Patent Publication Number: US-6669598-B2

Title: Line pressure control for a continuously variable transmission

Description:
TECHNICAL FIELD 
     This invention relates to transmission controls and, more particularly, to electro-hydraulic controls for a continuously variable transmission. 
     BACKGROUND OF THE INVENTION 
     Continuously variable transmissions include a continuously variable unit (CVU), such as a belt and pulley mechanism, and a gearing mechanism, such as a planetary gear arrangement. The gearing mechanism is conventionally controlled by torque transmitting mechanisms (i.e., clutches or brakes) that are selectively operated by hydraulic fluid. The continuously variable unit requires a high pressure to ensure sufficient clamping forces for the belt and pulley mechanism. The effective ratio of the CVU is determined by the radius at which the belt engages the pulleys. In most instances, the ratio can be varied from an underdrive to an overdrive. 
     The amount of clamping pressure required is a function of the input torque to the transmission and the ratio at which the variable transmission unit is operating. If the clamping pressure is low, there is a possibility of belt slippage, and even a small amount of belt slippage can be detrimental to the CVU. The ratio of the CVU is changed by reducing or increasing the pressure acting on one of the sheave halves of one of the pulleys, generally the input pulley, while the pressure at the other pulley is maintained substantially constant. If the control pressure is excessive at either pulley, there is an efficiency loss within the transmission and possible damage to or overstressing of the components of the CVU. 
     The control pressure level required to engage the torque transmitting mechanisms is generally lower than the pressure required to control the CVU. The amount of pressure required is essentially a function of torque being transmitted and size of the conventional clutch hardware, consisting of a movable piston and a clutch pack. If the control pressure is below the required value, slippage of the friction plates can occur, which will shorten the life of the torque transmitting mechanisms. 
     The hydraulic circuit generally includes a pressure regulating valve that must be capable of regulating the correct clamping pressure and the correct torque transmitting engagement pressure to avoid a shortened life for either the variable transmission unit or the torque transmitting unit. 
     The pressure within the circuit is generated by a positive displacement pump. The amount of pressure that the pump can generate is a function of the pump speed, flow demand of the transmission, and leakage within the circuits. The more flow the transmission requires, the lower the line pressure that can be generated. Generally, two regulator valves are employed, one for the CVU control and one for the torque transmitting mechanisms. The valves are usually disposed in flow relation such that the CVU line pressure circuit has priority. All of the hydraulic fluid not used by the CVU control is passed to the regulator valve for the gearing section controls. Thus, the regulator valve for the gearing section controls must be sized to accommodate large amounts of fluid flow at times during the operation of the transmission, particularly during ratio changes when the pressure at the control pulley is being reduced. This means that the flow priority is set in such a way that if the pump is not able to supply the requested line pressure for the CVU, part of the transmission flow demand is reduced through the regulator valve in order to achieve the desired line pressure. 
     For example, in order to preserve the required line pressure to the sheaves in a belt and pulley type transmission, the line pressure regulator valve may reduce the flow to the oil cooler. The oil flowing to the sheave is said to have the higher priority than the oil going to the oil cooler. Generally, this sacrifice of flow to one part of the transmission in order to maintain the pressure in another part of the transmission should happen only during extreme or transient conditions, such as the development of a large leak or a rapid ratio change within the transmission. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an improved hydraulic control system for a continuously variable transmission. 
     In one aspect of the invention, the hydraulic control system regulates the line pressure to the CVU pulleys, clutches, solenoids and torque converter of a continuously variable transmission. 
     In another aspect of the present invention, separate regulator valves provide a first line pressure and a second line pressure to thereby minimize oil flow demand and improve the transient shift performance and fuel economy. 
     In yet another aspect of the present invention, excess pump flow is only directed to the pressure regulator controlling the first line pressure which enables the remaining valves within the control system to be reduced in size and weight. 
     In still another aspect of the present invention, the hydraulic control system prioritizes the pressure in the first line pressure above all other pressure demands and directs this line pressure to the sheaves of the continuously variable transmission. 
     In still another aspect of the present invention, the hydraulic control system minimizes the effects of transient flow demands and the pressure output of the solenoid controls by prioritizing the first line pressure above all other pressure demands. 
     In a further aspect of the present invention, the control system minimizes slippage within the torque transmitting mechanisms by prioritizing second line pressure flow to the clutches above cooler flow demands. 
     The present invention employs a single variable bleed solenoid (VBS) valve which controls both the first line pressure regulator valve and the second line pressure regulator valve. The use of a single variable bleed solenoid to perform this function reduces the cost of the transmission control. The hydraulic system has an actuator feed limit valve which protects the variable bleed solenoid valve from over pressurization. A second line pressure feed limit valve is incorporated within the control to limit both the torque transmitting mechanisms and the torque converter from over pressurization by limiting the output of the second line pressure regulator valve prior to distribution of fluid to either of these devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the arrangement of FIGS. 1A,  1 B and  1 C. 
     FIGS. 1A and 1B are a schematic representation of a hydraulic control system. 
     FIG. 1C is a cross-sectional elevational view of a continuously variable transmission utilizing the control system of FIGS. 1A and 1B. 
     FIG. 2 is a plurality of curves showing the relation between the VBS signal pressure, system pressures and CVU input torque. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A control system  10  is shown in FIGS. 1A and 1B and a continuously variable transmission  12  is shown in FIG.  1 C. The control system  10  includes a binary pump  14  which has an inlet or suction port  16  and two outlet ports  18  and  20 . Port  18  is the primary pump output port and port  20  is a secondary output pump port. These ports  18  and  20  can have different pressures during the operation of the control system  10 . The control system  10  further includes a primary pressure regulator valve  22 , a secondary regulator valve  24 , a variable bleed solenoid (VBS)  26 , a primary overpressure regulator valve  28 , a pressure transducer  30 , a primary feed limit valve  32 , an actuator feed limit valve  34 , a variable bleed solenoid (VBS)  36 , an on/off solenoid  38 , a ratio control mechanism  40 , a secondary line pressure limit valve  42 , a clutch boost valve  44 , a torque converter clutch regulator apply valve  46 , a clutch control valve  48 , a torque converter clutch enable valve  50  and a manual control valve  52 . 
     The pressure regulator valve  22  includes a valve body portion  22 A having found therein a valve bore portion  22 B. A valve spool  22 C is slidably disposed in the valve bore  22 B and is urged rightward by a control spring  22 D. The valve body portion  22 A includes a pair of line pressure ports  22 E, a pair of tier two feed ports  22 F, a second line pressure port  22 G, a variable bleed solenoid (VBS) signal port  22 H, and a suction or exhaust port  22 I. The ports  22 E are continually connected with the output port  18  of the pump  14  through a primary or first line pressure passage  54 . The port  22 G is continually connected with the output port  20  of the pump  14  through a secondary pressure passage  56 . The port  22 I is continually connected with a suction or inlet line  57  that is continually connected with the inlet port  16  of the pump  14 . 
     The secondary regulator valve  24  includes a valve body portion  24 A in which is formed a valve bore  24 B. A spool valve  24 C is slidably disposed with the valve bore  24 B and is urged rightward as seen in FIG. 1B by a control spring  24 D. Valve body  24 A has an inlet port  24 E which is in continuous communication with the port  22 F of the valve  22  through a feed passage  58 . 
     The valve body  24 A also has formed therein a pair of second line pressure passages  24 F that are in continuous communication with a second line pressure passage  60 . The valve body  24 A further includes a second line limit port  24 G which is in continuous communication with a second line limit passage  62 , and a limited converter feed port  24 H which is in communication with a limited converter feed passage  64 . The limited converter feed passage  64  is also in communication with the passage  62  through a restriction or orifice  66 . 
     The valve body  24 A includes a variable bleed signal port  24 J that is in continuous communication with a variable bleed signal line  68  that is also connected with the port  22 H of the valve  22  and with a signal port  26 A of the variable bleed solenoid valve  26 . 
     The variable bleed solenoid valve  26  also has an inlet port  26 B that is in communication with an actuator feed limit passage  70 . The variable bleed solenoid  26 , the variable bleed solenoid  36 , and the on/off solenoid valve  38  are conventional solenoid valves that are well known in the art of hydraulic control mechanisms. 
     The overpressure regulator valve  28  includes a valve body portion  28 A, an inlet port  28 B and a ball check assembly  28 C. The ball check assembly  28 C is set to open the passage  54  which is connected to port  28 B to exhaust when the pressure in the passage  54  exceeds a maximum predetermined value. The function of the valve therefore is to limit the maximum pressure that can be achieved in passage  54 . 
     The pressure transducer  30  is a conventional electro-hydraulic device that is actuated by the pressure in passage  58  and puts out an electrical signal relative to that pressure level. The pressure transducer  30 , the variable bleed solenoid valve  26 , the variable bleed solenoid valve  36 , the on/off solenoid valve  38 , and the ratio control mechanism  40  are all in electrical communication with a conventional electrical control unit (ECU), not shown, which may include, as is well known, a programmable digital computer that distributes electrical signals to these mechanisms in accordance with commands given by the electrical control unit in response to sensor signals received from both the transmission  12  and the pressure transducer  30 . 
     The primary limit valve  32  includes a valve body portion  32 A having formed therein a valve bore  32 B in which a valve spool  32 C is slidably disposed. The valve spool  32 C is urged rightward, as seen in FIG. 1A, by a control spring  32 D. The valve body portion  32 A includes a primary feed port  32 E connected with a primary feed passage  72  and a pair of primary feed limit ports  32 F that are in communication with a primary limit feed passage  74 . The primary feed limit passage  74  also communicates with the first line pressure passage  54  through an orifice or restriction  76 . 
     The actuator feed limit valve  34  includes a valve body portion  34 A in which is formed a valve bore  34 B that has slidably disposed therein a spool valve  34 C. The spool valve  34 C is urged rightward, as viewed in FIG. 1A, in the valve bore  34 B by a control spring  34 D. The valve body portion  34 A has an inlet port  34 E that is in fluid communication with the first line pressure passage  54  and a pair of actuator limit feed passages  34 F that are in fluid communication with the actuator feed limit passage  70 . The actuator feed limit valve is operable to provide a controlled pressure to the variable bleed solenoid  26 , the variable bleed solenoid  36 , and the on/off solenoid  38 . 
     The variable bleed solenoid  36  has an inlet port  36 A which is in fluid communication with the actuator feed limit passage  70  and an outlet port  36 B that is in fluid communication with a variable bleed signal passage (VBS)  78 . The VBS signal passage  78  provides a variable control signal generated at the variable bleed solenoid  36 . 
     The on/off solenoid valve  38  has an inlet port  38 A and an outlet port  38 B. The inlet port  38 A is in fluid communication with the actuator feed limit passage  70 . The output port  38 B is in fluid communication with a TCC enable signal passage  80 . 
     The ratio control mechanism  40  includes a hydraulic valve  40 A, a conventional stepper motor  40 B and a sheave follower  40 C. The valve  40 A includes a valve body  40 D that has formed therein a valve bore  40 E in which is slidably disposed a valve spool  40 F. The valve body  40 D includes an inlet port  40 G that is in fluid communication with the line pressure passage  54  and an outlet port  40 H that is in communication with the primary feed passage  72 . 
     The stepper motor  40 B has connected therewith a lever arm  40 I which is pinned with the valve spool  40 F and slidably engages a cam follower  40 J which is a portion of the sheave follower  40 C. The cam follower  40 J engages a sheave  166  which is a component of an input pulley  140 . 
     The limit valve  42  includes a valve body portion  42 A that has formed therein a valve bore  42 B in which is slidably disposed a valve spool  42 C. The valve body  42 A has an inlet port  42 D that is in fluid communication with the second line pressure passage  60  and a pair of outlet ports  42 E that are in fluid communication with the second line limit passage  62 . The function of the line limit valve  42  is to limit the pressure in passage  62  to a level determined by the area of the valve spool  42 C and the force in a control spring  42 F. Whenever the pressure in passage  60  is below this value the valve spool  42 C will permit unrestricted flow between the passages  60  and  62 . 
     The clutch boost valve  44  includes a valve body portion  44 A that has formed therein a valve bore  44 B in which is slidably disposed a valve spool  44 C. The valve spool  44 C is urged rightward in the valve bore  44 B by a control spring  44 D. The valve body  44 A has an inlet port  44 E and an outlet port  44 F. The inlet port  44 E is in communication with the passage  62  and the output port  44 F is in fluid communication with a clutch boost passage  82 . 
     The torque converter clutch regulator apply valve  46  includes a valve body portion  46 A that has formed therein a valve bore  46 B in which is slidably disposed a valve spool  46 C. The valve spool  46 C is urged leftward, as seen in FIG. 1B, by a control spring  46 D. The valve body  46 A has an inlet port  46 E that is in fluid communication with the passage  62 , a pair of outlet ports  46 F that are in fluid communication with a regulated apply passage  84 , and a variable bleed solenoid signal port  46 G that is in fluid communication with the VBS signal passage  78 . 
     The clutch control valve  48  includes a valve body portion  48 A having formed therein a valve bore  48 B in which is slidably disposed a valve spool  48 C. The valve body portion  48 A has formed therein an inlet port  48 D that is in fluid communication with passage  62 , a pair of outlet ports  48 E that are in fluid communication with a clutch control passage  86 , a first signal port  48 F that is in communication with the VBS signal passage  78  and a second signal port  48 G that is in communication with the clutch boost passage  82 . A control spring  48 H urges the valve spool  48 C rightward in the valve bore  48 B, as seen in FIG.  1 B. The valve spool  48 C is also urged rightward by fluid pressure in the clutch boost passage  82  and is urged leftward by control pressure in the VBS signal passage  78  and by pressure at the outlet ports  48 E which act on the end of the valve spool  48 C. Thus, the pressure in the clutch control passage  86  is a function of clutch boost pressure in passage  82 , variable bleed solenoid signal pressure in passage  78  and the outlet pressure of the clutch control valve  48 . 
     The torque converter clutch enable valve  50  includes a valve body portion  50 A in which is formed a valve bore  50 B that has a valve spool  50 C slidably disposed therein. The valve spool  50 C is urged rightward in the valve bore  50 B by a control spring  50 D. The valve spool  50 C is urged leftward by pressure in the passage  80  which is admitted to the right end of valve spool  50 C through a control port  50 E. The valve body  50 A has a clutch control inlet port  50 F, a regulated apply port  50 G, a pair of converter feed ports  50 H, a limited clutch control feed port  50 I, a second line limit port  50 J, a torque converter release outlet port  50 K, and a torque converter apply outlet port  50 L. The port  5 OF is in fluid communication with the clutch control passage  86 , the port  50 G is in fluid communication with the regulated apply passage  84 , the ports  50 H are in fluid communication with the converter feed passage  64 , the port  50 I is in fluid communication with the clutch feed passage  88 , the port  50 J is in fluid communication with a second line limit passage  62 , the port  50 K is in fluid communication with a torque converter release passage  90 , and the port  50 L is in fluid communication with a torque converter apply passage  92 . 
     The manual control valve  52  includes a valve body portion  52 A that has a valve bore  52 B in which is slidably disposed a spool valve  52 C. The spool valve  52 C is preferably connected with a conventional manual shift control mechanism which will move the valve spool valve  52 C linearly within the valve bore  52 B. The valve bore  52 B includes an inlet port  52 D which is in fluid communication with the clutch feed passage  88 , a drive port  52 E which is in fluid communication with a drive passage  94  and a reverse port  52 F which is in fluid communication with a reverse passage  96 . When the valve spool  52 C is moved to the reverse position, fluid pressure delivered through the passage  88  will flow through a bypass port  52 G to thereby permit communication of fluid from the inlet port  52 D to the reverse port  52 F and the passage  96 . A plurality of exhaust ports are also present which will exhaust the reverse passage during neutral, drive and park and will exhaust the drive passage at the end valve spool  52 C during neutral, reverse or park. 
     The drive passage  94  has disposed therein a ball check and orifice apparatus  98  which is a conventional timing device which will permit rapid apply and controlled flow release. The reverse passage  96  has a similar timing mechanism  100  for the reverse clutch apply and release. The drive passage  94  is in fluid communication with the second line limit passage  62  through a restriction  182 , and the reverse passage is in fluid communication with the second line limit passage  62  through a restriction  104 . This will ensure that the clutches are pre-filled with fluid at their normal engagement speeds. 
     The transmission  12  includes a torque converter  106 , a planetary gear arrangement  108 , a continuously variable unit (CVU)  110  and a final drive mechanism  112 . The torque converter  106  is a conventional hydrodynamic device having an engine driven impeller  114 , a fluid driven turbine  116  and a stator  118 . 
     A torque converter clutch  120  is disposed between the turbine  116  and an input shell  122 . The input shell  122  is drivingly connected between an engine  124  and the impeller  114  in a conventional manner. The planetary gear arrangement  108  includes a sun gear  126 , a ring gear  128  and a planet carrier assembly  130 . The planet carrier assembly  130  includes a carrier member  132  and a plurality of meshing pinions  134  and  136  meshing with the sun gear  126  and ring gear  128 , respectively. 
     The carrier member  132  is driven by a shaft  138  that is drivingly connected with the turbine  116  and the torque converter clutch  120 . The sun gear  126  is drivingly connected with the input pulley  140  of the CVU  110 . The ring gear  128  is operatively connected with a torque transmitting mechanism or brake  142  which, when applied, will hold the ring gear  128  stationary. The carrier  132  and the sun gear  126  are operatively interconnected by a torque transmitting mechanism or clutch  144  which, when applied, will secure the ring gear member  128  and the planet carrier assembly member  130  of the planetary gear arrangement  108  together such that the planetary system will rotate as a single unit. 
     The input pulley  140  is connected through a friction belt assembly  146  with an output pulley  148 . The output pulley  148  is connected through a pair of transfer gears  150  and  152  with a conventional final drive differential  154  that is a component of the final drive mechanism  112 . 
     The brake  142  has an apply cavity  156  that is in fluid communication with the reverse passage  96  and the clutch  144  has an apply cavity  158  that is in fluid communication with the passage  94 . Thus, the forward and reverse clutch and brake are controlled in their engagement by the valve  50  and the manual valve  52 . The torque converter clutch enable valve  50  is operative to supply clutch control pressure in passage  86  to the clutch feed passage  88  when the valve is in the spring set position shown, and from the second line limit passage  62  to the clutch feed passage  88  when the valve spool  50 C is in the pressure set position as a result of a pressure signal in the port  50 E. 
     The sheave  166  of the pulley  140  has a dual chamber piston  160  that receives fluid pressure through the passage  74 . The pressure in the dual chamber piston  160  controls the force with which the sheave halves are held against the belt  146 . The output pulley  148  has a control piston  162  that includes a chamber  164  that is in fluid communication with the passage  54 . Therefore, fluid pressure in the passage  54  provides the force to hold the sheave halves of pulley  148  against the belt  146 . 
     The belt  146  is shown in two positions in FIG.  1 C. In the position where the portion  146 A of the belt  146  is at the extreme outboard end or diameter of the input pulley  140  is the overdrive position, and in the position where the portion  146 B of the belt  146  is at the extreme inner diameter of the input pulley  140  is the maximum underdrive condition. During transmission operation in forward or reverse, the pressure is applied to the control piston  160  to urge the belt  146  from the position  146 B toward the position  146 A. As the belt is moved outward on the input pulley, it is moved inward on the output pulley, thereby decreasing the speed of the input pulley relative to the output speed of the transmission  12 . 
     The pressure at the chamber  164  is determined by the primary pressure regulator valve  22  as a result of the VBS signal in passage  68 . The pressure at the piston  160  and passage  74  is determined by the ratio control mechanism  40  and is limited in its maximum amount by the primary feed limit valve  32 . The ratio control mechanism  40  has an output pressure that is proportional to the input request of the stepper motor  40 B and the position of the sheave follower  40 C. 
     As seen in FIG. 1C, the ratio control mechanism  40  is disposed on the transmission and the sheave follower  40 C is actuated by the sheave halve  166  of the input pulley  140 . As the stepper motor  40 B requests a ratio change, the valve  40 A will affect the primary feed pressure in passage  72  accordingly, thereby changing the pressure in piston  160 , such that the sheave  166  will be moved in one direction or the other depending on the pressure change and the sheave follower  40 C will move the lever control  401  to return the valve spool  40 F to a position wherein the pressure in the passage  54  is metered to the primary feed passage  72  at a level that maintains the desired ratio. 
     The pressure regulator valve  22  operates in three modes: a primary mode, a secondary mode, and a priority mode. During the primary mode, fluid flow in pump  14  is delivered from port  18  to passage  54  where it enters through the port  22 E to a differential area  22 K on the valve spool  22 C. The fluid pressure operating on the differential area  22 K operates in opposition to the variable bleed solenoid signal in passage  68  and the force in spring  22 D to move the valve spool  22 C leftward, such that the passage  56  is opened through port  22 G to the suction port  22 I, thereby returning the fluid to the suction port  16  of the pump  14  through the passage  57 . Thus, the flow from the port  20  of the pump  14  is passed directly back to suction and therefore does not create any resistance or energy absorption in the pump  14 . During this operation, a portion of the inlet of the fluid in passage  54  is directed through the regulator valve  22  and out one of the ports  22 F to the passage  58  where it is directed to the secondary pressure regulator valve  24 . 
     During the secondary mode, the system pressure requirements and flow requirements are sufficiently high so that the valve spool  22 C is closed to port  22 G such that output flow from the port  20  will pass through a pump switching ball  170  to the passage  54  which is then utilized in the differential area  22 K to counteract the VBS signal pressure at port  22 H and the force in spring  22 D. During this operation, a portion of the inlet of the fluid in passage  54  is directed through the regulator valve  22  out one of the ports  22 F to the passage  58  where it is directed to the secondary pressure regulator valve  24 . 
     During the priority mode of operation, the valve spool  22 C is moved sufficiently to the right due to the pressure in port  22 H and the force in spring  22 D, such that the passage  54  is disconnected from one of the ports  22 F and is passed through a restriction  22 J to the passage  58  to maintain a minimum flow amount to the secondary regulatory valve  24 . The overpressure relief valve  28  protects against instantaneous or momentary spikes of pressure which occur when the valve spool  22 C does not respond quickly enough to affect the change in system pressure when transient conditions or shift conditions occur quickly. 
     The pressure regulator valve  24  controls a pressure in line  60  which provides feed oil for the torque transmitting mechanisms, the torque converter, and the oil cooler. The pressure regulator valve  24  allows the pressure in passage  60  to be maintained at a lower value than the pressure in passage  54  during most driving conditions. The lower pressure reduces leakage which increases flow available for transient maneuvers and improves the fuel economy by allowing the secondary pump port  20  to be switched out of operation at an earlier time. 
     The valve  24  performs two functions. It regulates the pressure in passage  60  by modulating between port  24 E and exhaust  24 I until the force balance is achieved between pressure in passage  60 , the variable bleed solenoid signal in passage  68  and the valve spring  24 D. Secondly, the valve  24  forces the limited converter passage  64  feed oil to a lower priority than the oil in passage  60 . To reduce the number of components, the same variable bleed solenoid  26  is used to provide signal pressure to both valve  22  and valve  24 . 
     FIG. 2 shows that the variable bleed solenoid signal pressure required to provide adequate sheave torque at full overdrive ratio (OD) is less than the variable bleed signal pressure required to provide full underdrive ratio (UD). The pressure in passage  62 , which operates the torque transmitting mechanisms, is not a function of ratio but merely a function of input torque. Since the same signal passage  68  is used to modulate both valve  22  and valve  24 , valve  24  must reach pressure for full torque transmitting mechanism capacity at the variable bleed signal for input torque and the overdrive ratio. This means that line pressure in passage  60  continues to rise as the variable bleed solenoid signal rises above the value from maximum input torque at full overdrive ratio. 
     The valve  42  is positioned downstream of the regulator valve  24  to limit the maximum value of the pressure in the passage  62  to a value at or below the maximum acceptable value at the torque transmitting mechanisms and the torque converter components. This preserves the life of these units. The valve  42  modulates between ports  42 D and the exhaust until the force balance is achieved between the pressure in passage  62  acting on the end of the valve  42  and the spring  42 F. The resulting pressure in passage  62  is shown in FIG.  2 . 
     The valve  24  also provides a priority function for controlling the pressure in passage  64 , which is a limited converter feed oil, in order to maintain the pressure in passage  60  at an acceptable level. The pressure regulator valve  24  accomplishes this priority relationship by feeding the passage  64  with fluid from passage  62  only after the flow requirements for passage  60  have been achieved. This is provided by routing the feed path for passage  64  through the valve  24 . If sufficient pressure in passage  60  cannot be generated, the spring  24 D and the variable bleed solenoid pressure signal will push the valve spool  24 C to the right, stopping the flow of fluid in passage  60  to exhaust. If the pressure in passage  60  is still deficient, the valve spool  24 C will move further to the right until it restricts the port  24 G which supplies the limited torque converter feed passage  64 . The flow of fluid in the passage  64  will be reduced until the force balance is achieved or until port  24 G is fully closed. The orifice  66  is provided to ensure that passage  64  is never completely closed in order to provide some cooler flow under extreme or transient operating conditions. 
     The valve  34  functions to feed the solenoids. These solenoids are fed with the highest priority oil, that is the oil in passage  54 . Since the pressure in passage  54  is often greater than the maximum allowable at the solenoids, the valve  34  is included to limit the maximum pressure feeding solenoids to below the maximum value recommended by the manufacturer. 
     The line pressure transducer  30  provides two important functions. The line pressure transducer  30  is located in passage  58  between the valves  22  and  24  to perform these functions. First, the transducer  30  provides accurate feedback to the ECU regarding the actual pressure in passage  54 . This allows closed loop control of pressure in passage  54  resulting in improved fuel economy due to reduced pressure safety factors. Second, the placement of the transducer  30  in the passage  58  provides an accurate diagnostic signal when the transmission is operating while prioritizing the fluid distribution from passage  60 . Conditions that could cause this type of operation would include increased pump leakage or other circuit leakage. This signal could be used by the ECU to take diagnostic action, such as holding the ratio constant or increasing idle speed and other operating functions. 
     The clutch control valve  48  regulates the pressure in passage  86  when modulating between the port  48 D and an exhaust port until a force balance is achieved against the spring  48 H and the variable solenoid pressure at the port  48 F. 
     During a shifting event or interchange, pressure in passage  86  is modulated as a function of the area ratios of the clutch regulator valve  48 , the spring  48 H and the pressure of the variable signal solenoid in port  48 F. The area ratio is referred to as the valve gain. The area ratio referred to is the area at the end of the valve spool  48 C and the differential area presented to port  48 F. The larger the gain between the pressure in passage  86  and the pressure in passage  78 , the larger the clutch control pressure varies relative to changes and variations in the pressure in passage  78 . The variations in the pressure in passage  86  create variations in shift feel which might be unacceptable to the operator. 
     Conventional practice would size the clutch control valve  48  gain for maximum pressure requirements of the torque transmitting mechanism for its worse case of holding torque. Often, holding torques are much higher than shifting torques. During shifting torques, the clutch is slipping. Only while the clutch is slipping will errors in clutch pressure be manifested as unpleasant shift feel to the operator. Because of this, it is desirable to set the valve gain to be the smallest that will accomplish the shifting events. This leaves the problem of supplying additional pressure for the torque holding events. 
     This control system allows the gain of the clutch control valve  48  to be optimally sized for shifting events while providing boosted pressure for holding events. The boost valve  44  sends either exhaust or pressure in passage  82  to the spring end of the valve  48 , depending on the level of pressure in passage  62  acting on the end of the valve  44 . When the output of the valve  44  is connected with exhaust, the force balance takes place as normal and the clutch regulator pressure in passage  86  is a function of the gain of valve  48 , the pressure in passage  78 , and the force in spring  48 H. When the output of the valve  44  is equal to the pressure in passage  62 , the valve  48  is pushed to the open position and the port  48 D is opened to the port  48 E. The pressure in passage  86  is raised to the level of pressure in passage  62  during this event. The level of the pressure in passage  62  is independently set through the modulating controls of valve  24  to provide enough pressure for the holding event. 
     The valve  46  regulates a controlled pressure in passage  84  by modulating between a pressure at port  46 E and an exhaust port until the force balance is achieved against the spring  46 D and the pressure of the variable solenoid valve in port  46 G. During torque converter clutch operation, the pressure in passage  84  is modulated as a function of the area ratio of the regulator valve  46 , the spring  46 D and the variable bleed solenoid pressure at port  46 G. Using a variable bleed solenoid  36  instead of a standard or more conventional pulse-width-modulated solenoid provides a less noisy signal and has less variation changes in line pressure. The result is improved torque converter clutch operation. 
     The torque converter clutch valve  50  is a shift valve with two discreet positions: a spring set position and a pressure set position. The spring pushes the valve to its de-energized or spring set position. When the torque converter solenoid valve  38  is electrically activated, the signal in passage  80  is sent to the port  50 E pushing the valve spool against the spring  50 D into its energized or pressure set position. In de-energized or spring set position, the valve  50  feeds the passage  88  with pressure from passage  86 . The valve  50  also feeds the passage  90  with pressure from passage  64  and connects the passage  92  with an oil cooler circuit  172 . In this position, the valve also closes the port  50 G. In the energized or pressure set position, the valve  50  feeds the passage  88  with pressure from passage  62 , exhausts the passage  90  through an orifice  174 , feeds the passage  92  with fluid in the passage  84 , and connects the passage  64  with the oil cooler circuit  172 . 
     The valve  50  can supply modulated control pressure fluid to the passage  88  from the passage  86  when the valve  50  is in the de-energized position and application of the torque transmitting mechanism is required, but the torque converter clutch is to be released. An example of this would be using the pressure in passage  88  to engage one of the torque transmitting mechanisms of the transmission to begin a vehicle launch. The pressure in passage  86  is controlled by the pressure in the passage  78  which comes from the VBS valve  36 . Under such conditions, the torque converter clutch is required to be released in order to prevent stalling of the engine and to allow torque multiplication through the torque converter to improve launch performance. Since the open torque converter generates heat under such conditions, the return flow in the passage  92  is connected to the oil cooler circuit  172 . 
     When the valve  50  is in the energized or pressure set position, the valve  50  is used to apply the torque converter clutch. Pressure in passage  84  is directed to the pressure in passage  92 , which energizes the torque converter clutch. The pressure in passage  90  which is return oil from the torque converter clutch is exhausted through an orifice  174  to aid in the apply feel of the torque converter clutch. Since the solenoid valve  36  is being modulated according to the requirements of the torque converter clutch system, the pressure in passage  86  will be modulated at an incorrect level for the torque transmitting mechanism that has been engaged. For this reason, the pressure in passage  62  is connected with the passage  88 . Since the torque converter heat generated is minimal when the torque converter clutch is applied, the passage  64  bypasses the torque converter and is connected directly to the oil cooler circuit  172 . 
     Conventional practice would be to provide a separate regulator valve, control valve, on/off solenoid, and variable bleed solenoid valves for both torque transmitting mechanism control and torque converter clutch control. With the present system, these two criteria are met through the multiplexing of two systems such that a single set of valves will provide both functions. The malfunction mode protection and the modulation requirements for the two systems are very similar. The modulation modes are mutually exclusive since the torque converter clutch is not applied while the torque transmitting mechanism is being applied and vice versa. This allows the reuse of the VBS valve  36 , the solenoid valve  38 , and the valve  50  in order to reduce cost, manufacturing and assembly time, and also improve the overall reliability of the system. 
     The modulation requirements for the pressure in passages  92  and  88  are quite similar. Both require regulated pressures as a function of valve gain, spring load, and an electronically-controlled modulating pressure signal provided by the VBS valve  36 . Therefore, the reuse of the valving circuit during both torque converter clutch engagement and torque transmitting mechanism engagement is available. 
     The loss of function mode requirements are also very similar for the pressure in the passage  92  and the pressure in passage  88 . For the torque converter clutch system, it is not acceptable for a single element malfunction to occur which results in loss of converter flow to the torque converter with the torque converter clutch not applied, nor to stall the engine as a result of the torque converter clutch being applied at low vehicle speeds. The hardware required to satisfy these two requirements is an independent, electronically-controlled shift valve in series with an apply regulator valve. For the pressure in passage  88 , it is not acceptable for a single element malfunction to result in a loss of ability to apply the torque transmitting mechanism. The hardware required to satisfy this requirement is an independent electronically-controlled shift valve in series with a clutch regulator valve. This protection is provided in both instances by the valves  50 ,  38 , and  36 . In addition, the line activated boost valve  44  provides independent protection. 
     If the electrical system should become inoperable, the valve  50  will assume the spring set position as established by the spring  50 D. The clutch control pressure in the passage  86  will be maintained at a maximum value as established by the spring  48 H and the bias pressure in the passage as applied at the port  48 G. The valve  50  will deliver the pressurized fluid in the passage  86  to the passage  88  which is communicated with the valve  52  which in turn will distribute pressurized fluid to the torque transmitting mechanism that has been selected by the operator. The system pressures as established by the regulator valves  22  and  24  will be at a maximum value, and the ratio of the CVU  110  will remain unchanged. This will permit the operator to drive the vehicle to a repair station where the electrical function can be restored.