950G Series II Wheel Loader Caterpillar

Introduction to Power Shift Transmission Control Systems

The following information covers the transmission valves that are used to control power shift transmissions. The valves operate the hydraulic clutches in order to control the power flow through the transmission.

Power Train Hydraulic System (Backhoe Loader)

Illustration 1	g01085968

The power train hydraulic system supplies the oil and controls the oil flow to the hydraulic clutches. The power train hydraulic system also provides lubrication oil in order to cool the transmission components.

Illustration 1 shows the power train hydraulic system in a backhoe loader. The transmission control valve (at the top of Illustration 1) controls the engagement of the forward and reverse clutches in the transmission. The speed selection is done with shifting forks, but the direction is determined with clutches.

Oil flows into the transmission control valve. A flow control spool controls the amount of oil that can flow in. The remaining oil is bypassed to the torque converter. Valve spools that are inside the transmission control valve determine which clutches will be filled with oil.

Cooled oil is sent to the transmission case in order to lubricate the bearings, the gears, and the clutches before returning to the sump. The cooled oil flows through the clutches in order to prevent excessive heat build-up when the clutches slip. Clutch slip occurs momentarily as the clutch is engaging each time the transmission shifts.

Power Train Hydraulic System (Track-Type Tractor)

Illustration 2	g01085969

Illustration 2 shows the power train hydraulic system in a track-type tractor. The transmission control valve controls the oil flow to three forward clutches and three reverse clutches.

Oil from the two-section pump flows through the filter to the transmission control valve.

Cooled oil is sent to the transmission case in order to lubricate the bearings, the gears, and the clutches before returning to the sump.

Transmission Control Valve (Track-type Tractor)

Illustration 3	g01085971

The transmission control valve controls oil flow to the clutches. The transmission control valve that is shown in Illustration 3 has a speed selection spool and a directional spool. Cables connect the spools to the transmission control lever in the cab. The position of the spools determines which clutches are open to supply oil and which clutches are open to drain.

Oil enters the circuit from the pump. As the pressure increases, the pressure differential valve will allow oil into the direction clutch circuit. The pressure differential valve will meter the oil in order to keep the oil pressure constant in the direction clutch circuit.

Oil that flows to the modulating relief valve will meter the pressure in the engaged speed clutch. Excess oil from the modulating relief valve flows to the torque converter circuit.

Oil flows through an orifice to the area behind the load piston. The load piston and the modulating relief valve work together in order to cause the pressure in the clutch to increase slowly. This is called modulation.

The transmission control valves in various machines achieve clutch fill and modulation in various ways. Cables and levers that are attached to the transmission control valve spools (Illustration 2 and 3) move the spools, which direct oil to the clutches. The remaining valves in this module use electric controls and solenoids to control oil to the clutches.

Countershaft Transmission Control Valve

Illustration 4	g01085972

The components and operation of the transmission control valve in a countershaft transmission will be discussed using the control valve that is shown in 4 through 14. This transmission control valve body houses six electrically actuated solenoids that route oil flow to the directional selector spools and to the speed selector spools.

Illustration 5	g01085974

The upper portion of the transmission control valve contains the three direction selector spools (arrows in Illustration 5). The direction selector spools shift in order to allow direction clutch pressure oil (P2) in order to be routed to one of the three direction clutch packs. When a directional solenoid is activated, the appropriate directional selector spool routes pressurized P2 oil to a directional clutch. no. 1 (Forward Low), no. 2 (Forward High), and no. 3 (Reverse) are the directional clutches. The P2 supply oil to the selector spools is in parallel for Forward and Reverse but separated for Forward High and Forward Low. This is to prevent engagement of more than one directional clutch at a time.

Illustration 6	g01085975

The load piston (1) and the modulating relief valve (2) ) are located in the center section of the control valve (Illustration 6). The load piston works with the modulating relief valve in order to provide a controlled pressure rise (modulation) in the clutches and to limit maximum P2 pressure. The modulating relief valve also sends excess oil to the torque converter.

Illustration 7	g01085977

The lower portion of the transmission control valve contains the three-speed selector spools (arrows in Illustration 7). The speed selector spools shift in order to allow speed clutch pressure oil (P1) to be routed to one of the three-speed clutch packs. When a speed solenoid is activated, the appropriate speed selector spool directs pressurized P1 oil to a speed clutch. no. 4, no. 5, and no. 6 are the speed clutches.

P1 supply oil is directed separately through the three selector spools in order to prevent engagement of more than one speed clutch at one time. Supply oil is first available to the no. 4 solenoid and first speed selector spool. Then the no. 5 solenoid and second speed selector spool. Finally to the no. 6 solenoid and third speed selector spool. Therefore, in any default situation, the transmission will either shift down or shift to a neutral condition.

Illustration 8	g01085979

The shift solenoids (Illustration 8) consist of the stem and the coil. All six solenoids including the stems and coils are interchangeable. Loss of electrical power to any speed or directional solenoid will neutralize the transmission by releasing the corresponding clutch. During normal operation, supply oil is directed to the end of the solenoid stem. When the solenoid is activated, a pin shifts upward inside the stem. The pin unseats a ball that allows oil to pass through the oil passage to the appropriate selector spool.

Illustration 9	g01085980

The pressure differential valve and the spring (Illustration 9) are located between the transmission control valve and the separator plate. The pressure differential valve maintains P1 pressure at a specified pressure that is greater than P2.

Transmission Control Valve in Neutral

Illustration 10	g01085981

The dump valve that is shown on the right in Illustration 10) provides smoother shifts by rapidly venting the load piston chamber oil pressure between shifts.

When the engine is running and the speed selector is in NEUTRAL, oil flows from the pump through the transmission control valve to the three-speed selector solenoids. The P1 oil also fills the slugs at the ends of the spools. This keeps the spools in the disengaged position.

Oil also flows to the pressure differential valve and to the selector spool in the dump valve. P1 oil in the dump valve is used to open a drain passage for load piston oil. When the P1 pressure reaches the specified amount, the pressure differential valve opens. Supply oil begins to flow into the P2 circuit.

Some of the P2 oil flows to the dump valve and shifts the selector spool down. P1 oil is blocked. The oil from the load piston chamber will not be open to the drain line. The rest of the P2 oil that flows through the pressure differential valve flows into the transmission control valve, then directly to the slug cavity of the modulating relief valve.

P2 oil flows through the P2 inlet orifice in the transmission control valve body. The oil again divides and flows in three directions. Partial flow is directed to the slug cavity of the check valve, through the screened orifice, and into the load piston cavity. Oil that enters the load piston cavity flows to the dump valve. Because the selector spool is shifted downward, the oil from the load piston cavity is blocked.

P2 oil flows to the slug cavity of the REVERSE selector spool, then to the FORWARD LOWsolenoid and the FORWARD HIGH solenoid.

The oil also is sent to the slug cavity of the FORWARD LOW selector spool. Then, the oil is sent to the slug cavity of the FORWARD HIGH selector spool. From the FORWARD HIGH selector spool, flow is directed to the REVERSE solenoid.

In NEUTRAL, no solenoids are energized. No speed or direction clutches are engaged. The oil in the slug cavities of the direction selector spools keeps the clutches disengaged.

The modulating relief valve meters excess pump flow to the P3torque converter circuit. The backflow check valve separates the torque converter circuit from the lower pressure that is maintained by the modulation relief valve.

Transmission Control Valve (Speed Clutch Fill)

Illustration 11	g01085985

When a shift is made from NEUTRAL to first speed REVERSE, solenoids no. 2 and no. 4 are energized.

When the no. 4 solenoid is energized, oil is sent to the selector spool for the first speed (no. 6) clutch. The selector spool moves to the right. This directs P1 oil to the first speed clutch.

During the speed clutch fill (Illustration 11), P1 pressure decreases and the spring closes the poppet in the pressure differential valve. The directional spool will not move until the speed clutch is filled.

When P2 pressure drops, the check valve shifts in order to open the load piston cavity to the drain. When P2 drops further, the selector spool in the dump valve shifts up.

When the selector spool shifts up, the P1 pressure is directed to the slug chamber in the dump spool. P1 pressure in the slug cavity moves the dump spool and piston down against the piston spring. This drains the load piston cavity through the passage to the dump valve. The dump valve is used to provide a smoother shift. The oil that is in the load piston cavity does not drain to the tank rapidly enough through the load piston cavity drain.

P2 pressure also decreases through the screened orifice in the modulating relief valve and around the selector spools.

When the modulating relief valve moves to the right, the torque converter supply passage is blocked.

When the load piston and the modulating relief valve have shifted to the reset position, residual pressure in the P3 circuit is vented through a drain passage.

Transmission Control Valve (Start of Modulation)

Illustration 12	g01085987

After the speed clutch has filled, the P1 pressure raises until the pressure differential valve opens. When the pressure differential valve opens, oil flows into the P2 circuit (Illustration 12). Flow is directed to the modulating relief valve, the directional clutch selector spools, and the directional solenoids.

Since the no. 2 solenoid is energized, oil flows to the REVERSE direction selector spool. The selector spool shifts to the right. As the selector spool shifts, oil flows to the no. 3 clutch. The no. 3 clutch begins to fill.

Oil in the P2 circuit also flows to the dump valve, to the slug cavity of the modulating relief valve, to the slug cavity of the check valve and through a screened orifice to the load piston cavity.

When the no. 3 clutch is full, pressure in the P2 circuit begins to increase. This shifts the check valve and closes the drain for the load piston cavity.

P2 pressure is not high enough to shift the selector spool in the dump valve. The P1 oil continues to hold the dump spool and piston, so the load piston cavity is open to drain. An orifice in the dump spool is sized in order to provide a controlled delay in closing the load piston drain through the dump valve.

When P2 pressure is high enough to shift the selector spool in the dump valve, P1 oil is blocked. P1 pressure continues to act upon the end of the dump spool through the orifice in the spool. The dump spool slowly moves up to the closed position.

When the load piston drain is closed, the modulation cycle begins.

As pressure increases in the modulating relief valve, a passage to the torque converter circuit opens. At this time, the torque converter circuit is still open to drain through the load piston spring cavity.

By the time the direction clutch has filled, the load piston has moved slightly to the left.

Transmission Control Valve in First Reverse

Illustration 13	g01085988

Now, the modulating relief valve begins to move slowly to the right at a steady rate (Illustration 13).

The load piston starts moving to the left at a steady rate. As the pressure continues to increase, the load piston starts to cover the torque converter drain passage. P3 pressure starts to build and oil flows through the backflow check valve to the torque converter circuit.

The load piston and the modulating relief valve will work together in order to maintain a steady pressure in the clutch.

The load piston continues to move to the left. This blocks the torque converter drain passage. When the load piston reaches the travel limit, the load piston is in a position to meter oil to drain.

The modulating relief valve stops moving to the right. This does not completely shut off P2 from P3. The modulating relief valve is also metering oil to drain and P1, P2, and P3 are at normal operating pressures.

Transmission Control Valve in Fourth Forward

Illustration 14	g01085989

When fourth speed FORWARD is selected (Illustration 14), the no. 6 speed solenoid and no. 1 directional solenoid are activated. Solenoid no. 1 is energized only for fourth speed FORWARD.

The shifting sequence for all speeds and directions are the same.

Also, notice that P2 oil flow is no longer available to the no. 2 directional solenoid.

Individual Clutch Modulation (ICM)

Illustration 15	g01085991

Another type of hydraulic shift control system that is found on some machines is called Individual Clutch Modulation (ICM). An ICM transmission differs in that each clutch is modulated individually in order to provide smoother shifting under load. Speed and direction shifts are accomplished by individual control valves, which hydraulically engages a specific clutch pack.

The transmission hydraulic system consists mainly of the valves that make up the transmission hydraulic control unit. The upshift solenoid and the downshift solenoid are controlled by the Transmission Electronic Control Module (ECM) when a shift is needed. The ECM monitors several factors in order to determine when a shift will be made.

When a shift solenoid is activated, oil is sent to the rotary actuator. The vane that is in the center of the rotary actuator is mechanically connected to the rotary selector spool in the selector valve group. The position of the rotary selector spool will determine which stations of the pressure control valve are filled and which stations are drained. The pressure control valve has a station for each clutch. Each station has valves that modulate the oil flow in order to keep a steady pressure inside the clutch.

Illustration 16	g01085992

The ICM transmission hydraulic control unit is located on top of the transmission case. The upshift and downshift solenoids (arrows) are also visible in Illustration 16.

Illustration 17	g01085993

The following components are located within the transmission hydraulic control unit: pressure control valve, selector valve group and rotary actuator.

Illustration 18	g01085994

The downshift and upshift solenoids (Illustration 18) are located on top of the transmission. The solenoids are the connection between the electrical system and the hydraulic systems of the transmission. When the downshift or upshift solenoids are activated electrically, transmission oil is sent to the rotary actuator.

Illustration 19	g01085995

The rotary actuator (Illustration 19) is part of the transmission hydraulic control group. The upshift or downshift solenoids send oil into one of the passages. The rotor in the rotary actuator turns, which turns the rotary selector spool in the selector valve group. The selector spool allows pilot oil to flow to the appropriate valve in the pressure control valve.

Illustration 20	g01085999

The selector valve group (Illustration 20) is part of the transmission hydraulic controls. The selector valve group controls the system oil pressure. The selector valve group controls the oil flow to the solenoids and to the pressure control valve.

The incoming oil first flows past the priority reduction valve. This valve modulates in order to control the oil pressure in the transmission hydraulic control unit. The oil then flows to the neutralizer valve. The neutralizer valve will prevent oil from flowing to the rotary selector spool if the engine is started with the transmission selector lever in a gear other than Neutral.

The position of the rotary selector spool is controlled by the rotary actuator and the shift solenoids. The rotary selector spool determines which selector pistons in the pressure control valve receive pilot oil and which selector pistons are drained.

Illustration 21	g01086002

Detent springs (arrows) are used to help the rotary selector spool maintain the proper positions.

Illustration 22	g01086007

The pressure control valve (Illustration 22) is part of the transmission hydraulic control unit. The pressure control valve contains the pressure modulation reduction valves. There is one valve for each clutch in the transmission. The modulation reduction valves for the transmission clutches provide separate control of the pressure and time that it takes to engage and disengage each clutch. Each load piston body has a letter identification on the body for disassembly and assembly purposes.

Modulation Reduction Valve (Clutch Fill)

Illustration 23	g01086008

All of the modulation reduction valves of the transmission pressure control valve operate in a similar way. Therefore, only the basic operation of one valve is discussed.

When a shift is started, a pilot passage receives pilot oil at the correct sequence from the rotary selector spool. This also causes the selector piston and the load piston to move against the force of the springs. This causes the modulation reduction valve to move against the force of a spring. Movement of the valve closes the passage from the clutch to the drain. The modulation reduction valve opens the passage from the pump to the clutch.

Oil also fills the area between the selector piston and the load piston.

Modulation Reduction Valve (Clutch Engaged)

Illustration 24	g01086009

After the clutch is full of oil, the pressure of the pump oil increases in the selected clutch. This causes the load piston to again move against the force of the springs. Clutch oil also flows through an orifice in the modulation reduction valve and opens the check ball. Then the oil goes into the slug chamber at the end of the modulation reduction valve.

The pressure at the end of the modulation reduction valve works against the pressure at the end of the load piston. The pressure increases until the load piston is moved all the way to the left against the stop. The pressure in the clutch is now at the maximum.

Two factors control the amount of time that is required for the pressure in the clutch to achieve the maximum amount:

• The size of the load piston orifices

• The force of the springs

Note all color codes when assembling an ICM pressure control valve.

The force of the springs can be changed by the removal or the addition of shims in the load piston.

When the clutch is full, the modulation reduction valve will move to the right and left in order to keep the pressure in the passage constant.

Modulation Reduction Valve (Clutch Released)

Illustration 25	g01086010

When the clutch needs to be released, the position of the rotary selector spool will cause the pilot pressure to drain. The springs will move the selector piston against the stop. The passage between the load piston and the selector piston will be open to drain. The springs will move the load piston against the stop.

The modulation reduction valve will shift. Movement of the modulation reduction valve closes the passage from the pump to the clutch. This opens the passage from the clutch to the drain.

A decay orifice is located in the drain passage in order to control the amount of time that is required for the pressure in the clutch to reach zero. These orifices are color coded. The clutch that is used for reverse does not have a decay orifice.

Electronic Clutch Pressure Control (ECPC)

Illustration 26	g01086011

Another method of electronic clutch engagement is called Electronic Clutch Pressure Control (ECPC). With ECPC, the transmission shifting function is controlled by the Power Train Electronic Control Module. The Power Train ECM responds to the operator shifting requests by controlling the amount of electrical current that is sent to the proportional solenoids.

The ECM selects the transmission clutches to be engaged and the clutch pressure is modulated electronically. The proportional solenoid valves control the modulation of the clutch pressure. The ECM uses the transmission speed, engine speed, and the power train oil temperature signals in order to control smooth engagement of the clutches.

Electronic clutch modulation allows the ECM to control the time that is required to fill a clutch with oil and the rate of the clutch pressure modulation.

Illustration 27	g01086013

The ECPC transmission clutches are hydraulically engaged and the spring released. The transmission modulating valve solenoid is energized (Illustration 27) in order to send supply oil to the clutch. As current is applied to the solenoid, magnetic force moves the rod to the right which pushes the ball closer to the orifice. The ball begins to restrict the amount of oil to drain. As the pressure at the left end of the spool increases, the spool shifts to the right and directs supply oil in order to engage the clutch.

When the solenoid is de-energized, no force is acting on the rod and supply oil flows through the spool and the orifice to drain. The spring that is located on the right end of the valve spool moves the spool to the left. The valve spool opens the passage between the clutch and the tank. The valve spool blocks the passage between the clutch and the supply oil. Oil flow to the clutch is blocked. Oil from the clutch flows to the drain and the clutch is released.

Illustration 28	g01086015

Illustration 28 shows the ECPC transmission modulation cycle. The vertical axis represents current and clutch pressure. The current that is represented is from the Power Train ECM to the modulating solenoid valve. The pressure that is represented is supplied to each individual clutch. When the clutch is filled and the piston is in contact with the plates, the current and pressure are directly proportional, and are represented on the same axis. The horizontal axis represents time in intervals that relate to the hydraulic pressure that is supplied to the clutch.

The pulse time is caused by an initial high current that is applied to the valve in order to begin pressurizing the clutch when a clutch is engaged. The ramp level begins a reduction in the current that is applied to the valve which lowers the current in order to the hold level.

When the current is at the hold level, the clutch is full. The clutch pressure then follows the current that is applied to the solenoid.

At the end of the hold time, the current increases as the clutch is engaging. This time is called the desired slip time. The pressure ramp is called modulation.

Modulation continues until the clutch is fully engaged and the maximum clutch pressure is reached. The clutch pressure will stay at the maximum value for a short time. This condition is called the full on time. The clutch pressure is then reduced to the clutch engagement level. The clutch is fully engaged, but at a lower pressure. This pressure reduction increases the clutch seal life.

Transmission Electronic Control Systems

Illustration 29	g02614878

All Caterpillar electronic control systems can be broken down into three general circuits: input components, electronic controls and output components. A diagram of the ICM Transmission Electronic Control System is shown in Illustration 29.

Input components in the system function as sensors of various machine conditions. The input components react electrically to the following changes: pressure, temperature, position and speed. As these changes occur, the input components send electrical signals to the electronic controls.

Electronic controls are sealed assemblies which receive the electrical signals from the input components as information to an internal program. The electronic controls then supply electrical signals according to that program to the output components.

Output components are designed to be seen, to be heard, or to move when supplied with the necessary electrical signals from the electronic controls.

The components in this diagram are arranged in three basic categories: Input components, electronic control, and output components. In this example, the input components consist of a tractor transmission switch, a transmission speed sensor, a shift lever switch, and a hold switch. The input components send information in the form of electrical signals to the electronic control. The electronic control reads the information from the input components. The electronic control sends electrical current to one of the output components. The two output components are an upshift solenoid and a downshift solenoid.

The tractor transmission switch tells the electronic control the speed range (gear) in which the transmission is operating. The transmission speed sensor detects the speed of the transmission output shaft which is directly proportional to the ground speed of the machine. The shift lever switch is positioned by the machine operator. The shift lever switch tells the electronic control the position of the transmission selector lever. When activated by the operator, the hold switch prevents automatic upshifts and downshifts unless an engine underspeed condition exists.

The electronic control is the main component in the electronic system. The electronic control is programmed to compare the information that is provided by the input components. When the conditions for an upshift or a downshift are correct, the electronic control supplies electrical current to the appropriate solenoid.

The shift solenoids directly connect the electronic system to the transmission hydraulic system. When an upshift or a downshift is indicated, the corresponding solenoid is momentarily energized. This opens a valve in the base of the solenoid which permits oil to flow to the appropriate transmission control valve spool. The control valve spool then initiates the shift.

Electronically controlled transmissions have incorporated the most favorable features of mechanical, hydraulic, and electronic systems. Some features of electronically controlled transmissions are listed below:

• Mechanical linkages eliminated.

• System adjustments performed electronically.

• Design changes and updates performed with software.

• Reduced operator fatigue.

• Smoother shifts.

• Simplified troubleshooting.