Abstract:
In a vehicular automatic transmission of an engaging pressure electronically controlled type, a failures detecting apparatus is installed to detect an occurrence of failure of a hydraulic switching device such as a hydraulic switching valve and includes: a gear range determinator to determine in which gear range the present gear range of the automatic transmission falls; a failure detection start determinator to determine a start of the failure detection of the hydraulic switching device according to a result of determination by the gear range determinator; a memory storing parts of a plurality of engaging elements which are selected as the combinations of engagement for the respective gear ranges and other parts than those for the respective gear ranges in a table form; a hydraulic switching device checking device to intentionally output an engagement command signal to command to engage to each of the other parts of engaging elements which are not to be engaged and which are read from the memory; and a hydraulic switching device abnormality state determinator to determine through the checking device whether the hydraulic switching device is in an abnormal state depending on whether the hydraulic is transmitted to a corresponding one of the other parts of engaging elements in response to the engagement command signal.

Description:
BACKGROUND OF THE INVENTION: 
     a) Field of the Invention 
     The present invention relates generally to a vehicular automatic transmission equipped with a failure detecting apparatus. The present invention, more particularly, relates to a technique on a hydraulic circuit of the vehicular automatic transmission of an engaging pressure electronically controlled type in which the hydraulic circuit is simplified, the number of assembled parts are reduced, and a reduction in size of a control valve body is achieved. 
     b) Description of the Related Art 
     A Japanese Patent Application First Publication No. Heisei 8-121586 published on May 14, 1996 exemplifies a previously proposed hydraulic control apparatus for the vehicular automatic transmission of the engaging pressure electronically controlled type. 
     In the previously proposed vehicular automatic transmission disclosed in the above-identified Japanese Patent Application Publication, as a fail-safe valve of an LR (low-and-reverse) brake which is clutched when a gear range is a first-speed D (Drive) range and is released when the gear range is a second-speed, a third-speed, or a fourth-speed D range, a forceful hydraulic drain structure is disposed which forcefully drains the hydraulic supplied to the LR brake at a time of the gear range is in the D range second-speed, third-speed, and fourth-speed at which the hydraulic in at least one of either a 2-nd brake pressure P 2 ND or an OD (Overdrive) clutch pressure POD is developed, the 2-nd brake pressure P 2 ND being the hydraulic for a 2-nd-brake to be clutched at the time of D range 2-nd or 4-th speed and the OD clutch pressure POD being the hydraulic for an OD clutch to be engaged at the time of D range 3-rd or 4-th speed. 
     SUMMARY OF THE INVENTION: 
     However, in the previously proposed hydraulic control apparatus for the vehicular automatic transmission, no detection means is provided to detect whether a first spool which constitutes the forceful hydraulic drain structure is stuck (or a sticky slip occurs). If the first spool is stuck at a position where the OD clutch pressure POD enters, the LR brake would be engaged and would be interlocked due to the stick on the first spool if the abnormality occurs in a hydraulic system on the LR brake pressure and the undesired hydraulic is developed. 
     It is, hence, an object of the present invention to provide the vehicular automatic transmission of the engaging pressure electronically controlled type with which a failure detecting apparatus is equipped and the failure detecting apparatus can achieve such a fail-safe operation as to forcefully drain an engagement element pressure at a most appropriate timing neither giving an ill effect on a gear shift control nor developing the interlock under an occurrence of the failure and which can check to see if this fail-safe operation is always achieved without failure. 
     The above-described object can be achieved by providing a vehicular automatic transmission, comprising: a plurality of engaging elements; a shift gear mechanism in which combinations of engagement and release of the respective engaging elements are selected by means of hydraulic control section so as to perform a multiple range of gear shift; a hydraulic switching device that is installed in the hydraulic control section to prevent the shift gear mechanism from being interlocked; and a failure detecting apparatus to detect an occurrence of failure of the hydraulic switching device, the failure detecting apparatus including: a gear range determinator to determine in which gear range the present gear range of the automatic transmission falls; a failure detection start determinator to determine a start of the failure detection of the hydraulic switching device according to a result of determination by the gear range determinator; a memory storing parts of the engaging elements which are selected as the combinations of engagement for the respective gear ranges and other parts than those for the respective gear ranges in a table form; a hydraulic switching device checking device to intentionally output an engagement command signal to command to engage to each of the other parts of engaging elements which are not to be engaged and which are read from the memory; and a hydraulic switching device abnormality state determinator to determine through the checking device whether the hydraulic switching device is in an abnormal state depending on whether the hydraulic is transmitted to a corresponding one of the other parts of engaging elements in response to the engagement command signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic connection diagram of a gear-train in a vehicular automatic transmission to which a failure detecting apparatus in a preferred embodiment according to the present invention is applicable. 
     FIG. 2 is a logic table on an engagement of each speed range at an R (reverse) range and a D (drive) range in the vehicular automatic transmission to which the failure detecting apparatus according to the present invention is applicable. 
     FIG. 3 is a hydraulic control system configuration view of the vehicular automatic transmission to which the failure detecting apparatus in the preferred embodiment is applicable. 
     FIG. 4A is a schematic circuit block diagram of an electronic control system in the hydraulic control system in the hydraulic control system shown in FIG.  3 . 
     FIG. 4B is a schematic circuit block diagram of an automatic transmission control unit in the electronic control system shown in FIG.  4 A. 
     FIG. 5 is an operational flowchart representing a failure detection procedure executed in the automatic transmission control unit shown in FIG. 4A or  4 B. 
     FIG. 6 is a logic table representing a power supply or no power supply state of a low-and-reverse brake solenoid described in the preferred embodiment shown in FIG.  3 . 
     FIG. 7 is a characteristic graph representing an output hydraulic (power supply quantity) and a power supply time duration in the preferred embodiment. 
     FIG. 8 is a logic table representing the power supply state to a 2-4 brake solenoid described in the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will hereinafter be made to the drawings in order to facilitate a better understanding of the present invention. 
     FIG. 1 shows one example of a gear train of a vehicular automatic transmission to which a hydraulic control apparatus in a preferred embodiment according to the present invention is applicable. 
     In FIG. 1, E denotes an engine output axle, I denotes a transmission input axle, and O denotes a transmission output axle. A torque converter T/C is interposed between the engine output axle E and the transmission input axle I. A first planetary gear group G 1  and a second planetary gear group G 2  are interposed between the input and output axles I and O of the automatic transmission. 
     The first planetary gear group G 1  is a simple planetary gear group having a first pinion P 1 , a first carrier C 1 , a first pinion P 1 , a first carrier C 1 , a first sun gear S 1 , and a first ring gear R 1  and a second planetary gear group G 2  is a simple planetary gear group G 2  having a second pinion P 2 , a second carrier C 2 , a second sun gear S 2 , and a second ring gear R 2 . 
     The transmission input axle I and the second sun gear S 2  are directly coupled. A reverse clutch R/C is installed in a midway through a member linking the transmission input axle I to the first sun gear S 1 . 
     A 2-4 brake 2-4/B of a multi-plate brake structure is installed so as to enable this member to be fixed onto the casing. A high clutch H/C is installed in a midway through a member linking the transmission input axle I to the first carrier C 1 . A Low Clutch L/C is installed in a midway through a member linking the first carrier C 1  to the second ring gear R 2 . A Low &amp; Reverse brake L &amp; R/B of a multi-plate structure so as to enable this member to be fixed onto the casing is installed. A one-way clutch OWC disposed in parallel to the Low &amp; Reverse brake L &amp; R/B. The first ring gear R 1  and the second carrier C 2  are directly coupled. The transmission output axle O is linked to the second carrier C 2 . 
     FIG. 2 shows an engagement logic table at each gear range in a reverse range (also called, R range) and in a drive range (also called, D range). 
     It is noted that Ï mark denotes the engagement state and x mark denotes the release state. 
     At the time of R range, the reverse clutch R/C and the low &amp; reverse brake L &amp; R/B are engaged to each other. 
     The Low clutch L/C is engaged at the first-speed of D range. 
     At the second speed of D range, both the low clutch L/C and 2-4 brake 2-4/B are engaged. 
     At the third-speed of D range, both low clutch L/C and a high clutch H/C are engaged. 
     In the fourth-speed of D range, the high clutch H/C and 2-4 brake 2-4/B are clutched. It is noted that during the first-speed (engine braking is effected at the first-speed range) in a HOLD mode in a low range (hereinafter, L range), the low clutch L/C and the low &amp; reverse clutch (L &amp; R/B) are engaged. 
     FIG. 4A shows a gear control system of the vehicular automatic transmission to which the hydraulic control apparatus according to the present invention is applicable. 
     In FIG. 4A, a reference numeral  1  denotes a line pressure hydraulic passage,  2  denotes a manual valve,  3  denotes a D range pressure hydraulic passage, and  4  denotes an R range pressure hydraulic passage. 
     The manual valve  2  is a valve which can be switched according to a select operation into the D range in which a line pressure oil passage  1  and the D range pressure oil passage  3  are connected or into the D range in which the line pressure oil passage  1  and the D range pressure oil passage  3  are connected, and into the R range in which the line pressure oil passage  1  and the R range pressure passage  4  are connected. 
     In addition, in FIG. 4A, a reference numeral  5  denotes a pilot valve and  6  denotes a pilot pressure oil passage. 
     When the pilot valve  5  serves to perform a pressure decrease control over the line pressure from the line pressure oil passage  1  to a constant pilot pressure. 
     In addition, a reference numeral  7  shown in FIG. 4A denotes a duty ratio control low clutch solenoid which supplies a control pressure to a low clutch amplifier valve  8 , produces a low clutch pressure from a D range pressure in a low clutch amplification valve  8 , and introduces the low clutch via a low clutch pressure oil pressure passage  9 . 
     A reference numeral  10  denotes a duty ratio control high clutch solenoid. The duty ratio control high clutch solenoid  10  supplies a control pressure for a high clutch amplifier valve  11 . Then, the high clutch amplifier valve  11  produces the low clutch pressure from the D range pressure at the low clutch amplifier valve  8  and introduces the low clutch pressure to the low clutch L/C via the low clutch pressure oil passage  9 . A high clutch oil pressure switch  13  is installed on the high clutch pressure oil passage  12 . At the same time when the hydraulic is supplied to the high clutch, the hydraulic is supplied to a high clutch hydraulic switch  13  to turn the switch  13  ON. 
     A reference numeral  14  denotes a duty-ratio control 2-4 brake solenoid. The control pressure is supplied to a 2-4 brake solenoid. The control pressure is supplied to a 2-4 brake amplifier solenoid  15 . A 2-4 brake pressure is produced at the 2-4 brake amplifier valve  15  from the D range pressure PD and is introduced via the 2-4 brake pressure oil passage  16  to the 2-4 brake 2-4/B. A 2-4 brake oil pressure switch  17  is installed on 2-4 brake pressure oil passage  16 . At the same time when the hydraulic is supplied to the 2-4 brake, the hydraulic is supplied to the 2-4 brake hydraulic switch  17  to be turned ON, thus 1&gt;ON. 
     A low &amp; reverse brake solenoid  18  supplies the control pressure for the low &amp; reverse brake amplifier valve  19 . The low &amp; reverse brake pressure is produced from the line pressure and is introduced to the low &amp; reverse brake L &amp; R/B via the low &amp; reverse brake pressure oil passage  20 . 
     A pressure control solenoid  22  of an on-and-off type switches the line pressure into two stages of a high pressure and a low pressure. 
     A duty-ratio lock-up solenoid  23  serves to control the engagement and release of the lock-up clutch. 
     An AT control unit (ATCU)  24  carries out various kinds of control operations including a gear shift control on the basis of the input information and outputs a solenoid drive current for each solenoid  7 ,  10 ,  14 ,  18 ,  22 , and  23  according to the result of processing. 
     Then, a 2-4 brake first fail-safe valve  25  is hydraulic operating valve in which both of a fuel pressure PFP which is at any time acted upon one end of a spool(a hydraulic having the same value as a maximum pressure of a high clutch H/C engaged at a high-speed stage of the Drive range) and a low clutch pressure (L/CP) which is acted upon the other end of the spool serve as operation signal pressures. A 2-4 bake second fail-safe valve  26  is the hydraulic operating valve in which both of the fuel pressure PFP which is acted upon the one end of the spool and the high clutch pressure PH/C which is acted upon the other end of the spool serve as the operation signal pressures. 
     At a third-speed of the Drive range at which both of the low clutch and the high clutch pressures are simultaneously developed, the high clutch pressure is applied to the 2-4 brake second fail-safe valve  26  so that the low clutch pressure is applied to the 2-4 brake first fail-safe valve  25 . Hence, since the D range pressure is drained, a 2-4 brake pressure is forcefully drained. 
     In FIG. 3, a reference numeral  27  denotes a low-and-reverse brake first fail-safe valve and a reference numeral  28  denotes a low-and-reverse brake second fail-safe valve. In each of the low-and-reverse (L &amp; R) brake first fail-safe valve  27  and the low-and-reverse brake second fail-safe valve  28 , the fuel pressure PFP which is at any time acted upon the one end of the spool and either of the high clutch pressure PH/C or 2-4 brake pressure P 2 - 4 B serve as the operation signal pressures. In addition, each of the L &amp; R brake first and second fail-safe valves  27  and  28  drains forcefully a line pressure so that the low-and-reverse brake pressure is drained at the 2 nd -speed, 3 rd -speed, and 4 th -speed of the D range at which either one or both of the high clutch pressure P 2 - 4 /B is developed so that the L &amp; R brake pressure is drained. 
     FIG. 4A is a schematic circuit block diagram representing an electronic control system of the hydraulic control operation for the automatic transmission in the preferred embodiment according to the present invention. 
     An engine control unit  29  transmits a throttle opening angle TH and an engine revolution speed Ne in terms of a serial transmission control unit  24  (abbreviated as ATCU). It is noted that a torque down (reduction of torque)communication is carried out between both engine control unit  29  and the automatic transmission unit ATCU  24 . A turbin revolution speed Nt and an output axle revolution speed No from a turbine speed sensor  30  and an output axle speed sensor  38  in a power train P/T are inputted to the automatic transmission control unit (ATCU)  24 . 
     An inhibitor switch  31  installed within the automatic transmission shown in FIG. 1 supplies a range signal to the ATCU  24  and a hold switch  32  installed within the automatic transmission shown in FIG. 1 outputs a hold switch signal to the ATCU  24 . A high clutch hydraulic switch  13 , a 2-4 brake hydraulic switch  17 , and a low-and-reverse (L&amp;R) brake switch  21  each installed in a control valve unit output switch signals representing hydraulic supply states on their corresponding engageable elements to the ATCU  24 . An oil temperature signal is inputted from an oil temperature sensor  36 . 
     A solenoid driver current is outputted from the ATCU  24  to each corresponding one of solenoids  7 ,  10 ,  14 ,  18 ,  22 , and  23  and a speed display signal is outputted to a speedometer  37  disposed on an instrument panel. 
     FIG. 4B shows an internal circuit block diagram of the ATCU (automatic transmission control unit)  24 . 
     In FIG. 4B, the ATCU  24  generally includes: a microprocessor (MPU)  24   a , a timer interrupt controller  24   b ; a RAM (Random Access Memory)  24   c ; a ROM  24   d (Read Only Memory);an Input Port  24   e ; an Output Port  24   f ; an input communication controller included in the input port  24   e ; and an output communication controller included in the output port  24   f.    
     Next, an action in the hydraulic control apparatus in the preferred embodiment will be described below. 
     [Failure Detection Processing] 
     FIG. 5 is an operational flowchart on a failure detection processing of the hydraulic control apparatus in the preferred embodiment. 
     At a step  101 , the ATCU  24  determines the present gear stage according to the vehicular velocity, throttle opening angle TH, and the select position information (range). 
     At the next step  102 , the ATCU  25  determines whether the automatic transmission A/T falls in a gear shift position or a gear range select operation is being carried out. 
     If Yes at the step  101 , the present routine is ended. 
     If No at step  102 , the routine goes to a step  103 . 
     At step  103 , the ATCU  24  determines whether it is the immediate after the present gear stage is in a predetermined gear range. 
     If No at step  103 , the present routine is ended. 
     If Yes at step  103 , the present routine goes from step  103  to a step  104 . 
     At step  104 , the ATCU  24  determines if the present gear range falls in a forward run range. 
     If No at step  104 , the present routine is ended. 
     If Yes at step  104 , the present routine goes to a step  105 . 
     At step  105 , the ATCU  24  determines whether the vehicular velocity V is equal to or higher than a predetermined vehicular velocity Vpre. 
     If V&lt;Vpre (No) at step  105 , the present routine is ended. 
     If V≧Vpre (Yes) at step  105 , the present routine goes to a step  106 . 
     At step  106 , the ATCU  24  determines whether the present oil temperature T is equal to or higher than a predetermined oil temperature Tpre. 
     If T&lt;Tpre (No) at step  106 , the present routine is ended. 
     At a step  107 , the ATCU  24  outputs an engagement signal (a power supply to the corresponding solenoid (this is intermediate value) to any one of the clutches not to be engaged from a solenoid output table in case where the failure in any of the fail-safe values is detected. 
     At a step  108 , the ATCU  24  determines whether the hydraulic switch of one of the clutches not to be engaged is turned on. 
     If not turned on (No) at step  108 , the routine goes to a step  111 . 
     If turned on (yes) at step  108 , the routine goes to a step  109 . 
     At step  109 , the ATCU  24  determines whether a predetermined period of time has passed from a time at which the hydraulic switch ON state of one of the clutches which is not to be engaged. If No at step  109 , the routine goes to step  112 . If Yes at step  109 ,the routine goes to a step  110 . 
     At step  110 , a failure counter is incremented by one {(failure counter)+1} since the conditions at steps  108  and  109  are satisfied. 
     At step  111 , the content of the failure counter is cleared to zero since the hydraulic switch ON condition at step  108  is not satisfied. 
     Then, at a step  112 , the ATCU  24  determines if the number of counts in the failure counter is equal to or greater than a predetermined number of times Tipre. 
     If (failure counter)≧Tipre (Yes) at step  112 , the routine goes to step  113 . If (Failure counter)&lt;Tipre (No) at step  112 , the routine goes to a step  114 . 
     At step  113 , the ATCU  24  determines that the fail-safe valve has failed and the routine goes to a step  115 . 
     At step  114 , the ATCU  24  determines that the fail-safe valve operated normally. 
     At step  115 , the ATCU  24  warns the vehicular occupant of the occurrence of failure in the fail-safe valve. 
     [Low &amp; Reverse Brake Fail-safe Valve Failure Determination] 
     A case where the above-described failure detection processing is applied to a failure determination on the low-and-reverse brake first fail-safe valve  21  and the low-and-reverse second fail-safe valve  28  will be described in details below. 
     The gear stage is determined according to the vehicular velocity V, the opening angle of the throttle valve TH, and the select position information, as described at step  101  shown in FIG. 5, in order to confirm that the automatic transmission is neither in the gear shift operation nor in the select operation of the gear range. 
     This determination process is carried out because the hydraulic is not stable during the gear shift operation or during the select operation and, thus, the abnormality determination is difficult. 
     Next, the reason for the confirmation of whether the present time is immediate after the predetermined gear range and the present gear range falls in the forward run range (in this case, 2nd-speed, 3rd-speed, or the 4th-speed) will be described below. 
     That is to say, when the 2-4 brake (2-4/B) and the low-and-reverse brake (L &amp; R/B) are simultaneously engaged or the high clutch and the low-and-reverse brake are simultaneously engaged, the interlock phenomenon occurs. 
     To prevent this interlock, when the 2-4 brake pressure is developed or the high clutch pressure is developed, the low-and-reverse brake first and second fail-safe valves  27  and  28  are operated to forcefully drain the low-and-reverse brake pressure. In other words, when the gear range indicates 2nd-speed, 3rd-speed, or 4th-speed, the low-and-reverse solenoid  18  is operated to engage the low-and-reverse brake. However, if the fail-safe valve  27  or  28  is normal, the low-and-reverse brake is not always engaged. 
     When the above-described condition is satisfied, an engagement command signal is outputted to the low-and-reverse brake solenoid  18 , as shown in FIG. 6, when the gear range indicates 2nd, 3rd, or 4th-speed. 
     FIG. 7 shows a time relationship between the power supply and the power supply duration of time. 
     In FIG. 7, although a lateral axis denotes the time and a longitudinal axis denotes the hydraulic, this hydraulic means that if the OFF output is carried out for the solenoid, what degree of the hydraulic is developed under the application of the normal hydraulic. Hence, at the 2nd-speed, 3rd-speed, or 4th-speed, if the low-and-reverse brake first and second fail-safe valves  27  and  28  operates normally, the hydraulic is not supplied to the low-and-reverse brake (L &amp; R/B). 
     In FIG. 7, a denotes the hydraulic at which no hydraulic switch is turned on and b denotes the hydraulic at which the hydraulic switch is always turned on. 
     It is noted that the outputted hydraulic at which the low-and-reverse brake hydraulic switch  21  is turned on is the hydraulic to a degree slightly higher than a minimum pressure required to engage the low-and-reverse brake. Hence, even if the hydraulic were developed, the hydraulic is so weak that the clutch drags and no interlock occurs and the failure can be detected. 
     In addition, c in FIG. 7 denotes a time duration longer than a chattering time duration (a time duration which enables the influence of an oil vibration due to the occurrence in the duty ratio period to be eliminated), d in FIG. 7 denotes a ramp pressure, e denotes a delay time duration, and f denotes a time out of a ramp time. 
     It is noted that the time out f of the ramp time is to prevent an endless loop of the hydraulic control in a case where a gradient of the ramp pressure is small. 
     In a case where the hydraulic to b has not reached even if it takes the time of f, a region of c is forcefully transferred as the hydraulic of b. 
     Under the above-described state of c when the low-and-reverse brake hydraulic switch  21  turns to ON from OFF state (OFF→ON) and the turn on state of the switch  21  is detected continuously for 30 milliseconds, a once abnormal state is determined. If the twice abnormality determinations are made, the ATCU  24  can determine the failure in the fail-safe valve. Hence, the ATCU  24  can perform the warning for the vehicular driver of the warning. 
     The continuous twice failure detection to determine whether the fail-safe valve is abnormal is to prevent an erroneous detection. 
     In addition, when, in a midway through c, the automatic transmission control unit  24  detects that the low-and-reverse brake hydraulic switch  21  is turned from the ON state to the OFF state, the ATCU  24  does not issue the command to increment the failure counter by one in order to prevent the erroneous detection. 
     If is of course that the failure detection processing should be halted when the gear shift or the select control is carried out during the execution of such a processing as described above and the priority higher than the present control is taken for the other control. 
     While the gear range is the same, each fail-safe valve does not stick in a reverse direction spontaneously. It is not necessary to carry out the failure detection processing many times at irregular interval of time. 
     The specific failure detection of each of the two low-and-reverse (L &amp; R) brake fail-safe valves  27  and  28  will be reviewed from the flowchart shown in FIG.  5 . 
     Since, at the 3rd-speed range, the 2-4 brake pressure P 2 - 4 /B is not applied but the high clutch pressure PH/C is applied to the low-and-reverse brake first fail-safe valve  27 , the line pressure is drained. At this stage, the power is supplied to the low-and-reverse solenoid  18 . This power supply quantity corresponds to a pressure such that the above-described corresponding low-and-reverse brake hydraulic switch is turned on. At this time, if the low-and-reverse brake hydraulic switch  21  is turned on, the automatic transmission control unit  24  determines the failure of the low-and-reverse first fail-safe valve  27  and carries out the warning to the vehicular occupant. 
     Similarly, since, when the 2nd-speed range occurs, the high clutch pressure PH/C is not applied but the 2-4 brake pressure P 2 - 4 /B is applied to the above-described corresponding brake second fail-safe valve  28 , the line pressure is drained. At this stage, the power supply to the 2-4 brake solenoid  14  as shown in FIG. 4 is carried out and the 2-4 brake hydraulic switch  17  is turned from OFF state to ON state (OFF→ON), the automatic transmission control unit  24  determines the occurrence of failure in the low-and-reverse brake second fail-safe valve  28  and carries out the warning to the vehicular occupant. 
     [Failure Determination of 2-4 brake fail-safe valve] 
     A case where the above-described processing shown in FIG. 5 is applied to the failure determination of the 2-4 brake first and second fail-safe valves  25  and  26  will be described below. 
     However, a basic process flow is generally the same as described with respect to the case of the above-described low-and-reverse brake first and second fail-safe valves  27  and  28 . Hence, a difference point from the case of the valves  27  and  28  will be described. 
     The reason for the confirmation that the automatic transmission falls in the forward run range (in this case, 3rd-speed) immediately after the predetermined gear range is entered will be described below. 
     That is to say, when the 2-4 brake occurs, the high clutch and the low clutch are simultaneously engaged, the interlock phenomenon occurs. 
     To prevent this interlock, when the high clutch pressure PH/C and the low clutch pressure PL/C are developed simultaneously, the 2-4 brake first and second fail-safe valve  25  and  26  cause the 2-4 brake pressure to forcefully be drained. 
     In details, when the gear stage is at the 3-rd speed range, the 2-4 brake solenoid  14  is operated. Even if 2-4 brake is tried to be engaged, the 2-4 brake is always not engaged provided that the fail-safe valves  25  and  26  are normal. 
     When the above-described condition is satisfied, the engagement command signal is outputted to the 2-4 brake solenoid  14  at the time of 3rd-speed range as shown in FIG.  8 . 
     The specific failure detection on the 2-4 brake fail-safe valves  25  and  26  will be described below with reference to the flowchart shown in FIG.  5 . 
     Under the 3rd-speed range, the 2-4 brake pressure P 2 - 4 /B is not applied but the high clutch pressure PH/C is applied so that the low clutch pressure PL/C is applied to the 2-4 brake first fail-safe valve  25 . The application of the low clutch pressure PL/C causes the D range pressure at the 2-4 brake first fail-safe valve to be drained. 
     At this stage, the power is supplied to the 2-4 brake solenoid  14 . The power supply quantity, at this time, corresponds to the pressure enough to turn on the above-described corresponding hydraulic switch (the 2-4 brake hydraulic switch  17 ). 
     When the 2-4 brake hydraulic switch  17  is turned on, the automatic transmission control unit  24  determines that either the 2-4 brake first or second fail-safe valve  25  or  26  has failed and issues the warning to the vehicular occupant. 
     According to the failure detection on the above-described fail-safe valves, the automatic transmission control unit  24  always monitors that each fail-safe valve is operated without failure in order to generate the interlock even if a winding of each solenoid is broken. 
     Such a failure that cannot conventionally be detected (so-called, a sleeping fail) becomes undetectable. Furthermore, the occurrence of failure is informed to the vehicular occupant. A frequency of the interlock can be reduced by carrying out a fail-safe on a double failure when another failure occurs. A safety level can be improved. 
     It is noted that, although, in the preferred embodiment described above, two valves are exemplified as the fail-safe valves, a single valve or three or more valves may constitute the fail-safe valves. 
     The entire contents of a Japanese Patent Application No. Heisei 11-223337 (filed in Japan on Aug. 6, 1999) are herein incorporated by reference. Although the invention has been described above by reference to certain embodiment of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in the light of the above teachings. The scope of the invention is defined with reference to the following claims.