Abstract:
A cost for manufacturing an automatic transmission can be reduced since a separate solenoid valve for controlling a damper clutch is not required when a hydraulic control system of an automatic transmission for a vehicle includes: a regulator valve that forms a line pressure by regulating a hydraulic pressure generated by a hydraulic pump; a torque converter control valve that receives a hydraulic pressure from the regulator valve and supplies a torque converter operating pressure to a torque converter; and a damper clutch control valve that receives the hydraulic pressure of the torque converter control valve and selectively supplies the torque converter operating pressure and a damper clutch operating pressure, wherein the damper clutch control valve is controlled by a control pressure supplied from a switch valve that is controlled an operating pressure of an overdrive clutch that operates at third and fourth forward speeds.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0071884, filed in the Korean Intellectual Property Office on Jul. 18, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (a) Field of the Invention 
         [0003]    The present invention relates to a hydraulic control system of an automatic transmission for a vehicle. More particularly, the present invention relates to a hydraulic control system of an automatic transmission for a vehicle that can be exempt from a damper clutch control solenoid valve such that a cost may be reduced. 
         [0004]    (b) Description of the Related Art 
         [0005]    Typically, an automatic transmission automatically controls its shifting operation. For such an automatic shifting control, a shift control apparatus controls hydraulic pressure of a plurality of solenoid valves depending on various vehicle running states such as a vehicle speed and a throttle valve opening. 
         [0006]    If a driver moves a select lever to a desired shift range, a manual valve in a hydraulic control system realizes a line conversion. In this case, a hydraulic pressure supplied from an oil pump is supplied to friction elements operating in respective shift-speeds according to a duty control of solenoid valves. 
         [0007]    As an example of such an automatic transmission, a hydraulic control system of an automatic transmission that enables four forward speeds and one reverse speed is hereinafter described with reference to  FIG. 8 . As shown in  FIG. 8 , a hydraulic pressure generated at a hydraulic pump  102  is controlled to a regulated pressure by a regulator valve  104 . The regulated pressure of the regulator valve  104  is supplied to a manual valve  106 , a reducing valve  108 , a low-reverse brake (L-R/B) solenoid valve S 1  (hereinafter called a first solenoid valve S 1 ), a low-reverse brake (L-R/B) pressure control valve  10  (hereinafter called a first pressure control valve  110 ), and a damper clutch control valve  112 . As shown in  FIG. 8 , the first solenoid valve S 1  receives the regulated pressure of the regulator valve  104  as a control pressure. In addition, the first pressure control valve  10  is controlled by the first solenoid valve S 1 . 
         [0008]    In addition, the manual valve  106  realizes hydraulic line conversion according to a range conversion of the select lever (not shown). By the manual valve  106 , the hydraulic pressure supplied from the regulator valve  104  is supplied to a second brake (2ND/B) pressure control valve  114  (hereinafter called a second pressure control valve  114 ), an underdrive clutch (UD/C) pressure control valve  116  (hereinafter called a third pressure control valve  116 ), and an overdrive clutch (OD/C) pressure control valve  118  (hereinafter called a fourth pressure control valve  118 ). 
         [0009]    In addition, the reduced hydraulic pressure of the reducing valve  108  is supplied to a second brake (2ND/B) solenoid valve S 2  (hereinafter called a second solenoid valve S 2 ), an underdrive clutch (UD/C) solenoid valve S 3  (hereinafter called a third solenoid valve S 3 ), and an overdrive clutch (OD/C) solenoid valve S 4  (hereinafter called a fourth solenoid valve S 4 ) that respectively control the second, third, and fourth pressure control valves  114 ,  116 , and  118 . 
         [0010]    In addition, the reduced pressure of the reducing valve  108  is also supplied to a proportional control solenoid valve S 5  that controls the regulator valve  104 , such that the line pressure may be controlled by controlling the proportional control solenoid valve S 5  depending on a driving condition of a vehicle. 
         [0011]    The hydraulic pressures controlled at the first, second, third, and fourth pressure control valves  110 ,  114 ,  116 , and  118  according to the control of the first, second, third, and fourth solenoid valves S 1 , S 2 , S 3 , and S 4  are supplied to frictional elements, directly or through first and second fail-safe valves  120  and  122 . 
         [0012]    That is, the first solenoid valve S 1  and the first pressure control valve  110  are connected with a low-reverse brake (L-R/B) that operates at a first forward speed and a reverse speed, through the first fail-safe valve  120 . The second solenoid valve S 2  and the second pressure control valve  114  is connected with a second brake (2ND/B) that operates at second and fourth forward speeds, through the second fail-safe valve  122 . 
         [0013]    In addition, the third solenoid valve S 3  and the third pressure control valve  116  are directly connected with an underdrive clutch (UD/C) that operates at first, second, and third forward speeds. The fourth solenoid valve S 4  and the fourth pressure control valve  118  are directly connected with an overdrive clutch (OD/C) that operates at third and fourth forward speed. 
         [0014]    A reverse clutch (R/C) that operates at the reverse speed is directly connected with the manual valve  106 . 
         [0015]    In addition, the regulated hydraulic pressure of the regulator valve  104  is also supplied to a torque converter control valve  124  that controls a hydraulic pressure supplied to a torque converter (TC), and also supplied to damper clutch control valve  112  that controls a hydraulic pressure supplied to a damper clutch for enhancing power delivery efficiency of the torque converter (TC). 
         [0016]    The damper clutch control valve  112  controls an on/off operation of the damper clutch according to a control of a damper clutch control solenoid valve (DCCSV). 
         [0017]    By such an arrangement, the first, second, third, and fourth solenoid valves S 1 , S 2 , S 3 , and S 4  are controlled by a transmission control unit (TCU) (not shown) so as to control the first, second, third, and fourth pressure control valves  100 ,  114 ,  116 , and  118 , such that each frictional element is supplied with or released from the supplied pressure depending on shift-speeds so as to realize multi-speeds. 
         [0018]    Such a hydraulic control system employs a solenoid valve for controlling a damper clutch control valve, which causes an increase of a cost for an automatic transmission. Therefore, if an automatic transmission may be exempt from the solenoid valve for controlling the damper clutch control valve while maintaining substantially equivalent functioning of the transmission, it may contribute to reduction of the cost for manufacturing the transmission. 
         [0019]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY OF THE INVENTION 
       [0020]    The present invention has been made in an effort to provide a hydraulic control system of an automatic transmission for a vehicle having an advantage of a reduced cost for manufacture. 
         [0021]    An exemplary embodiment of the present invention provides a hydraulic control system of an automatic transmission for a vehicle including: a regulator valve that forms a line pressure by regulating a hydraulic pressure generated by a hydraulic pump; a torque converter control valve that receives a hydraulic pressure from the regulator valve and supplies a torque converter operating pressure to a torque converter; and a damper clutch control valve that receives the hydraulic pressure of the torque converter control valve and selectively supplies the torque converter operating pressure and a damper clutch operating pressure, wherein the damper clutch control valve is controlled by a control pressure supplied from a switch valve that is controlled an operating pressure of an overdrive clutch that operates at third and fourth forward speeds. 
         [0022]    The damper clutch control valve may selectively supply the torque converter operating pressure and the line pressure to the torque converter. 
         [0023]    The exemplary hydraulic control system may further include a first solenoid valve that operates to output a hydraulic pressure at one or more predetermined speeds other than the third and fourth forward speeds, wherein the switching valve may selectively supply a forward range pressure of forward speeds and an output pressure of the first solenoid valve to the damper clutch control valve as a control pressure. 
         [0024]    The switching valve may include a valve body and a valve spool installed therein. The valve body may include a first port that receives an inflow of the forward range pressure, a second port that supplies the hydraulic pressure received through the first port to the damper clutch control valve as its control pressure, a third port that receives an output pressure of the first solenoid valve, a fourth port that receives a low-reverse brake operating pressure from the first pressure control valve, a fifth port that supplies the hydraulic pressure received through the fourth port to the first fail-safe valve, a sixth port that exhausts the hydraulic pressure received through the fifth port, and a seventh port that receives a overdrive clutch operating pressure as its control pressure. The valve spool may operate by hydraulic pressures supplied through the first, third, and seventh ports. 
         [0025]    The valve spool of the switching valve may include: a first land that receives a hydraulic pressure supplied through the first port, and selectively enables fluid communication between the first and second ports; a second land that selectively enables fluid communication between the second and third ports, cooperatively with the first land; a third land that selectively enables fluid communication between the fourth and fifth ports, cooperatively with the second land; and a fourth land that receives a hydraulic pressure supplied through the seventh port, and selectively enables fluid communication between the fifth and sixth ports, cooperatively with the third land. 
         [0026]    The fourth land may be formed of a larger diameter than the first land. 
         [0027]    The damper clutch control valve may include a valve body and a valve spool installed therein. The valve body of the damper clutch control valve may include a first port that selectively receives the forward range pressure from the switching valve or the output pressure of the first solenoid valve; a second port that always receives the line pressure; a third port that bypasses the hydraulic pressure supplied through the second port; a fourth port that receives the hydraulic pressure from the torque converter control valve; a fifth port that selectively supplies the hydraulic pressure received through the fourth port to the torque converter; a sixth port that supplies the damper clutch operating pressure to the torque converter or receives the hydraulic pressure exhausted from the torque converter; a seventh port that selectively fluid-communicates with the sixth port so as to exhaust the hydraulic pressure received through the sixth port; an eighth port that selectively fluid-communicates with the sixth port so as to supply the damper clutch operating pressure that is received from the third port; and a ninth port that selectively receives a control pressure. The valve spool may operate by hydraulic pressures supplied through the first and ninth ports. 
         [0028]    The valve spool of the damper clutch control valve may include: a first land that is applied with the control pressure received through the first port; a second land that selectively enables fluid communication between the second and third ports, cooperatively with the first land; a third land that selectively enables fluid communication between the fourth and fifth ports, cooperatively with the second land; a fourth land that selectively enables fluid communication between the sixth and seventh ports, cooperatively with the third land; and a fifth land that is applied with the control pressure of the ninth port and selectively enables fluid communication between the sixth and eighth ports, cooperatively with the fourth land. 
         [0029]    According to such an exemplary embodiment of the present invention, the damper clutch can be controlled at third and fourth forward speeds by using a switching valve without additionally employing a separate solenoid valve for controlling a damper clutch. Therefore, a less number of solenoid valves are required and thus the cost for manufacturing an automatic transmission is substantially reduced. 
         [0030]    The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description of the Invention, which together serve to explain by way of example the principles of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
           [0032]      FIG. 1  is a schematic diagram of a hydraulic control system according to an exemplary embodiment of the present invention. 
           [0033]      FIG. 2  is a schematic diagram of a switching valve according to an exemplary embodiment of the present invention. 
           [0034]      FIG. 3  is a schematic diagram of a damper clutch control valve according to an exemplary embodiment of the present invention. 
           [0035]      FIG. 4  is a partial diagram of the hydraulic control system that shows a hydraulic pressure flow of the damper clutch in the neutral N range. 
           [0036]      FIG. 5  is a partial diagram of the hydraulic control system that shows a hydraulic pressure flow of the damper clutch at the first forward speed. 
           [0037]      FIG. 6  is a partial diagram of the hydraulic control system that shows a hydraulic pressure flow of the damper clutch while the damper clutch is not operated at the third and fourth forward speeds. 
           [0038]      FIG. 7  is a partial diagram of the hydraulic control system that shows a hydraulic pressure flow of the damper clutch while the damper clutch is operated at the third and fourth forward speeds. 
           [0039]      FIG. 8  shows an example of a conventional hydraulic control system. 
       
    
    
       [0040]    It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
         [0041]    In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0042]    Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0043]      FIG. 1  is a schematic diagram of a hydraulic control system according to an exemplary embodiment of the present invention. A hydraulic pressure generated at a hydraulic pump  2  is controlled to be a regulated pressure by a regulator valve  4 . The regulated pressure of the regulator valve  4  is directly supplied to a manual valve  6 , a reducing valve  8 , a low-reverse brake (L-R/B) solenoid valve S 1  (hereinafter called a first solenoid valve S 1 ), a low-reverse brake (L-R/B) pressure control valve  10  (hereinafter called a first pressure control valve  10 ), and a damper clutch control valve  12 . As shown in  FIG. 1 , the first solenoid valve S 1  receives the regulated pressure of the regulator valve  4  as a control pressure. In addition, the first pressure control valve  10  is controlled by the first solenoid valve S 1 . 
         [0044]    In addition, the manual valve  6  realizes hydraulic line conversion according to a range conversion of the select lever (not shown). By the manual valve  6 , the hydraulic pressure supplied from the regulator valve  4  is supplied to a second brake (2ND/B) pressure control valve (hereinafter called a second pressure control valve  14 ), an underdrive clutch (UD/C) pressure control valve (hereinafter called a third pressure control valve  16 ), and an overdrive clutch (OD/C) pressure control valve (hereinafter called a fourth pressure control valve  18 ). 
         [0045]    In addition, the reduced hydraulic pressure of the reducing valve  8  is supplied, as source pressures, to a second brake (2ND/B) solenoid valve S 2  (hereinafter called a second solenoid valve S 2 ), an underdrive clutch (UD/C) solenoid valve S 3  (hereinafter called a third solenoid valve S 3 ), and an overdrive clutch (OD/C) solenoid valve S 4  (hereinafter called a fourth solenoid valve S 4 ) that respectively control the second, third, and fourth pressure control valves  14 ,  16 , and  18 . 
         [0046]    In addition, the reduced pressure of the reducing valve  8  is also supplied to a proportional control solenoid valve S 5  that controls the regulator valve  4 , such that the line pressure may be controlled by controlling the proportional control solenoid valve S 5  depending on a driving condition of a vehicle. 
         [0047]    The hydraulic pressures controlled at the first, second, third, and fourth pressure control valves  10 ,  14 ,  16 , and  18  according to the control of the first, second, third, and fourth solenoid valves S 1 , S 2 , S 3 , and S 4  are supplied to frictional elements, directly or through a switching valve  20  and first and second fail-safe valves  22  and  24 . 
         [0048]    That is, the first solenoid valve S 1  and the first pressure control valve  10  are connected with a low-reverse brake (L-R/B) that operates at a first forward speed and a reverse speed, through the switching valve  20  and the first fail-safe valve  22 . The second solenoid valve S 2  and the second pressure control valve  14  is connected with a second brake (2ND/B) that operates at second and fourth forward speeds, through the second fail-safe valve  24 . 
         [0049]    In addition, the third solenoid valve S 3  and the third pressure control valve  16  are directly connected with an underdrive clutch (UD/C) that operates at first, second, and third forward speeds. The fourth solenoid valve S 4  and the fourth pressure control valve  18  are directly connected with an overdrive clutch (OD/C) that operates at third and fourth forward speed. 
         [0050]    A reverse clutch (R/C) that operates at the reverse speed is directly connected with the manual valve  106 . 
         [0051]    In addition, the regulated hydraulic pressure of the regulator valve  4  is also supplied to a torque converter control valve  26  that controls a hydraulic pressure supplied to a torque converter (TC), and also supplied to damper clutch control valve  12  that controls a hydraulic pressure supplied to a damper clutch for enhancing power delivery efficiency of the torque converter (TC). 
         [0052]    By such an arrangement, the first, second, third, and fourth solenoid valves S 1 , S 2 , S 3 , and S 4  are controlled by a transmission control unit (TCU) (not shown) so as to control the first, second, third, and fourth pressure control valves  10 ,  14 ,  16 , and  18 , such that each frictional element is supplied with or released from the supplied pressure depending on shift-speeds so as to realize multi-speeds. 
         [0053]    According to such a hydraulic control system of an exemplary embodiment of the present invention, a conventional damper clutch control solenoid valve DCCSV (refer to  FIG. 8 ) is removed. Instead, the hydraulic control system is configured such that a part of a forward range pressure is supplied to the damper clutch control valve  12  as its control pressure, according to controlling of the switching valve  20  interposed between the first pressure control valve  10  and the first fail-safe valve  22 . 
         [0054]    For this purpose, the switching valve  20  of an exemplary embodiment of the present invention is arranged as shown in  FIG. 2 . That is, a valve body of the switching valve  20  includes first, second, third, fourth, fifth, sixth, and seventh ports  200 ,  202 ,  204 ,  206 ,  208 ,  210 , and  212 . The first port  200  receives an inflow of the forward range pressure. The second port  202  supplies the hydraulic pressure received through the first port  200  to the damper clutch control valve  12  as its control pressure. The third port  204  receives an output pressure of the first solenoid valve S 1 . The fourth port  206  receives a low-reverse brake (L-R/B) operating pressure from the first pressure control valve  10 . The fifth port  208  supplies the hydraulic pressure received through the fourth port  206  to the first fail-safe valve  22 . The sixth port  210  exhausts the hydraulic pressure received through the fifth port  208 . The seventh port  212  receives a overdrive clutch (OD/C) operating pressure as its control pressure. 
         [0055]    In addition, a valve spool of the switching valve  20  installed in the valve body includes first, second, third, and fourth lands  214 ,  216 ,  218 , and  220 . The first land  214  receives a hydraulic pressure supplied through the first port  200 , and selectively enables fluid communication between the first and second ports  200  and  202 . The second land  216  selectively enables fluid communication between the second and third ports  202  and  204 , cooperatively with the first land  214 . The third land  218  selectively enables fluid communication between the fourth and fifth ports  206  and  208 , cooperatively with the second land  216 . The fourth land  220  receives a hydraulic pressure supplied through the seventh port  212 , and selectively enables fluid communication between the fifth and sixth ports  208  and  210 , cooperatively with the third land  218 . The first, second, and third lands  214 ,  216 , and  218  are formed of the same diameter, and the fourth land  220  is formed of a diameter larger than that of the first, second, and third lands  214 ,  216 , and  218 . 
         [0056]    In addition, the damper clutch control valve  12  that receives the control pressure from the second port  202  of the switching valve  20  is arranged as shown in  FIG. 3 . That is, a valve body of the damper clutch control valve  12  includes first, second, third, fourth, fifth, sixth, seventh, eighth, and ninth ports  120 ,  122 ,  124 ,  126 ,  128 ,  130 ,  132 ,  134 , and  136 . The first port  120  selectively receives the forward range pressure from the second port  202  of the switching valve  20  or the output pressure of the first solenoid valve S 1 . The second port  122  always receives the line pressure. The third port  124  bypasses the hydraulic pressure supplied through the second port  122 . The fourth port  126  receives the hydraulic pressure from the torque converter control valve  26 . The fifth port  128  selectively supplies the hydraulic pressure received through the fourth port  126  to the torque converter TC. The sixth port  130  supplies the damper clutch operating pressure to the torque converter TC or receives the hydraulic pressure exhausted from the torque converter TC. The seventh port  132  selectively fluid-communicates with the sixth port  130  so as to exhaust the hydraulic pressure received through the sixth port  130 . The eighth port  134  selectively fluid-communicates with the sixth port  130  so as to supply the damper clutch operating pressure that is received from the third port  124 . The ninth port  136  selectively receives a control pressure. 
         [0057]    In addition, a valve spool of the damper clutch control valve  12  that is installed in the valve body thereof includes first, second, third, and fourth lands  140 ,  142 ,  144 , and  146 . The first land  140  is applied with the control pressure received through the first port  120  from the switching valve  20 . The second land  142  selectively enables fluid communication between the second and third ports  122  and  124 , cooperatively with the first land  140 . The third land  144  selectively enables fluid communication between the fourth and fifth ports  126  and  128 , cooperatively with the second land  142 . The fourth land  146  selectively enables fluid communication between the sixth and seventh ports  130  and  132 , cooperatively with the third land  144 . The fifth land  148  is applied with the control pressure of the ninth port  136  received from the manual valve  6  and selectively enables fluid communication between the sixth and eighth ports  130  and  134 , cooperatively with the fourth land  146 . A spring  138  is arranged between the valve body and the first land  140 . 
         [0058]    In such a hydraulic control system, the frictional elements of the reverse clutch R/C, the low-reverse brake L-R/B, the second brake 2ND/B, the underdrive clutch UD/C, and the overdrive clutch OD/C receive hydraulic pressures so as to realize four forward speeds and one reverse speed by being operated according to a schedule of the following table. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Operational chart for frictional elements 
               
             
          
           
               
                   
                 L-R/B 
                 2ND/B 
                 UD/C 
                 OD/C 
                 R/C 
               
               
                   
                   
               
             
          
           
               
                 1 st  speed 
                 ◯ 
                   
                 ◯ 
                   
                   
               
               
                 2 nd  speed 
                   
                 ◯ 
                 ◯ 
               
               
                 3 rd  speed 
                   
                   
                 ◯ 
                 ◯ 
               
               
                 4 th  speed 
                   
                 ◯ 
                   
                 ◯ 
               
               
                 Reverse speed 
                 ◯ 
                   
                   
                   
                 ◯ 
               
               
                 Neutral position 
                 ◯ 
               
               
                   
               
             
          
         
       
     
         [0059]    In order to realize the operation schedule of the frictional elements according to the above table, the first, second, third, and fourth solenoid valves S 1 , S 2 , S 3 , and S 4  are operated according to a schedule of the following table. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Operational chart for solenoid valves 
               
             
          
           
               
                   
                 S1 
                 S2 
                 S3 
                 S4 
               
               
                   
                   
               
             
          
           
               
                   
                 1 st  speed 
                 OFF 
                 ON 
                 OFF 
                 ON 
               
               
                   
                 2 nd  speed 
                 ON 
                 OFF 
                 OFF 
                 ON 
               
               
                   
                 3 rd  speed 
                 ON/OFF 
                 ON 
                 OFF 
                 OFF 
               
               
                   
                 4 th  speed 
                 ON/OFF 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                 Reverse speed 
                 OFF 
                 ON 
                 ON 
                 ON 
               
               
                   
                 Neutral position 
                 OFF 
                 ON 
                 ON 
                 ON 
               
               
                   
                   
               
             
          
         
       
     
         [0060]    In such an arrangement, the damper clutch control valve  12  is not applied with any control pressure in the neutral N range, as shown in  FIG. 4 . Therefore, the valve spool of the damper clutch control valve  12  is moved to the right in the drawing by the elastic force of the spring  138 , and the hydraulic pressure circulates the torque converter in a direction of arrows shown in  FIG. 4 . In this case, the damper clutch DC is in a non-operation state. 
         [0061]    In the first forward speed, as shown in  FIG. 5 , the forward range pressure is supplied from the manual valve  6  to the switching valve  20  through the first port  200 , and therefore, the valve spool thereof is moved to the left in the drawing since the hydraulic pressure received through the first port  200  is applied to the first land  214 . 
         [0062]    Accordingly, the first and second ports  200  and  202  fluidly communicate with each other such that the forward range pressure is supplied to the first port  120  of the damper clutch control valve  12  as the control pressure. In this case, the valve spool of the damper clutch control valve  12  remains moved to the right in the drawing. Therefore, the hydraulic pressure circulates the torque converter in a direction of arrows shown in  FIG. 5 , and the damper clutch DC is in a non-operation state. 
         [0063]    In the third and fourth forward speeds, the fourth solenoid valve S 4  is controlled off, and thus the overdrive clutch OD/C is operated. Therefore, the overdrive clutch operating pressure is supplied to the seventh port  212  of the switching valve  20  as its control pressure. Since the fourth land  220  of the switching valve  20  has a larger diameter than the first land  214 , the valve spool of the switching valve  20  remains moved to the right in the drawing even though the forward range pressure is applied to the first land  214  through the first port  220 . In such third and fourth forward speeds, the controlling on/off operation of the first solenoid valve S 1  does not affect formation of desired shift-speed. 
         [0064]    In such third and fourth speeds, when the damper clutch is not required to be operated, the first solenoid valve S 1  is turned off by the transmission control unit (TCU). In this case, the output pressure of the first solenoid valve S 1  is supplied to the first port  120  of the damper clutch control valve  12  through the third and second ports  204  and  202  of the switching valve  20 . Therefore, the valve spool of the damper clutch control valve  12  remains moved to the right in the drawing, and thus the hydraulic pressure circulates the torque converter in the arrow direction of  FIG. 6 . In this case, the damper clutch DC is in a non-operation state. 
         [0065]    When the damper clutch DC is desired to be operated in the third and fourth forward speeds depending on driving conditions, the first solenoid valve S 1  is turned on. Then as shown in  FIG. 7 , the damper clutch control valve  12  does not receive any control pressure from the switching valve  20  while it receives the forward range pressure through the ninth port  136 . Therefore, the valve spool of the damper clutch control valve  12  moves to the left in the drawing, overcoming the elastic force of the spring  138 . 
         [0066]    Accordingly, the second and third ports  122  and  124  fluidly communicate with each other, and the sixth and eighth ports  130  and  134  fluidly communicate with each other. Therefore, the line pressure at the second port  122  is supplied to the torque converter as the damper clutch operating pressure sequentially through the third, eighth, and sixth ports  124 ,  134 , and  130 , and thus the damper clutch (DC) is operated. 
         [0067]    In this case, the hydraulic pressure supplied through the fourth port  126  is exhausted through the seventh port  132  as a lubrication hydraulic pressure. 
         [0068]    While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.