission charging pressure relief valve is part of the transmission hydraulic control valve. The
relief valve limits the maximum pressure in the transmission charging circuit. Transmission charging
pressure can be measured at the tap (5). Transmission charging pressure measured at pressure tap (5)
should be:

Converter Drive - Low Idle: > 2515 kPa (365 psi)

High Idle: < 3065 kPa (445 psi)
Direct Drive - 1300 rpm: 2310 ± 70 kPa (335 ± 10 psi)

Shown is the "D" Station" in CONVERTER DRIVE. In CONVERTER DRIVE the lockup clutch
solenoid is de-energized and there is no pilot oil to the selector piston. The selector piston is all the
way to the right in the valve body and the load piston is all the way to the right in the selector piston.
The modulation reduction valve blocks the flow of oil to the two-stage relief valve.

The "D" Station does not have a load piston orifice or a load piston plug. Instead, a blocker plate is
used to prevent oil from flowing between the load piston and the selector piston. The load piston
always moves with the selector piston.
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Shown is the "D" Station" in DIRECT DRIVE. In DIRECT DRIVE the lockup clutch solenoid is
energized and pilot oil flows from the lockup solenoid to the selector piston. Pilot oil moves the
selector piston to the left. The load piston spring is compressed and moves the reduction valve spool to
the left against the force of the return spring. This movement opens the supply passage and allows
pressure oil to flow to the two-stage relief valve. Pressure oil also opens the ball check valve and fills
the cavity to the right of the slug. Pressure in the slug cavity balances the force of the load piston
spring and the reduction valve to control the pressure to the twostage relief valve.

Adding shims between the spring and the load piston will increase the pressure to the two-stage relief
valve and will lower the DIRECT DRIVE rail pressure.

The transmission gear switch (1) provides input signals to the Transmission/Chassis ECM. The
transmission gear switch inputs (also referred to as the "actual gear inputs") consist of six wires. Five
SOTREQ S/A 67 TREINAMENTO CORPORATIVO - CTG
 Power Train

of the six wires provide codes to the Transmission/Chassis ECM. Each code is unique for each
position of the transmission gear switch. Each transmission gear switch position results in two of the
five wires sending a ground signal to the Transmission/Chassis ECM. The other three wires remain
open (ungrounded). The pair of grounded wires is unique for each gear position. The sixth wire is the
"Ground Verify" wire, which is normally grounded. The Ground Verify wire is used to verify that the
transmission gear switch is connected to the Transmission/Chassis ECM. The Ground Verify wire
allows the Transmission/Chassis ECM to distinguish between loss of the transmission gear switch
signals and a condition in which the transmission gear switch is between gear detent positions.

Earlier transmission gear switches use a wiper contact assembly that does not require a power supply
to Pin 4 of the switch. Current transmission gear switches are Hall-Effect type switches. A power
supply is required to power the switch. A small magnet passes over the Hall cells, which then provide
a non-contact position switching capability. The Hall-Effect type switch uses the same 24 volt power
supply used to power the Transmission/Chassis ECM. The solenoid outputs provide +Battery voltage
to the upshift solenoid (2), the downshift solenoid (3) or the lockup solenoid (4) based on the input
information from the operator and the machine. The solenoids are energized until the transmission
actual gear switch signals the Transmission/Chassis ECM that a new gear position has been reached.

The Transmission Output Speed (TOS) sensor (arrow) is located on the transfer gear housing on the
input side of the transmission. Although the sensor is physically located near the input end of the
transmission, the sensor is measuring the speed of the transmission output shaft. The sensor is a Hall-
Effect type sensor. Therefore, a power supply is required to power the sensor. The sensor receives 10
Volts from the Transmission/Chassis ECM. The sensor output is a square wave signal of
approximately 10 Volts amplitude. The frequency in Hz of the square wave is exactly equal to twice
the output shaft rpm. The signal from this sensor is used for automatic shifting of the transmission.
The signal is also used to drive the speedometer and as an input to other electronic controls.

An 8T-5200 Signal Generator/Counter can be used to shift the transmission during diagnostic tests.
Disconnect the harness from the lockup solenoid and the speed sensor and attach the Signal Generator
to the speed sensor harness. Depress the ON and HI frequency buttons. Start the engine and move the
shift lever to the highest gear position. Rotate the frequency dial to increase the ground speed and the
transmission will shift.

NOTE: A 196-1900 adapter is required to increase the frequency potential from the signal generator
when connecting to the ECM's used on these trucks. When using the signal generator, the lockup
clutch will not engage above SECOND GEAR because the Engine Output Speed (EOS) and the
Converter Output Speed (COS) verification speeds will not be correct for the corresponding ground
speed signal.

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Primary pressure checked by removing plug

This schematic shows the conditions present when the clutch primary pressures are checked. When the
load piston (inner) plug is removed from the valve station cover, the chamber between the end of the
load piston and the selector piston is opened to drain. This condition prevents the clutch pressure from
increasing above its primary pressure setting.

Correct adjustment of the primary pressures is very important because they control clutch phasing and
fill times. High primary pressure settings will cause fast, rough shifts that can result in damage to the
power train. 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 excessive clutch slippage.

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Electronic Clutch Pressure Control (ECPC)

The clutch modulating valves are mounted on the rear of the transmission on the left side of the
transmission. There is one modulating valve for each of the eight transmission clutches. The pressure
taps in the modulating valves test the clutch pressures for the following:

- Clutch 1 (1) (forward high)

- Clutch 2 (2) (forward low)

- Clutch 3 (3) (reverse)

- Clutch 4 (4) (second speed)

- Clutch 5 (5) (third speed)

- Clutch 6 (6) (first speed)

- Clutch 7 (7) (low range)

- Clutch 8 (8) (high range)

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This illustration shows the transmission hydraulic system with the engine running and the directional
control switch in the NEUTRAL position.

When the engine is running, flow from the transmission scavenge pump is sent from the transmission
sump to the differential housing. Oil flow from the transmission charge pump is sent from the
differential housing through the transmission filter to the eight transmission modulating valves.
Transmission charge pump flow is also sent to the transmission relief valve, parking brake solenoid
valve, and the differential lock solenoid valve. The transmission relief valve limits the oil pressure to
the modulating valves. When NEUTRAL is selected, the Transmission/Chassis ECM energizes No. 5
and No. 8 solenoids. The modulating valve controls the oil flow to the clutches.

When the solenoids are energized, the electromagnetic force moves the pin against the ball. The ball
moves to the right against the seat. The oil flow through the center of the valve spool is blocked. The
oil pressure increases at the left end of the spool and the valve spool moves to the right compressing
the spring. Oil flow is then directed to the clutches.

From the transmission relief valve, oil flows to the power train oil cooler and the power train oil cooler
relief valve. The relief valve limits the oil pressure to the cooler. When the oil pressure to the cooler
exceeds 520 kPa (75 psi), the relief valve opens and sends the excess oil pressure to the outlet side of
the oil cooler.

Oil flows through the power train oil cooler and on to the transmission for cooling and lubrication
purposes. The lubrication system of the transmission has a relief valve to limit the oil pressure. When
the oil pressure in the lubrication system exceeds 380 kPa (55 psi), the relief valve opens and sends the
excess oil pressure to the transmission sump.

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This illustration shows the transmission hydraulic system with the engine running, the directional
control switch in the FORWARD position, and FIRST SPEED selected.

When FIRST SPEED FORWARD is selected, the Transmission/Chassis ECM energizes the No. 6,
and No. 7 solenoids before energizing the No. 1 solenoid. The modulating valves control the oil flow
to the clutches.

Transmission Modulating Valve - No Commanded Signal

In this illustration, the transmission modulating valve is shown with no current signal applied to the
solenoid. The Transmission/Chassis ECM controls the rate of oil flow through the transmission
modulating valves to the clutches by changing the signal current strength to the solenoid. With no
current signal applied to the solenoid, the transmission modulating valve is DE-ENERGIZED and oil
flow to the clutch is blocked. The transmission modulating valve is located on the transmission control
valve.

Pump oil flows into the valve body around the valve spool and into a drilled passage in the center of
the valve spool. The oil flows through the drilled passage and orifice to the left side of the valve spool
to a drain orifice. Since there is no force acting on the pin assembly to hold the ball against the drain
orifice, the oil flows through the spool and the drain orifice past the ball to the tank.

The spring located on the right side of the spool in this view holds the valve spool to the left. The
valve spool opens the passage between the clutch passage and the tank passage and blocks the passage
between the clutch passage and the pump supply port. Oil flow to the clutch is blocked. Oil from the
clutch drains to the tank preventing clutch engagement.

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Transmission Modulating Valve - Commanded Signal Below Maximum

In this illustration, the modulating valve is shown with a signal to the solenoid that is below the
maximum current. Clutch engagement begins when the Transmission/Chassis ECM sends an initial
current signal to ENERGIZE the solenoid. The amount of commanded current signal is proportional to
the desired pressure that is applied to the clutch during each stage of the engagement and
disengagement cycle.

The start of clutch engagement begins when the current signal to the solenoid creates a magnetic field
around the pin. The magnetic force moves the pin against the ball in proportion to the strength of the
current signal from the ECM.

The position of the ball against the orifice begins to block the drain passage of the oil flow from the
left side of the valve spool to the tank. This partial restriction causes the pressure at the left end of the
valve spool to increase. The oil pressure moves the valve spool to the right against the spring. As the
pressure on the right side of the valve spool overrides the force of the spring, the valve spool shifts to
the right.

The valve spool movement starts to open a passage on the right end of the valve spool for pump
supply oil to fill the clutch. Oil also begins to fill the spring chamber on on the right end of the spool.

In the initial clutch filling stage, the ECM commands a high current pulse to quickly move the valve
spool to start filling the clutch. During this short period of time, the clutch piston moves to remove the
clearances between the clutch discs and plates to minimize the amount of time required to fill the
clutch. The ECM then reduces the current signal which reduces the pressure setting of the proportional
solenoid valve. The change in current signal reduces the flow of oil to the clutch. The point where the
clutch plates and discs start to touch is called TOUCH-UP.

Once TOUCH-UP is obtained, the ECM begins a controlled increase of the current signal to start the
MODULATION cycle. The increase in the current signal causes the ball and pin to further restrict oil
through the drain orifice to tank causing a controlled movement of the spool to the right. The spool
movement allows the pressure in the clutch to increase.

During the MODULATION cycle, the valve spool working with the variable commanded current
signal from the ECM acts as a variable pressure reducing valve.
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The sequence of partial engagement is called desired slippage. The desired slippage is controlled by
the application program stored in the ECM.

Transmission Modulating Valve - Commanded Signal At Maximum

In this illustration, the modulating valve is shown with a maximum current signal commanded to the
solenoid. When the modulation cycle stops, the Transmission/Chassis ECM sends the maximum
specified current signal to fully engage the clutch.

The constant current signal pushes the pin firmly against the ball in the solenoid valve. The pin force
against the ball blocks more oil from flowing through the drain orifice. This restriction causes an
increase in pressure on the left side of the valve spool. The valve spool moves to the right to allow
pump flow to fully engage the clutch.

In a short period of time, maximum pressure is felt at both ends of the proportional solenoid valve
spool. This pressure along with the spring force on the right end of the spool causes the valve spool to
move to the left until the forces on the right end and the left end of the valve spool are balanced.

The valve spool movement to the left (balanced) position reduces the flow of oil to the engaged clutch.
The ECM sends a constant maximum specified current signal to the solenoid to maintain the desired
clutch pressure.

The different maximum specified pressures for each clutch is caused by different maximum current
signals being sent by the ECM to each individual modulating valve. The different maximum signal
causes a difference in the force pushing the pin against the ball to block leakage through the drain
orifice in each solenoid valve. The different rate of leakage through the spool drain orifice provides
different balance positions for the proportional solenoid valve spool. Changing the valve spool
position changes the flow of oil to the clutch and the resulting maximum clutch pressure.

The operation of the proportional solenoid to control the engaging and releasing of clutches is not a
simple on and off cycle. The ECM varies the strength of the current signal through a programmed
cycle to control movement of the valve spool.

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