Patent Publication Number: US-8522946-B2

Title: Hydraulic control device

Description:
INCORPORATION BY REFERENCE 
     The disclosures of Japanese Patent Application Nos. 2010-113272 and 2011-073683 filed on May 17, 2010 and Mar. 29, 2011, respectively, including the specifications, drawings and abstracts are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a hydraulic control device that controls a hydraulic pressure to be supplied to a hydraulically driven friction engagement element in an automatic transmission that changes between shift speeds by switching an engagement state of the friction engagement element. 
     DESCRIPTION OF THE RELATED ART 
     An example of this type of hydraulic control device proposed in the related art establishes a specific shift speed when an electrical failure occurs and all solenoids are de-energized (see Japanese Patent Application Publication. No. JP-A-2005-265101, for example). The device includes normally-closed solenoid valves SL 1 , SL 2 , and SL 4  and a sequence valve that when a shift lever is in a drive (D) range, connects the solenoid valve SL 1  to a C 1  clutch, the solenoid valve SL 2  to a C 2  clutch, and the solenoid valve SL 4  to a B 2  brake during normal times, and connects a D-range oil passage of a manual valve, which outputs a line pressure when the shift lever is in the D range, to respective servos for either the C 1  clutch or the C 2  clutch and the B 2  brake during a failure. Consequently, the vehicle can travel continuously with either a third gear or a fifth gear established even during a failure. 
     SUMMARY OF THE INVENTION 
     While a specific shift speed may be established to allow the vehicle to travel continuously using the configuration described above during an electrical failure, it may be preferable to establish a neutral state during other types of failures. In this case, it is desirable to establish the neutral state more reliably and efficiently, because the establishment of the neutral state can also cope with the failures. 
     It is a main object of a hydraulic control device according to the present invention to establish the neutral state more reliably and efficiently. 
     In order to achieve the foregoing main object, the hydraulic control device according to the present invention adopts the following means. 
     A first aspect of the present invention provides a hydraulic control device that controls a hydraulic pressure to be supplied to a hydraulic servo for a hydraulically driven friction engagement element in an automatic transmission that changes between shift speeds by switching an engagement state of the friction engagement element, including: 
     a pump that generates a hydraulic pressure; 
     a first pressure regulation mechanism that includes a normally-open solenoid and that regulates the hydraulic pressure from the pump to generate a line pressure; 
     a second pressure regulation mechanism that includes a normally-closed solenoid and that receives and regulates the line pressure to output the regulated pressure; 
     a signal pressure output mechanism that includes a normally-closed solenoid to output a signal pressure; and 
     a switching mechanism that is connected to oil passages for the respective mechanisms and an oil passage for the hydraulic servo and that includes a signal pressure input oil passage to which at least the signal pressure from the signal pressure output mechanism can be input, the switching mechanism allowing communication between the oil passage for the first pressure regulation mechanism and the oil passage for the hydraulic servo and blocking communication between the oil passage for the second pressure regulation mechanism and the oil passage for the hydraulic servo when the signal pressure is not input to the signal pressure input oil passage, and blocking communication between the oil passage for the first pressure regulation mechanism and the oil passage for the hydraulic servo and allowing communication between the oil passage for the second pressure regulation mechanism and the oil passage for the hydraulic servo when the signal pressure is input to the signal pressure input oil passage. 
     The hydraulic control device according to the first aspect of the present invention includes the first pressure regulation mechanism that includes a normally-open solenoid and that regulates the hydraulic pressure from the pump to generate a line pressure, the second pressure regulation mechanism that includes a normally-closed solenoid and that receives and regulates the line pressure to output the regulated pressure, the signal pressure output mechanism that includes a normally-closed solenoid to output a signal pressure, and the switching mechanism that includes a signal pressure input oil passage to which at least the signal pressure from the signal pressure output mechanism can be input, the switching mechanism allowing communication between the oil passage for the first pressure regulation mechanism and the oil passage for the hydraulic servo for the friction engagement element and blocking communication between the oil passage for the second pressure regulation mechanism and the oil passage for the hydraulic servo when the signal pressure is not input to the signal pressure input oil passage, and blocking communication between the oil passage for the first pressure regulation mechanism and the oil passage for the hydraulic servo and allowing communication between the oil passage for the second pressure regulation mechanism and the oil passage for the hydraulic servo when the signal pressure is input to the signal pressure input oil passage. Consequently, a predetermined shift speed can be established by supplying the line pressure from the first pressure regulation mechanism to the hydraulic servo for the friction engagement element when the various solenoids are not energized, and a neutral state can be established by supplying no hydraulic pressure to the hydraulic servo for the friction engagement element when only the solenoid of the signal pressure output mechanism, of the various solenoids, is energized. As a result, the neutral state can be established more reliably and efficiently. In this case, the hydraulic pressure from the second pressure regulation mechanism may further be input to the signal pressure input oil passage as the signal pressure. 
     According to a second aspect of the present invention, in the hydraulic control device, the switching mechanism may include a first switching valve that includes a signal pressure input oil passage, the first switching valve switching between a state in which communication between the oil passage for the first pressure regulation mechanism and the oil passage for the hydraulic servo is allowed and communication between the oil passage for the second pressure regulation mechanism and the oil passage for the hydraulic servo is blocked and a state in which communication between the oil passage for the first pressure regulation mechanism and the oil passage for the hydraulic servo is blocked and communication between the oil passage for the second pressure regulation mechanism and the oil passage for the hydraulic servo is allowed, and a second switching valve that includes a signal pressure input oil passage connected to the oil passage for the second pressure regulation mechanism, the second switching valve allowing communication between the oil passage for the first pressure regulation mechanism and the signal pressure input oil passage of the first switching valve when a signal pressure from the second pressure regulation mechanism is input, and blocking communication between the oil passage for the first pressure regulation mechanism and the signal pressure input oil passage of the first switching valve and allowing communication between the oil passage for the signal pressure output mechanism and the signal pressure input oil passage of the first switching valve when the signal pressure from the second pressure regulation mechanism is not input. 
     According to a third aspect of the present invention, in the hydraulic control device, the switching mechanism may include a dedicated solenoid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic configuration of an automobile  10 ; 
         FIG. 2  shows an operation table of a speed change mechanism  30 ; 
         FIG. 3  illustrates the relationship between the respective rotational speeds of various rotary elements of the speed change mechanism  30  for various shift speeds; 
         FIG. 4  shows a schematic configuration of a hydraulic circuit  40 ; 
         FIG. 5  shows a schematic configuration of the hydraulic circuit  40 , centered on a primary regulator valve  44  and a secondary regulator valve  45 ; 
         FIG. 6  shows an exemplary relationship among a current Islt applied to a linear solenoid valve SLT (an electromagnetic coil), an engine speed Ne, and a flow rate Q of hydraulic oil supplied to a cooler and a portion to be lubricated; 
         FIG. 7  is a block diagram showing functional blocks of an AT ECU  90 ; 
         FIG. 8  is a flowchart showing an exemplary main control section process routine executed by a main control section  92 ; and 
         FIG. 9  is a flowchart showing an exemplary monitoring section process routine executed by a monitoring section  94 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will be described below. 
       FIG. 1  shows a schematic configuration of an automobile  10 .  FIG. 2  shows an operation table of a speed change mechanism  30 . 
     As shown in  FIG. 1 , the automobile  10  includes an engine  12 , an engine electronic control unit (engine ECU)  16 , an automatic transmission  20 , an automatic transmission electronic control unit (AT ECU)  90 , and a main electronic control unit (main ECU)  80 . The engine  12  is an internal combustion engine that outputs power generated by explosive combustion of a hydrocarbon fuel such as gasoline and light oil. The engine ECU  16  receives an operating state of the engine  12  from various sensors such as a crank angle sensor  18  that detects a crank angle to control an operation of the engine  12 . The automatic transmission  20  is connected to a crankshaft  14  of the engine  12  and to axles  18   a  and  18   b  for left and right wheels  19   a  and  19   b  to transmit power from the engine  12  to the axles  18   a  and  18   b . The AT ECU  90  controls the automatic transmission  20 . The main ECU  80  controls the entire vehicle. The main ECU  80  receives inputs of a shift position SP from a shift position sensor  82  that detects the operation position of a shift lever  81 , an accelerator operation amount Acc from an accelerator pedal position sensor  84  that detects the depression amount of an accelerator pedal  83 , a brake switch signal BSW from a brake switch  86  that detects depression of a brake pedal  85 , a vehicle speed V from a vehicle speed sensor  88 , and so forth, via an input port. In addition, the main ECU  80  is connected to the engine ECU  16  and the AT ECU  90  to be discussed later via a communication port to exchange various control signals and data with the engine ECU  16  and the AT ECU  90 . 
     As shown in  FIG. 1 , the automatic transmission  20  includes a torque converter  24 , the stepped speed change mechanism  30 , and a hydraulic circuit  40  (see  FIG. 4 ). The torque converter  24  is provided with a lock-up clutch including a pump impeller  24   a  on the input side connected to the crankshaft  14  of the engine  12  and a turbine runner  24   b  on the output side. The stepped speed change mechanism  30  includes an input shaft  21  connected to the turbine runner  24   b  of the torque converter  24  and an output shaft  22  connected to the axles  18   a  and  18   b  via a gear mechanism  26  and a differential gear  28 , and outputs to the output shaft  22  power input to the input shaft  21  while changing the speed. The hydraulic circuit  40  serves as an actuator that drives the speed change mechanism  30 . In the embodiment, the hydraulic circuit  40  and the AT ECU  90  correspond to the hydraulic control device. In the embodiment, the torque converter  24  is interposed between the crankshaft  14  of the engine  12  and the speed change mechanism  30 . However, the present invention is not limited thereto, and various starting devices may be adopted. 
     The speed change mechanism  30  is formed as a 6-speed stepped speed change mechanism, and includes a single-pinion type planetary gear mechanism, a Ravigneaux type planetary gear mechanism, three clutches C 1 , C 2 , and C 3 , two brakes B 1  and  82 , and a one-way clutch F 1 . The single-pinion type planetary gear mechanism includes a sun gear  31  which is an externally toothed gear, a ring gear  32  which is an internally toothed gear disposed concentrically with the sun gear  31 , a plurality of pinion gears  33  meshed with the sun gear  31  and meshed with the ring gear  32 , and a carrier  34  that rotatably and revolvably holds the plurality of pinion gears  33 . The sun gear  31  is fixed to a case. The ring gear  32  is connected to the input shaft  21 . The Ravigneaux type planetary gear mechanism includes two sun gears  36   a  and  36   b  which are each an externally toothed gear, a ring gear  37  which is an internally toothed gear, a plurality of short pinion gears  38   a  meshed with the sun gear  36   a , a plurality of long pinion gears  38   b  meshed with the sun gear  36   b  and the plurality of short pinion gears  38   a  and meshed with the ring gear  37 , and a carrier  39  that couples the plurality of short pinion gears  38   a  and the plurality of long pinion gears  38   b  to each other and that rotatably and revolvably holds the gears  38   a  and the gears  38   b . The sun gear  36   a  is connected to the carrier  34  of the single-pinion type planetary gear mechanism via the clutch C 1 . The sun gear  36   b  is connected to the carrier  34  via the clutch C 3 , and connected to the case via the brake B 1 . The ring gear  37  is connected to the output shaft  22 . The carrier  39  is connected to the input shaft  21  via the clutch C 2 . The carrier  39  is also connected to the case via the one-way clutch F 1 , and connected to the case via the B 2  which is provided in parallel with the one-way clutch F 1 . 
     As shown in  FIG. 2 , the speed change mechanism  30  can switchably establish first to sixth forward speeds, a reverse speed, and a neutral state by turning on and off (engaging and disengaging) the clutches C 1  to C 3  and turning on and off the brakes B 1  and B 2  in combination. The reverse speed can be established by turning on the clutch C 3  and the brake B 2  and turning off the clutches C 1  and C 2  and the brake B 1 . The first forward speed can be established by turning on the clutch C 1  and turning off the clutches C 2  and C 3  and the brakes B 1  and B 2 . When the engine brake is in operation, the first forward speed can be established with the brake B 2  turned on. The second forward speed can be established by turning on the clutch C 1  and the brake B 1  and turning off the clutches C 2  and C 3  and the brake B 2 . The third forward speed can be established by turning on the clutches C 1  and C 3  and turning off the clutch C 2  and the brakes B 1  and B 2 . The fourth forward speed can be established by turning on the clutches C 1  and C 2  and turning off the clutch C 3  and the brakes B 1  and B 2 . The fifth forward speed can be established by turning on the clutches C 2  and C 3  and turning off the clutch C 1  and the brakes B 1  and B 2 . The sixth forward speed can be established by turning on the clutch C 2  and the brake B 1  and turning off the clutches C 1  and C 3  and the brake B 2 . The neutral state can be established by turning off all the clutches C 1  to C 3  and the brakes B 1  and B 2 .  FIG. 3  illustrates the relationship between the respective rotational speeds of the various rotary elements of the speed change mechanism  30  for the various shift speeds. In the drawing, the S 1  axis represents the rotational speed of the sun gear  33 , the CR 1  axis represents the rotational speed of the carrier  34 , the R 1  axis represents the rotational speed of the ring gear  32 , the S 2  axis represents the rotational speed of the sun gear  36   b , the S 3  axis represents the rotational speed of the sun gear  36   a , the CR 2  axis represents the rotational speed of the carrier  39 , and the R 2  axis represents the rotational speed of the ring gear  37 . 
     The clutches C 1  to C 3  and the brakes B 1  and B 2  in the speed change mechanism  30  are turned on and off (engaged and disengaged) by the hydraulic circuit  40  shown in  FIGS. 4 and 5 . As shown in  FIG. 4 , the hydraulic circuit  40  is formed by components including: a mechanical oil pump  42 ; a primary regulator valve  44 ; a linear solenoid valve SLT; a manual valve  46 ; a linear solenoid valve SL 1 ; a linear solenoid valve SL 2 ; a linear solenoid valve SL 3 ; a linear solenoid valve SL 5 ; a first clutch application relay valve  50 ; a second clutch application relay valve  55 ; a first solenoid relay valve  60  and a second solenoid relay valve  65 ; a C 3 -B 2  application control valve  70 ; a B 2  application control valve  75 ; a first on/off solenoid valve S 1 ; and a second on/off solenoid valve S 2 . The mechanical oil pump  42  sucks hydraulic oil via a strainer  41  and pumps the hydraulic oil into a line pressure oil passage  43  using power from the engine  12 . The primary regulator valve  44  regulates the pressure of the hydraulic oil pumped from the mechanical oil pump  42  to generate a line pressure PL. The linear solenoid valve SLT drives the primary regulator valve  44  by regulating the line pressure PL to generate a modulator pressure PMOD via a modulator valve (not shown) to output the modulator pressure PMOD as a signal pressure. The manual valve  46  is formed with an input port  46   a  that receives the line pressure PL, a D (drive)-position output port  46   b , and a R (reverse)-position output port  46   c . The manual valve  46  allows communication between the input port  46   a  and the D-position output port  46   b  and blocks communication between the input port  46   a  and the R-position output port  46   c  when the shift lever  81  is operated to the D position. Further, the manual valve  46  blocks communication between the input port  46   a  and the D-position output port  46   b  and allows communication between the input port  46   a  and the R-position output port  46   c  when the shift lever  81  is operated to the R position. The manual valve  46  blocks communication between the input port  46   a  and the D-position output port  46   h  and communication between the input port  46   a  and the R-position output port  46   c  when the shift lever  81  is operated to the N position. The linear solenoid valve SL 1  receives and regulates a drive pressure PD which is a pressure output from the D-position output port  46   b  to output the regulated pressure. The linear solenoid valve SL 2  receives and regulates the drive pressure PD to output the regulated pressure. The linear solenoid valve SL 3  receives and regulates the line pressure PL from the line pressure oil passage  43  to output the regulated pressure. The linear solenoid valve SL 5  receives and regulates the drive pressure PD to output the regulated pressure to the brake B 1 . The first clutch application relay valve  50  selectively switches between a normal mode, in which an SL 1  pressure which is a pressure output from the linear solenoid valve SL 1  is supplied to the clutch C 1 , an SL 2  pressure which is a pressure output from the linear solenoid valve SL 2  is supplied to the clutch C 2 , and an SL 3  pressure which is a pressure output from the linear solenoid valve SL 3  is supplied to one of the clutch C 3  and the brake B 2 , and a fail-safe mode, in which the drive pressure PD is supplied to one of the clutch C 1  and the clutch C 2  and the line pressure PL is supplied to the clutch C 3 . The second clutch application relay valve  55  switches between a third forward speed mode, in which the drive pressure PD is supplied to the clutch C 1  and the line pressure PL is supplied to the clutch C 3 , and a fifth forward speed mode, in which the drive pressure PD is supplied to the clutch C 2  and the line pressure PL is supplied to the clutch C 3 , when the first clutch application relay valve  50  is in the fail-safe mode. The first solenoid relay valve  60  and the second solenoid relay valve  65  output the modulator pressure PMOD to switch between the modes (the normal mode and the fail-safe mode) of the first clutch application relay valve  50 . The C 3 -B 2  application control valve  70  switches among a mode in which the SL 3  pressure is supplied to the clutch C 3 , a mode in which the line pressure PL is supplied to the clutch C 3  and a reverse pressure PR which is a pressure output from the R-position output port  46   c  is supplied to the brake B 2 , a mode in which the reverse pressure PR is supplied to the clutch C 3  and the brake B 2 , and a mode in which the SL 3  pressure is supplied to the brake B 2 . The B 2  application control valve  75  switches among a mode in which the SL 3  pressure from the C 3 -B 2  application control valve  70  is supplied to the brake B 2 , a mode in which the reverse pressure PR is supplied to the brake B 2 , and a mode in which the hydraulic pressure acting on the brake B 2  is drained. The first on/off solenoid valve S 1  drives the second solenoid relay valve  65  and the C 3 -B 2  application control valve  70 . The second on/off solenoid valve S 2  outputs a signal pressure (S 2  pressure) for switching between the modes of the first clutch application relay valve  50  via the first solenoid relay valve  60  and the second solenoid relay valve  65  instead of the modulator pressure PMOD. In the embodiment, only the linear solenoid valve SLT, of the solenoid valves SLT, SL 1  to SL 3 , SL 5 , S 1 , and S 2 , is formed as a normally-open solenoid valve which is open when a solenoid coil of the solenoid valve is de-energized, and the other solenoid valves SL 1  to SL 3 , SL 5 , S 1 , and S 2  are each formed as a normally-closed solenoid valve which is closed when a solenoid coil of the solenoid valve is de-energized. 
     As shown in  FIG. 4 , the first clutch application relay valve  50  includes a sleeve  51  formed with various ports, a spool  52  that slides in the sleeve  51  to allow and block communication between the various ports, and a spring  53  that presses an end surface of the spool  52 . The various ports formed in the sleeve  51  include: a first signal pressure port  51   a  that receives the modulator pressure PMOD and the S 2  pressure from the first solenoid relay valve  60  as a signal pressure for pressing an end surface of the spool  52  in the same direction as the urging force of the spring  53 ; a second signal pressure port  51   b  that inputs the modulator pressure PMOD to a space between lands of the spool  52  with different diameters as a signal pressure; an input port  51   c  that receives the line pressure PL; an output port  51   d  connected to a C 3 -B 2  communication oil passage  54  coupled to the C 3 -B 2  application control valve  70 ; an input port  51   e  that receives the SL 3  pressure of the linear solenoid valve SL 3 ; an input port  51   f  connected to a fifth forward speed communication oil passage  59   a  coupled to the second clutch application relay valve  55 ; an output port  51   g  connected to the clutch C 2  (hydraulic servo); an input port  51   h  that receives the SL 2  pressure of the linear solenoid valve SL 2 ; an output port  51   i  connected to a third forward speed communication oil passage  59   b  coupled to the second clutch application relay valve  55 ; an output port  51   j  connected to the clutch C 1  (hydraulic servo); an input port  51   k  that receives the SL 1  pressure of the linear solenoid valve SL 1 ; and a third signal pressure port  51   l  that receives an SLT pressure which is a pressure output from the linear solenoid valve SLT as a signal pressure for pressing an end surface of the spool  52  in the opposite direction to the urging force of the spring  53 . 
     In the first clutch application relay valve  50 , the signal pressure input to the second signal pressure port  51   b  presses the spool  52  in the same direction as the urging force of the spring  53  because of a pressure difference due to the difference in diameter between lands (difference in pressure receiving area). The spool  52  is moved in accordance with the balance relationship among the urging force of the spring  53 , a force produced by the signal pressure input to the first signal pressure port  51   a  to press the spool  52  in the same direction as the urging force of the spring  53 , a force produced by the signal pressure input to the second signal pressure port  51   b  to press the spool  52  in the same direction as the urging force of the spring  53 , and a force produced by the signal pressure input to the third signal pressure port  51   l  to press the spool  52  in the opposite direction to the urging force of the spring  53 . In the balance relationship among the forces, the modulator pressure PMOD is input to the second signal pressure port  51   b  at all times, and the signal pressure from the linear solenoid valve SLT for driving the primary regulator valve  44  is input to the third signal pressure port  51   l  at all times. Therefore, when the modulator pressure PMOD or the S 2  pressure is not input to the first signal pressure port  51   a , the pressing force from the third signal pressure port  51   l  exceeds the resultant force of the urging force of the spring  53  and the pressing force from the second signal pressure port  51   b  to move the spool  52  in the direction of contracting the spring  53  (toward the position indicated on the left half when  FIG. 4  is seen sideways). At this time, communication between the input port  51   e  on the linear solenoid valve SL 3  side and the output port  51   d  on the C 3 -B 2  communication oil passage  54  side is blocked, communication between the input port  51   h  on the linear solenoid valve SL 2  side and the output port  51   g  on the clutch C 2  side is blocked, communication between the input port  51   k  on the linear solenoid valve SL 1  side and the output port  51   j  on the clutch C 1  side is blocked, communication between the input port  51   f  on the fifth forward speed communication oil passage  59   a  side and the output port  51   g  on the clutch C 2  side is allowed, and communication between the input port  51   i  on the third forward speed communication oil passage  59   b  side and the output port  51   j  on the clutch C 1  side is allowed. On the other hand, when the modulator pressure PMOD or the S 2  pressure is input to the first signal pressure port  51   a , the resultant force of the urging force of the spring  53 , the pressing force from the first signal pressure port  51   a , and the pressing force from the second signal pressure port  51   b  exceeds the pressing force from the third signal pressure port  51   l  to move the spool  52  in the direction of expanding the spring  53  (toward the position indicated on the right half when  FIG. 4  is seen sideways). At this time, communication between the input port  51   e  on the linear solenoid valve SL 3  side and the output port  51   d  on the C 3 -B 2  communication oil passage  54  side is allowed, communication between the input port  51   h  on the linear solenoid valve SL 2  side and the output port  51   g  on the clutch C 2  side is allowed, communication between the input port  51   k  on the linear solenoid valve SL 1  side and the output port  51   j  on the clutch C 1  side is allowed, communication between the input port  51   f  on the fifth forward speed communication oil passage  59   a  side and the output port  51   g  on the clutch C 2  side is blocked, and communication between the input port  51   i  on the third forward speed communication oil passage  59   b  side and the output port  51   j  on the clutch C 1  side is blocked. 
     As shown in  FIG. 4 , the second clutch application relay valve  55  includes a sleeve  56  formed with various ports, a spool  57  that slides in the sleeve  56  to allow and block communication between the various ports, and a spring  58  that presses an end surface of the spool  57 . The various ports formed in the sleeve  56  include: a first signal pressure port  56   a  that receives the SL 2  pressure of the solenoid valve SL 2  as a signal pressure for pressing an end surface of the spool  57  in the opposite direction to the urging force of the spring  58 ; an output port  56   b  connected to the fifth forward speed communication oil passage  59   a ; a second signal pressure port  56   c  connected to the fifth forward speed communication oil passage  59   a  to input a hydraulic pressure in the oil passage  59   a  to a space between lands of the spool  57  with different diameters as a signal pressure; an output port  56   d  connected to the third forward speed communication oil passage  59   b ; an input port  56   e  that receives the drive pressure PD; a drain port  56   f ; an input port  56   g  that receives the modulator pressure PMOD; an output port  56   h  connected to a communication oil passage  59   c  coupled to the B 2  application control valve  75 ; and a third signal pressure port  56   i  that receives the S 2  pressure of the second on/off solenoid valve S 2  as a signal pressure for pressing an end surface of the spool  57  in the same direction as the urging force of the spring  58 . 
     In the second clutch application relay valve  55 , the signal pressure input to the second signal pressure port  56   c  presses the spool  57  in the opposite direction to the urging force of the spring  58  because of a pressure difference due to the difference in diameter between lands (difference in pressure receiving area). The spool  57  is moved in accordance with the balance relationship among the urging force of the spring  58 , a force produced by the signal pressure input to the first signal pressure port  56   a  to press the spool  57  in the opposite direction to the urging force of the spring  58 , a force produced by the signal pressure input to the second signal pressure port  56   c  to press the spool  57  in the opposite direction to the urging force of the spring  58 , and a force produced by the signal pressure input to the third signal pressure port  56   i  to press the spool  57  in the same direction as the urging force of the spring  58 . When the SL 2  pressure of the linear solenoid valve SL 2  is not input to the first signal pressure port  56   a , the urging force of the spring  58  moves the spool  57  in the direction of expanding the spring  58  (toward the position indicated on the left half when  FIG. 4  is seen sideways). At this time, communication between the input port  56   e  on the drive pressure PD side and the output port  56   d  on the third forward speed communication oil passage  59   b  side is allowed, and communication between the input port  56   c  on the drive pressure PD side and the fifth forward speed communication oil passage  59   a  is blocked. On the other hand, when the SL 2  pressure of the linear solenoid valve SL 2  is input to the first signal pressure port  56   a , the pressing force from the first signal pressure port  56   a  exceeds the urging force of the spring  58  to move the spool  57  in the direction of contracting the spring  58  (toward the position indicated on the right half when  FIG. 4  is seen sideways). At this time, communication between the input port  56   e  on the drive pressure PD side and the output port  56   d  on the third forward speed communication oil passage  59   b  side is blocked, and communication between the input port  56   e  on the drive pressure PD side and the fifth forward speed communication oil passage  59   a  is allowed. Once the SL 2  pressure of the linear solenoid valve SL 2  is input to the first signal pressure port  56   a , the drive pressure PD introduced into the fifth forward speed communication oil passage  59   a  is input to the second signal pressure port  56   c  via the input port  56   e  and the output port  56   b  so that the drive pressure PD presses the spool  57  in the opposite direction to the urging force of the spring  58 . Therefore, even if the SL 2  pressure is canceled thereafter, the spool  57  is held at the same position. 
     As shown in  FIG. 4 , the first solenoid relay valve  60  includes a sleeve  61  formed with various ports, a spool  62  that slides in the sleeve  61  to allow and block communication between the various ports, and a spring  63  that presses an end surface of the spool  62 . The various ports formed in the sleeve  61  include: a first signal pressure port  61   a  that receives the SL 2  pressure of the linear solenoid valve SL 2  as a signal pressure for pressing an end surface of the spool  62  in the opposite direction to the urging force of the spring  63 ; a second signal pressure port  61   b  that inputs the SL 1  pressure of the solenoid valve SL 1  to a space between lands of the spool  62  with different diameters as a signal pressure; an input port  61   c  connected to a communication oil passage  64  coupled to the second solenoid relay valve  65 ; an output port  61   d  coupled to the first signal pressure port  51   a  of the first clutch application relay valve  50 ; an input port  61   e  that receives the modulator pressure PMOD; and a second signal pressure port  61   f  that receives a hydraulic pressure in the communication oil passage  64  as a signal pressure for pressing an end surface of the spool  62  in the same direction as the urging force of the spring  63 . 
     In the first solenoid relay valve  60 , the signal pressure input to the second signal pressure port  61   b  presses the spool  62  in the opposite direction to the urging force of the spring  63  because of a pressure difference due to the difference in diameter between lands of the spool  62  (difference in pressure receiving area). The spool  62  is moved in accordance with the balance relationship among the urging force of the spring  63 , a force produced by the signal pressure input to the first signal pressure port  61   a  to press the spool  62  in the opposite direction to the urging force of the spring  63 , a force produced by the signal pressure input to the second signal pressure port  61   b  to press the spool  62  in the opposite direction to the urging force of the spring  63 , and a force produced by the signal pressure input to the third signal pressure port  61   f  to press the spool  62  in the same direction as the urging force of the spring  63 . When the SL 2  pressure of the linear solenoid valve SL 2  is not input to the first signal pressure port  61   a  and the SL 1  pressure of the linear solenoid valve SL 1  is not input to the second signal pressure port  61   b  either, the urging force of the spring  63  moves the spool  62  in the direction of expanding the spring  63  (toward the position indicated on the left half when  FIG. 4  is seen sideways). At this time, communication between the input port  61   e  on the modulator pressure PMOD side and the output port  61   d  on the side of the first signal pressure port  51   a  of the first clutch application relay valve  50  is blocked, and communication between the input port  61   c  on the communication oil passage  64  side and the output port  61   d  on the side of the first signal pressure port  51   a  of the first clutch application relay valve  50  is allowed. On the other hand, when the SL 2  pressure of the linear solenoid valve SL 2  is input to the first signal pressure port  61   a  or the SL 1  pressure of the linear solenoid valve SL 1  is input to the second signal pressure port  61   b , the pressing force of the SL 1  pressure or the pressing force of the SL 2  pressure exceeds the urging force of the spring  63  to move the spool  62  in the direction of contracting the spring  63  (toward the position indicated on the right half when  FIG. 4  is seen sideways). At this time, communication between the input port  61   e  on the modulator pressure PMOD side and the output port  61   d  on the side of the first signal pressure port  51   a  of the first clutch application relay valve  50  is allowed, and communication between the input port  61   c  on the communication oil passage  64  side and the output port  61   d  on the side of the first signal pressure port  51   a  of the first clutch application relay valve  50  is blocked. 
     As shown in  FIG. 4 , the second solenoid relay valve  65  includes a sleeve  66  formed with various ports, a spool  67  that slides in the sleeve  66  to allow and block communication between the various ports, and a spring  68  that presses an end surface of the spool  67 . The various ports formed in the sleeve  66  include: a signal pressure port  66   a  that receives the S 1  pressure of the first on/off solenoid valve S 1  as a signal pressure for pressing an end surface of the spool  67  in the opposite direction to the urging force of the spring  68 ; an input port  66   b  coupled to the output port  61   d  of the first solenoid relay valve  60 ; an output port  66   c  connected to a communication oil passage  69  coupled to the B 2  application control valve  75 ; an input port  66   d  that receives the reverse pressure PR; an input port  66   e  that receives the S 2  pressure of the second on/off solenoid valve S 2 ; an output port  66   f  connected to the communication oil passage  64  coupled to the first solenoid relay valve  60 ; and an input port  66   g  that receives the modulator pressure PMOD. 
     In the second solenoid relay valve  65 , when the S 1  pressure of the first on/off solenoid valve S 1  is not input to the signal pressure port  66   a , the urging force of the spring  68  moves the spool  67  in the direction of expanding the spring  68  (toward the position indicated on the left half when  FIG. 4  is seen sideways). At this time, communication between the input port  66   b  on the side of the output port  61   d  of the first solenoid relay valve  60  and the output port  66   c  on the communication oil passage  69  side is allowed, communication between the input port  66   e  on the second on/off solenoid valve S 2  side and the output port  66   f  on the communication oil passage  64  side is allowed, and communication between the input port  66   g  on the modulator pressure PMOD side and the output port  66   f  is blocked. On the other hand, when the S 1  pressure of the first on/off solenoid valve S 1  is input to the signal pressure port  66   a , the pressing force of the S 1  pressure exceeds the urging force of the spring  68  to move the spool  67  in the direction of contracting the spring  68  (toward the position indicated on the right half when  FIG. 4  is seen sideways). At this time, communication between the input port  66   b  on the side of the output port  61   d  of the first solenoid relay valve  60  and the output port  66   c  on the communication oil passage  69  side is blocked, communication between the input port  66   e  on the second on/off solenoid valve S 2  side and the output port  66   f  on the communication oil passage  64  side is blocked, and communication between the input port  66   g  on the modulator pressure PMOD side and the output port  66   f  is allowed. 
     As shown in  FIG. 4 , the C 3 -B 2  application control valve  70  includes a sleeve  71  formed with various ports, a spool  72  that slides in the sleeve  71  to allow and block communication between the various ports, and a spring  73  that presses an end surface of the spool  72 . The various ports formed in the sleeve  71  include: a signal pressure port  71   a  that receives the S 1  pressure of the first on/off solenoid valve S 1  as a signal pressure for pressing an end surface of the spool  72  in the opposite direction to the urging force of the spring  73 ; an output port  71   b  connected to a first communication oil passage  74   a  coupled to the B 2  application control valve  75 ; an input port  71   c  that receives the reverse pressure PR; an output port  71   d  connected to a second communication oil passage  74   b  coupled to the B 2  application control valve  75 ; an input port  71   e  connected to the C 3 -B 2  communication oil passage  54  on the first clutch application relay valve  50  side; an output port  71   f  connected to the clutch C 3  (hydraulic servo); an input port  71   g  that receives the reverse pressure PR; and a drain port  71   h.    
     In the C 3 -B 2  application control valve  70 , when the S 1  pressure of the first on/off solenoid valve S 1  is not input to the signal pressure port  71   a , the urging force of the spring  73  moves the spool  72  in the direction of expanding the spring  73  (toward the position indicated on the left half when  FIG. 4  is seen sideways). At this time, communication between the output port  71   b  on the side of the first communication oil passage  74   a  coupled to the B 2  application control valve  75  and the drain port  71   h  is allowed, communication between the input port  71   c  on the reverse pressure PR side and the output port  71   b  on the first communication oil passage  74   a  side is blocked, communication between the input port  71   e  and the output port  71   d  on the side of the second communication oil passage  74   b  coupled to the B 2  application control valve  75  is allowed, communication between the input port  71   e  on the side of the C 3 -B 2  communication oil passage  54  coupled to the first clutch application relay valve  50  and the output port  71   d  on the second communication oil passage  74   b  side is blocked, communication between the input port  71   e  and the output port  71   f  on the clutch C 3  side is allowed, and communication between the input port  71   g  on the reverse pressure PR side and the output port  71   f  is blocked. On the other hand, when the S 1  pressure of the first on/off solenoid valve S 1  is input to the signal pressure port  71   a , the pressing force of the S 1  pressure exceeds the urging force of the spring  73  to move the spool  72  in the direction of contracting the spring  73  (toward the position indicated on the right half when  FIG. 4  is seen sideways). At this time, communication between the output port  71   b  on the side of the first communication oil passage  74   a  coupled to the B 2  application control valve  75  and the drain port  71   h  is blocked, communication between the input port  71   c  on the reverse pressure PR side and the output port  71   b  on the first communication oil passage  74   a  side is allowed, communication between the input port  71   e  and the output port  71   d  on the side of the second communication oil passage  74   b  coupled to the B 2  application control valve  75  is blocked, communication between the input port  71   e  on the side of the C 3 -B 2  communication oil passage  54  coupled to the first clutch application relay valve  50  and the output port  71   d  on the second communication oil passage  74   b  side is allowed, communication between the input port  71   e  and the output port  71   f  on the clutch C 3  side is blocked, and communication between the input port  71   g  on the reverse pressure PR side and the output port  71   f  is allowed. 
     As shown in  FIG. 4 , the B 2  application control valve  75  includes a sleeve  76  formed with various ports, a spool  77  that slides in the sleeve  76  to allow and block communication between the various ports, and a spring  78  that presses an end surface of the spool  77 . The various ports formed in the sleeve  76  include: a first signal pressure port  76   a  that receives a hydraulic pressure (the modulator pressure PMOD) from the output port  56   h  of the second clutch application relay valve  55  as a signal pressure for pressing an end surface of the spool  77  in the opposite direction to the urging force of the spring  78 ; a second signal pressure port  76   b  that inputs a pressure output from the output port  66   c  (the communication oil passage  69 ) of the second solenoid relay valve  65  to a space between lands of the spool  76  with different diameters as a signal pressure; an input port  76   c  connected to the second communication oil passage  74   b  coupled to the C 3 -B 2  application control valve  70 ; an output port  76   d  connected to the brake B 2  (hydraulic servo); and an input port  76   e  connected to the first communication oil passage  74   a  coupled to the C 3 -B 2  application control valve  70 . 
     In the B 2  application control valve  75 , when a signal pressure is input to none of the first signal pressure port  76   a  and the second signal pressure port  76   b , the urging force of the spring  78  moves the spool  77  in the direction of expanding the spring  78  (toward the position indicated on the left half when  FIG. 4  is seen sideways). At this time, communication between the input port  76   c  on the second communication oil passage  74   b  side and the output port  76   d  on the brake B 2  side is allowed, and communication between the input port  76   e  on the first communication oil passage  74   a  side and the output port  76   d  is blocked. On the other hand, when a signal pressure is input to either of the first signal pressure port  76   a  and the second signal pressure port  76   b , the pressing force of the signal pressure exceeds the urging force of the spring  78  to move the spool  77  in the direction of contracting the spring  78  (toward the position indicated on the right half when  FIG. 4  is seen sideways). At this time, communication between the input port  76   c  on the second communication oil passage  74   b  side and the output port  76   d  on the brake B 2  side is blocked, and communication between the input port  76   c  on the first communication oil passage  74   a  side and the output port  76   d  is allowed. 
     As shown in  FIG. 5 , the hydraulic circuit  40  includes a secondary regulator valve  45  provided at a stage later than the primary regulator valve  44 . The primary regulator valve  44  receives hydraulic oil pumped from the mechanical oil pump  42  at an input port  44   a  and outputs part of the received hydraulic oil from a secondary port  44   b , which is coupled to the torque converter  24 , and a drain port  44   c  to regulate the line pressure PL. The secondary regulator valve  45  receives hydraulic oil from the secondary port  44   b  at an input port  45   a  and outputs part of the received hydraulic oil from a cooling/lubrication port  45   b , which is coupled to a cooler (COOLER) and a portion to be lubricated (LUBE), and a drain port  45   c  to regulate a secondary pressure. The primary regulator valve  44  and the secondary regulator valve  45  are driven by the normally-open linear solenoid valve SLT.  FIG. 6  shows an exemplary relationship among a current Islt applied to the linear solenoid valve SLT (an electromagnetic coil), an engine speed Ne, and a flow rate Q of hydraulic oil supplied to the cooler and the portion to be lubricated. As shown, the flow rate Q of hydraulic oil supplied to the cooler and the portion to be lubricated tends to become higher as the engine speed Ne becomes higher and as the current Islt becomes lower. 
     In the thus configured hydraulic circuit  40 , the neutral state can be established by turning on the second on/off solenoid valve S 2 . The first forward speed can be established by turning on the linear solenoid valve SL 1 . When the engine brake is in operation, the first forward speed can be established by further turning on the first on/off solenoid valve S 1  and turning on the linear solenoid valve SL 3 . The second forward speed can be established by turning on the linear solenoid valves SL 1  and SL 5 . The third forward speed can be established by turning on the linear solenoid valves SL 1  and SL 3 . The fourth forward speed can be established by turning on the linear solenoid valves SL 1  and SL 2 . The fifth forward speed can be established by turning on the linear solenoid valves SL 2  and SL 3 . The sixth forward speed can be established by turning on the linear solenoid valves SL 2  and SL 5 . 
     Now, a case where the shift lever  81  is operated to the D (drive) position is considered. In this case, the vehicle is normally running using any of the first to sixth forward speeds. Thus, as shown in the engagement table of  FIG. 2 , the first solenoid relay valve  60  is driven by either the SL 1  pressure from the linear solenoid valve SL 1  or the SL 2  pressure from the linear solenoid valve SL 2 , and the modulator pressure PMOD is input to the first signal pressure port  51   a  of the first clutch application relay valve  50 . Therefore, the first clutch application relay valve  50  is switched to the normal mode, in which the linear solenoid valve SL 1  (output port) is connected to the clutch C 1  via the input port  51   k  and the output port  51   j  of the first clutch application relay valve  50 , the linear solenoid valve SL 2  (output port) is connected to the clutch C 2  via the input port  51   h  and the output port  51   g , and the linear solenoid valve SL 3  is connected to the C 3 -B 2  communication oil passage  54  via the input port  51   e  and the output port  51   d . When the first on/off solenoid valve S 1  is turned off, communication between the input port  71   e  of the C 3 -B 2  application control valve  70  connected to the C 3 -B 2  communication oil passage  54  and the output port  71   f  connected to the clutch C 3  is allowed. Thus, the linear solenoid valve SL 3  is connected to the clutch C 3  via the input port  51   e  and the output port  51   d  of the first clutch application relay valve  50 , the C 3 -B 2  communication oil passage  54 , and the input port  71   e  and the output port  71   f  of the C 3 -B 2  application control valve  70 . Thus, the vehicle can travel with any of the first to sixth forward speeds established by driving corresponding ones of the linear solenoid valves SL 1  to SL 5 . When the engine brake is in operation, the first on/off solenoid valve S 1  is turned on to allow communication between the input port  71   e  of the C 3 -B 2  application control valve  70  on the C 3 -B 2  communication oil passage  54  side and the output port  71   d  on the second communication oil passage  74   b  side instead of communication between the input port  71   e  and the output port  71   f  on the clutch C 3  side. In this state, the second communication oil passage  74   b  is connected to the brake B 2  via the input port  76   c  and the output port  76   d  of the B 2  application control valve  75 . Therefore, the linear solenoid valve SL 3  is connected to the brake B 2  instead of the clutch C 3 . Thus, the brake B 2  can be turned on by supplying the SL 3  pressure from the linear solenoid valve SL 3  to the brake B 2 . 
     A case where all the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  are de-energized with the shift lever  81  operated to the D position is considered. In this case, the linear solenoid valves SL 1  and SL 2  do not output the SL 1  pressure and the SL 2  pressure, respectively. Thus, communication between the input port  61   e , to which the modulator pressure PMOD is input, of the first solenoid relay valve  60  and the output port  61   d , which is coupled to the first signal pressure port  51   a  of the first clutch application relay valve  50 , is blocked so that the modulator pressure PMOD is not input to the first signal pressure port  51   a . Therefore, the first clutch application relay valve  50  is switched to the fail-safe mode, in which the third forward speed communication oil passage  59   b  is connected to the clutch C 1  via the input port  51   i  and the output port  51   j  of the first clutch application relay valve  50 , the fifth forward speed communication oil passage  59   a  is connected to the clutch C 2  via the input port  51   f  and the output port  51   g , and the line pressure oil passage  43  is connected to the C 3 -B 2  communication oil passage  54  via the input port  51   c  and the output port  51   d . Since the first on/off solenoid valve S 1  is turned off, communication between the input port  71   e , which is connected to the C 3 -B 2  communication oil passage  54 , of the C 3 -B 2  application control valve  70  and the output port  71   f  connected to the clutch C 3  is allowed, and the line pressure oil passage  43  is connected to the clutch C 3  via the input port  51   c  and the output port  51   d  of the first clutch application relay valve  50 , the C 3 -B 2  communication oil passage  54 , and the input port  71   e  and the output port  71   f  of the C 3 -B 2  application control valve  70 . In the second clutch application relay valve  55 , the drive pressure PD is supplied to the third forward speed communication oil passage  59   b  when the SL 2  pressure is not output from the linear solenoid valve SL 2 , and the drive pressure PD is output to the fifth forward speed communication oil passage  59   a  when the SL 2  pressure is output from the linear solenoid valve SL 2 . Thus, in the fail-safe mode, when the vehicle is running using any of the first to third forward speeds, the third forward speed is established with the drive pressure PD supplied from the third forward speed communication oil passage  59   b  to the clutch C 1  and the line pressure PL supplied to the clutch C 3 . Also, when the vehicle is running using any of the fourth to sixth forward speeds, the fifth forward speed is established with the drive pressure PD supplied from the fifth forward speed communication oil passage  59   a  to the clutch C 2  and the line pressure PL supplied to the clutch C 3 . 
     Further, a case where only the second on/off solenoid valve S 2 , of the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2 , is energized and the other solenoid valves are de-energized with the shift lever  81  operated to the D position is considered. Also in this case, the linear solenoid valves SL 1  and SL 2  do not output the SL 1  pressure and the SL 2  pressure, respectively. Thus, the modulator pressure PMOD is not input to the first signal pressure port  51   a  of the first clutch application relay valve  50  from the first solenoid relay valve  60 . In the first solenoid relay valve  60 , communication between the output port  61   d  connected to the first signal pressure port  51   a  of the first clutch application relay valve  50  and the input port  61   c  connected to the communication oil passage  64  is allowed. In the second solenoid relay valve  65 , since the S 1  pressure is not input to the signal pressure port  66   a  from the first on/off solenoid valve S 1 , communication between the output port  66   f  connected to the communication oil passage  64  and the input port  66   e  connected to the second on/off solenoid valve S 2  is allowed. Thus, the second on/off solenoid valve S 2  is connected to the first signal pressure port  51   a  of the first clutch application relay valve  50  via the input port  66   e  and the output port  66   f  of the second solenoid relay valve  65 , the communication oil passage  64 , and the input port  61   c  and the output port  61   d  of the first solenoid relay valve  60 , and the S 2  pressure from the second on/off solenoid valve S 2  can switch the first clutch application relay valve  50  to the normal mode in which supply of the drive pressure PD to the clutches C 1  and C 2  and supply of the line pressure PL to the clutch C 3  are allowed. At this time, since all the linear solenoid valves SL 1 , SL 2 , SL 3 , and SL 5  are turned off, all the clutches C 1  to C 3  and the brakes  131  and B 2  are turned off to establish the neutral state. In this way, with the shift lever  81  operated to the D position, the third or fifth forward speed can be established when all the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  are de-energized, and the neutral state can be established when only the second on/off solenoid valve S 2  is energized and the other solenoid valves are de-energized. 
     Next, a case where all the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  are de-energized with the shift lever  81  operated to the R (reverse) position is considered. In this case, in the second solenoid relay valve  65 , the spool  67  is moved to the position indicated on the left half when  FIG. 4  is seen sideways to block communication between the input port  66   g  to which the modulator pressure PMOD is input and the output port  66   f  connected to the communication oil passage  64 . In the first solenoid relay valve  60 , communication between the input port  61   e  to which the modulator pressure PMOD is input and the output port  61   d  coupled to the first signal pressure port  51   a  of the first clutch application relay valve  50  is blocked. Therefore, in the first clutch application relay valve  50 , the spool  52  is moved to the position indicated on the left half when  FIG. 4  is seen sideways to allow communication between the input port  51   c  to which the line pressure PL is input and the output port  51   d  connected to the C 3 -B 2  communication oil passage  54 . In the C 3 -B 2  application control valve  70 , the spool  72  is moved to the position indicated on the left half when  FIG. 4  is seen sideways to allow communication between the input port  71   e  connected to the C 3 -B 2  communication oil passage  54  and the output port  71   f  connected to the clutch C 3  and communication between the input port  71   c  to which the reverse pressure PR is input and the output port  71   d  connected to the second communication oil passage  74   b . In the B 2  application control valve  75 , communication between the input port  76   c  connected to the second communication oil passage  74   b  and the output port  76   d  connected to the brake B 2  is allowed. Thus, the line pressure PL is supplied to the clutch C 3  and the reverse pressure PR is supplied to the brake B 2  to establish the reverse speed. 
     A case where only the first on/off solenoid valve S 1 , of the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2 , is energized with the shift lever  81  operated to the R position is considered. In the second solenoid relay valve  65 , the spool  67  is moved to the position indicated on the right half when  FIG. 4  is seen sideways to allow communication between the input port  66   g  to which the modulator pressure PMOD is input and the output port  66   f  connected to the communication oil passage  64  and communication between the input port  66   d  to which the reverse pressure PR is input and the output port  66   c  connected to the communication oil passage  69 . In the first solenoid relay valve  60 , communication between the input port  61   c  connected to the communication oil passage  64  and the output port  61   d  connected to the first signal pressure port  51   a  of the first clutch application relay valve  50  is allowed. Therefore, in the first clutch application relay valve  50 , the spool  52  is moved to the position indicated on the right half when  FIG. 4  is seen sideways to block communication between the input port  51   c  to which the line pressure PL is input and the output port  51   d  connected to the C 3 -B 2  communication oil passage  54 . In the C 3 -B 2  application control valve  70 , the spool  72  is moved to the position indicated on the right half when  FIG. 4  is seen sideways to allow communication between the input port  71   c  to which the reverse pressure PR is input and the output port  71   b  connected to the first communication oil passage  74   a  and communication between the input port  71   g  to which the reverse pressure PR is input and the output port  71   f  connected to the clutch C 3 . In the B 2  application control valve  75 , the reverse pressure PR is input to the second signal pressure port  76   b  via the output port  66   d  and the output port  66   c  of the second solenoid relay valve  65  and the communication oil passage  69 . Therefore, the spool  77  is moved to the position indicated on the right half when  FIG. 4  is seen sideways to allow communication between the input port  76   e  connected to the first communication oil passage  74   a  and the output port  76   d  connected to the brake B 2 . Thus, the reverse pressure PR is supplied to the clutch C 3  and the brake B 2  to establish the reverse speed. 
     Further, a case where only the second on/off solenoid valve S 2 , of the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2 , is energized and the other solenoid valves are de-energized with the shift lever  81  operated to the R (reverse) position is considered. In this case, in the second solenoid relay valve  65 , the spool  67  is moved to the position indicated on the left half when  FIG. 4  is seen sideways to block communication between the input port  66   g  to which the modulator pressure PMOD is input and the output port  66   f  connected to the communication oil passage  64 . In the first solenoid relay valve  60 , communication between the input port  61   e  to which the modulator pressure PMOD is input and the output port  61   d  connected to the first signal pressure port  51   a  of the first clutch application relay valve  50  is blocked. However, in the second solenoid relay valve  65 , communication between the input port  66   e  connected to the second on/off solenoid valve S 2  and the output port  66   f  connected to the communication oil passage  64  is allowed. In the first solenoid relay valve  60 , communication between the input port  61   c  connected to the communication oil passage  64  and the output port  61   d  connected to the first signal pressure port  51   a  of the first clutch application relay valve  50  is allowed. Therefore, the S 2  pressure is input to the first signal pressure port  51   a  from the second on/off solenoid valve S 2  to block communication between the input port  51   c  to which the line pressure PL is input and the output port  51   d  connected to the C 3 -B 2  communication oil passage  54 . Thus, the line pressure PL is not output to the clutch C 3 . In the B 2  application control valve  75 , communication between the input port  66   b  coupled to the output port  61   d  to which the S 2  pressure of the first solenoid relay valve  60  is output and the output port  66   c  connected to the communication oil passage  69  is allowed. In the second clutch application relay valve  55 , communication between the input port  56   i  connected to the second on/off solenoid valve S 2  and the output port  56   h  connected to the communication oil passage  59   c  is allowed. Therefore, in the B 2  application control valve  75 , the S 2  pressure from the communication oil passage  59   c  is input to the first signal pressure port  76   a , and the S 2  pressure from the communication oil passage  69  is input to the second signal pressure port  76   b , which moves the spool  77  to the position indicated on the right half when  FIG. 4  is seen sideways to allow communication between the input port  76   e  connected to the first communication oil passage  74   a  and the output port  76   d  connected to the brake B 2 . In the C 3 -B 2  application control valve  70 , communication between the output port  71   b  connected to the first communication oil passage  74   a  and the drain port  71   h  is allowed. Thus, the hydraulic pressure acting on the brake B 2  is drained via the input port  76   e  and the output port  76   d  of the B 2  application control valve  75 , the first communication oil passage  74   a , and the output port  71   b  and the drain port  71   h  of the C 3 -B 2  application control valve  70 . The neutral state is thus established. 
     The first clutch application relay valve  50  includes the third signal pressure port  51   l  to which a hydraulic pressure from the linear solenoid valve SLT is input as a signal pressure. Therefore, the spool  52  can be moved to the position indicated on the right half when  FIG. 4  is seen sideways to establish the neutral state, as in the case where the S 2  pressure from the second on/off solenoid valve S 2  is input to the first signal pressure port  51   a , by causing the linear solenoid valve SLT to output a low hydraulic pressure, that is, by applying a high current Ihi to the normally-open linear solenoid valve (an electromagnetic coil) SLT, instead of energizing only the second on/off solenoid valve S 2 . However, as shown in  FIG. 6 , when the high current Ihi is applied to the linear solenoid valve SLT, the hydraulic oil may not be sufficiently supplied to the cooler or the portion to be lubricated to result in insufficient cooling or insufficient lubrication. The configuration and the operation of the hydraulic circuit  40  have been described above. 
     As shown in the functional block diagram of  FIG. 7 , the AT ECU  90  includes a main control section  92  that governs processes performed in the entire automatic transmission  20 , a monitoring section  94  that monitors the state of the main control section  92 , an SLT circuit  98   a , an SL 1  circuit  98   b , an SL 2  circuit  98   e , an SL 3  circuit  98   d , an SL 5  circuit  98   e , an S 1  circuit  98   f , and an S 2  circuit  98   g  serving as drive circuits that drive the solenoid valves SLT, SL 1 , SL 2 , SL 3 , SL 5 , S 1 , and S 2 , respectively, and AND circuits  96   a  to  96   g  each having two input terminals to which a signal from the main control section  92  and a signal from the monitoring section  94  are input and an output terminal that outputs to the circuits  98   a  to  98   g , respectively, the logical sum of the signals input to the two input terminals. Each of the main control section  92  and the monitoring section  94  is formed as a microprocessor including a CPU as its main component, and includes a ROM that stores a processing program, a RAM that temporarily stores data, input and output ports, and a communication port in addition to the CPU. The main control section  92  and the monitoring section  94  may be formed as a single chip, or may be formed separately from each other. The main control section  92  receives the vehicle speed V detected by the vehicle speed sensor  88 , an input shaft speed Nin from a rotational speed sensor  29  attached to the input shaft  21  of the automatic transmission  20 , and so forth via an input port. The main control section  92  outputs solenoid command signals for driving the solenoid valves SLT, SL 1 , SL 2 , SL 3 , SL 5 , S 1 , and S 2  and so forth to corresponding ones of the AND circuits  96   a  to  96   g  via an output port. Also, the main control section  92  communicates with the main ECU  80 , and receives data such as the shift position SP from the shift position sensor  82 , the accelerator operation amount Acc from the accelerator pedal position sensor  84 , and the brake switch signal BSW from the brake switch  86  through communication via the main ECU  80 . The monitoring section  94  receives the vehicle speed V detected by the vehicle speed sensor  88  via an input port separately from the main control section  92 , and receives the solenoid command signals and so forth output from the main control section  92  via an input port. The monitoring section  94  outputs an on/off signal to the AND circuits  96   a  to  96   g , and outputs a solenoid command signal for driving the S 2  circuit  98   g  to the S 2  circuit  98   g  not via the AND circuit  96   g . Thus, when the main control section  92  outputs solenoid command signals with the monitoring section  94  outputting an on signal (a permission signal) to the AND circuits  96   a  to  96   g , the solenoid command signals are output to corresponding ones of the drive circuits  98   a  to  98   g . However, when the main control section  92  outputs solenoid command signals with the monitoring section  94  outputting an off signal (a prohibition signal) to the AND circuits  96   a  to  96   g , output of the solenoid command signals to the drive circuits is blocked. In the embodiment, as discussed above, only the linear solenoid valve SLT, of the solenoid valves SLT, SL 1  to SL 3 , SL 5 , S 1 , and S 2 , is formed as a normally-open solenoid valve, and the other solenoid valves SL 1  to SL 3 , SL 5 , S 1 , and S 2  are each formed as a normally-closed solenoid valve. Therefore, the neutral state can be established by outputting an on signal to the S 2  circuit  98   g  with the monitoring section  94  outputting an off signal (a prohibition signal) to block output of solenoid command signals from the main control section  92  to the drive circuits. 
     Next, operations of the automatic transmission  20  and the AT ECU  90  configured as described above, specifically operations of the main control section  92  and the monitoring section  94  of the AT ECU  90 , will be described. The operation of the main control section  92  will be described first, and the operation of the monitoring section  94  will be described thereafter.  FIG. 8  is a flowchart showing an exemplary main control section process routine executed by the main control section  92  of the AT ECU  90 . The routine is executed repeatedly at predetermined time intervals (for example, at intervals of several milliseconds). 
     When the main control section process routine is started, the main control section  92  first receives data required for control such as the accelerator operation amount Acc, the vehicle speed V from the vehicle speed sensor  88 , the input shaft speed Nin from the rotational speed sensor  29 , and a shift speed to be skipped (step S 100 ). The accelerator operation amount Acc is detected by the accelerator pedal position sensor  84  and input from the main ECU  80  through communication. The shift speed to be skipped is set in step S 170  to be discussed later. When such data are input, a target shift speed is set on the basis of the input accelerator operation amount Acc and vehicle speed V using a shift map (step S 110 ). Solenoid command signals are output to corresponding ones of the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  in accordance with the set target shift speed (step S 120 ). The input vehicle speed V is multiplied by a conversion coefficient k to compute an output shaft speed Nout, which is the rotational speed of the output shaft  22  (step S 130 ). The input input shaft speed Nin is divided by the computed output shaft speed Nout to compute an actual speed ratio γ (step S 140 ). It is determined whether or not the computed actual speed ratio γ falls within an allowable gear ratio range defined by a lower limit value (Gr*−α), which is obtained by subtracting a margin α from a post-shifting gear ratio Gr* which is obtained as the gear ratio of the target shift speed set in step S 110 , and an upper limit value (Gr*+α), which is obtained by adding the margin α to the post-shifting gear ratio Gr* (step S 150 ). If the effective gear ratio γ falls within the allowable rotational speed range, the routine is terminated. On the other hand, if the effective gear ratio γ does not fall within the allowable gear ratio range, it is further determined whether or not an electrical failure (solenoid electrical failure) such as a wire break or a short circuit is occurring in any of the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  or whether or not a valve stick (adhesion) is occurring in any of the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2 , the relay valves  60  and  65 , and the control valves  70  and  75  (step S 160 ). A solenoid electrical failure can be determined by comparing a solenoid current for the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  detected by a current sensor (not shown) with a corresponding solenoid command signal (on/off), for example. A valve stick can be determined by a hydraulic pressure sensor (not shown) attached to an oil passage in the hydraulic circuit  40  detecting a hydraulic pressure value that does not occur in the case where the valve position is normal, for example. If it is determined that either failure of a solenoid electrical failure and a valve stick is not occurring, the shift speed indicated by the target shift speed is set as a shift speed to be skipped (step S 170 ). The routine is thus terminated. Consequently, when the routine is executed subsequently, the shift speed to be skipped is input in step S 100  so that the target shift speed is selected from shift speeds that can be established other than the shift speed to be skipped in step S 110 . On the other hand, if it is determined that either failure of a solenoid electrical failure and a valve stick is occurring, all the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  are turned off (step S 180 ). The routine is thus terminated. As discussed above, with all the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  turned off, the third forward speed is exclusively established while the vehicle is running using any of the first to third forward speeds, and the fifth forward speed is exclusively established while the vehicle is running using any of the fourth to sixth forward speeds. Such setting of a shift speed to be skipped and exclusive establishment of a shift speed are performed on the assumption that the solenoid command signals set in step S 120  are normal and not erroneous. In case of an error in the command signals themselves, such an error is handled in a process performed by the monitoring section  94  to be discussed later. 
     Next, the operation of the monitoring section  94  will be described.  FIG. 9  is a flowchart showing an exemplary monitoring section process routine executed by the monitoring section  94  of the AT ECU  90 . The routine is executed repeatedly at predetermined time intervals (for example, at intervals of several milliseconds). 
     When the monitoring section process routine is started, the monitoring section  94  first receives data required for control such as the vehicle speed V from the vehicle speed sensor  88 , a current gear ratio Gr, and solenoid command signals output from the main control section  92  as a state of the automatic transmission  20  (step S 200 ). The current gear ratio Gr is obtained as the gear ratio of the currently established shift speed. Subsequently, the input solenoid command signals are analyzed to calculate a post-shifting gear ratio Gr* which is the gear ratio of a shift speed established after shifting performed on the basis of the solenoid command signals (step S 210 ). The calculated post-shifting gear ratio Gr* is multiplied by the conversion coefficient k and the vehicle speed V to calculate a post-shifting engine speed Ne* (step S 220 ). It is determined whether or not a gear ratio variation amount (Gr*−Gr), which is the deviation between the post-shifting gear ratio Gr* and the current gear ratio Gr, is equal to or less than an allowable gear ratio variation amount ΔGrlim (step S 230 ). Then, it is determined whether or not the post-shifting engine speed Ne* is equal to or less than an allowable rotational speed Nelim of the engine  12  (step S 240 ). The allowable gear ratio variation amount ΔGrlim defines the amount of variation in gear ratio with which a shift shock (deceleration) of the vehicle exceeds an allowable range. The allowable rotational speed Nelim is set as a rotational speed that is slightly lower than the upper limit rotational speed of the engine  12 . It may be said that the determinations in steps S 230  and S 240  are made to determine a shift command that is not made if the solenoid command signals from the main control section  92  are normal and not erroneous such as a downshift from the sixth forward speed to the first forward speed, for example. If the gear ratio variation amount is equal to or less than the allowable gear ratio variation amount ΔGrlim and the post-shifting engine speed Ne* is equal to or less than the allowable rotational speed Nelim, a permission signal (an on signal) is output to the AND circuits  96   a  to  96   g  (step S 250 ). The routine is thus terminated. Consequently, the solenoid command signals output from the main control section  92  are output to corresponding ones of the circuits  98   a  to  98   g  via the AND circuits  96   a  to  96   g  to drive the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2 . On the other hand, if it is determined that the gear ratio variation amount is not equal to or less than the allowable gear ratio variation amount ΔGrlim or it is determined that the post-shifting engine speed Ne* is not equal to or less than the allowable rotational speed Nelim, it is determined that an abnormality such as a data corruption due to a communication failure, for example, occurs in the solenoid command signals from the main control section  92 . Then, a prohibition signal (an off signal) is output to the AND circuits  96   a  to  96   g  (step S 260 ). An on signal (energization signal) is output to the S 2  signal (step S 270 ). The routine is thus terminated. Consequently, the solenoid command signals output from the main control section  92  are blocked by the AND circuits  96   a  to  96   g  so that the linear solenoid valves SLT and SL 1  to SL 5  and the first on/off solenoid valve S 1  are de-energized and only the second on/off solenoid valve S 2  is energized to establish the neutral state. Thus, even if an abnormality occurs in the main control section  92  so that abnormal solenoid command signals are output, no unexpected shock occurs in the vehicle and the upper limit rotational speed of the engine  12  is not exceeded. 
     The hydraulic control device according to the embodiment described above includes the normally-open linear solenoid valve SLT, the normally-closed linear solenoid valves SL 1  to SL 5 , the normally-closed first and second on/off solenoid valves S 1  and S 2 , the first clutch application relay valve  50  having the first signal pressure port  51   a  which switches between the normal mode, in which the linear solenoid valves SL 1  to SL 3  are connected to corresponding ones of the clutches C 1  to C 3 , and the fail-safe mode, in which the drive pressure PD is selectively supplied to the clutches C 1  and C 2  and the line pressure PL is supplied to the clutch C 3  to establish the third or fifth forward speed when all the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2  are de-energized, and the first and second solenoid relay valves  60  and  65  configured to supply the S 2  pressure from the second on/off solenoid valve S 2  to the first signal pressure port  51   a . Thus, the third or fifth forward speed can be established in the fail-safe mode by de-energizing all the solenoid valves SLT, SL 1  to SL 5 , S 1 , and S 2 , and the neutral state can be established by energizing only the second on/off solenoid valve S 2 . As a result, the neutral state can be established more reliably and efficiently. 
     In the embodiment, as shown in  FIG. 7 , the AT ECU  90  is configured such that the main control section  92  is connected to one of the input terminals of each of the AND circuits  96   a  to  96   g , the output terminal of each of the AND circuits  96   a  to  96   g  is connected to a corresponding one of the SLT circuit  98   a , the SL 1  circuit  98   b , the SL 2  circuit  98   c , the SL 3  circuit  98   d , the SL 5  circuit  98   e , the S 1  circuit  98   f , and the S 2  circuit  98   g , and the monitoring section  94  is connected to the other input terminal of each of the AND circuits  96   a  to  96   g  and to the S 2  circuit  98   g  not via the AND circuit  96   g  so that the monitoring section  94  outputs an off signal to each of the AND circuits  96   a  to  96   g  and an on signal to the S 2  circuit  98   g  to establish the neutral state when an abnormality is occurring in the main control section  92 . However, the present invention is not limited thereto, and any configuration in which the neutral state can be established irrespective of the solenoid command signals from the main control section  92  may be adopted. For example, the main control section  92  may be directly connected to the SLT circuit  98   a , the SL 1  circuit  98   b , the SL 2  circuit  98   c , the SL 3  circuit  98   d , the SL 5  circuit  98   e , the S 1  circuit  98   f , and the S 2  circuit  98   g , the monitoring section  94  may be directly connected to the S 2  circuit  98   g , and a blocking circuit that de-energizes the solenoid valves SLT and SL 1  to SL 5  and the first on/off solenoid valve S 1  in response to a signal from the monitoring section  94  may be provided separately. 
     in the embodiment, an abnormality in the solenoid command signals output from the main control section  92  is determined by the monitoring section  94  determining whether or not the gear ratio variation amount is less than the allowable gear ratio variation amount ΔGrlim and whether or not the post-shifting engine speed Ne* is less than the allowable rotational speed Nelim. However, the present invention is not limited thereto. For example, an abnormality in the solenoid command signals output from the main control section  92  may be determined by analyzing a combination of the currently established shift speed and the solenoid command signals output from the main control section  92  to the solenoid valves SLT, SL 1  to SL 5 , S and S 2  to determine whether or not a change that is not normally made if the main control section  92  is normal (such as a change from the sixth forward speed to the first forward speed and a change from the first forward speed to the sixth forward speed, for example) is to be made. Alternatively, it may be determined whether or not the solenoid command signals output from the main control section  92  are normal by comparing the number of clutches and brakes that need to be engaged for the currently established shift speed and the number of clutches and brakes that are actually engaged to determine whether or not both the numbers coincide with each other. The determination as to whether or not a clutch or a brake is actually engaged may be made using a detected value from a hydraulic pressure sensor attached to an oil passage connected to an oil chamber corresponding to the clutch or brake, or by detecting a feedback current reflecting a current applied to a corresponding solenoid. Instead of determining an abnormality in the solenoid command signals output from the main control section  92  as described above, an abnormality in the main control section  92  may be determined directly by monitoring an abnormality in the main control section  92  using a watchdog timer, monitoring an abnormality in the main control section  92  through echo back of communication data, or the like, for example. 
     In the embodiment, the first solenoid relay valve  60  switches between the mode in which the modulator pressure PMOD is output to the first signal pressure port  51   a  of the first clutch application relay valve  50  using either of the SL 1  pressure from the linear solenoid valve SL 1  and the SL 2  pressure from the linear solenoid valve SL 2  as the signal pressure and the mode in which the S 2  pressure from the second on/off solenoid valve S 2  is output to the first signal pressure port  51   a . However, the S 2  pressure from the second on/off solenoid valve S 2  may be directly output to the first signal pressure port  51   a . At this time, the second on/off solenoid valve S 2  may be turned on to output the S 2  pressure to the first signal pressure port  51   a  even while the vehicle is running using one of the first to sixth forward speeds. In this case, the first solenoid relay valve  60  may be omitted. 
     In the embodiment, the second clutch application relay valve  55  exclusively establishes the third forward speed when a failure occurs while the vehicle is running using a lower speed including the first to third forward speeds, and exclusively establishes the fifth forward speed when a failure occurs while the vehicle is running using a higher speed including the fourth to sixth forward speeds. However, the second clutch application relay valve  55  may be omitted so that a specific shift speed is exclusively established at all times when a failure occurs while the vehicle is running using any of the first to sixth forward speeds. 
     In the embodiment, the main control section  92  determines an abnormality in the automatic transmission  20  by determining whether or not the actual speed ratio γ falls within the allowable gear ratio range, whether or not a solenoid electrical failure is occurring, and whether or not a valve stick is occurring. However, some of such determinations may be omitted, and determinations other than such determinations may be executed in addition. 
     In the embodiment, the main control section  92  receives the vehicle speed V and the input shaft speed Nin and the monitoring section  94  receives the vehicle speed V and the current gear ratio Gr (currently established shift speed) as the state of the automatic transmission  20  (shifting state). However, the monitoring section  94  may use any data on the state of the automatic transmission  20  as long as the monitoring section  94  can determine an abnormality on the basis of the solenoid command signals set by the main control section  92 . 
     In the embodiment, the 6-speed speed change mechanism  30  which provides first to sixth forward speeds is incorporated. However, the present invention is not limited thereto, and an automatic transmission that provides any number of speeds such as 4-speed, 6-speed, and 8-speed automatic transmissions may be incorporated. The shift speeds established when all the solenoid valves SLT, SL 1 , SL 2 , SL 3 , SL 5 , S 1 , and S 2  are de-energized are not limited to the third and fifth forward speeds, and may be any shift speed. 
     The correspondence between the main elements of the embodiment and the main elements of the invention described in the “SUMMARY OF THE INVENTION” section will be described. In the embodiment, the clutches C 1  to C 3  and the brakes B 1  and B 2  correspond to the “friction engagement element”. The mechanical oil pump  42  corresponds to the “pump”. The primary regulator valve  44 , the linear solenoid valve SLT, and so forth correspond to the “first pressure regulation mechanism”. The linear solenoid valves SL 1  to SL 3  correspond to the “second pressure regulation mechanism”. The second on/off solenoid valve S 2  corresponds to the “signal pressure output mechanism”. The first clutch application relay valve  50 , the second clutch application relay valve  55 , the first solenoid relay valve  60 , the second solenoid relay valve  65 , the C 3 -B 2  application control valve  70 , the B 2  application control valve  75 , and so forth correspond to the “switching mechanism”. The first clutch application relay valve  50  corresponds to the “first switching valve”. The first solenoid relay valve  60  and the second solenoid relay valve  65  correspond to the “second switching valve”. The correspondence between the main elements of the embodiment and the main elements of the invention described in the “SUMMARY OF THE INVENTION” section does not limit the elements of the invention described in the “SUMMARY OF THE INVENTION” section, because such correspondence is an example given for the purpose of specifically describing the embodiment described in the “SUMMARY OF THE INVENTION” section. That is, the invention described in the “SUMMARY OF THE INVENTION” section should be construed on the basis of the description in that section, and the embodiment is merely a specific example of the invention described in the “SUMMARY OF THE INVENTION” section. 
     While a mode for carrying out the present invention has been described above by way of an embodiment, it is a matter of course that the present invention is not limited to the embodiment in any way, and that the present invention may be implemented in various forms without departing from the scope and sprit of the present invention. 
     The present invention may be applied to the automotive industry.