Patent Publication Number: US-7222005-B2

Title: Abnormal oil pressure reduction determination device for vehicle transmission

Description:
FIELD OF THE INVENTION 
   This invention relates to an abnormal oil pressure reduction (drop) determination device which can be used for a vehicle transmission controlled by oil pressure. 
   BACKGROUND OF THE INVENTION 
   In the prior art, the operation of a vehicle transmission is controlled by oil pressure. For example, in a belt type continuously variable transmission (CVT), torque capacity is adjusted by controlling the oil pressure supplied to a pulley. Torque capacity is the maximum torque which can be transmitted without causing the belt slip of the CVT. Tokkai-Hei 05-240331 published by the Japanese Patent Office in 1993 discloses a belt type CVT wherein, in a transient state where the accelerator pedal stroke (TVO) of the vehicle changes, or a speed change is performed, the oil pressure which adjusts torque capacity is controlled consistently. 
   SUMMARY OF THE INVENTION 
   However, in the prior art, even if the supply oil pressure drops temporarily due to some reason, an oil pressure command value is increased so that the required torque capacity is always obtained. For this purpose, an oil pump is operated excessively, and this impairs fuel consumption-performance. 
   It is therefore an object of this invention to provide an abnormal oil pressure reduction determination device for a vehicle transmission which can determine the situation where the oil pressure of the transmission is decreased abnormally for some reason. 
   In order to achieve the above object, this invention provides an abnormal oil pressure reduction determination device for use with a transmission of a vehicle, the transmission having an oil pressure control mechanism which performs speed change control using oil pressure. The abnormal oil pressure reduction determination device comprises an oil pressure sensor which detects a real oil pressure, a sensor which detects a vehicle running state, and a microcomputer-based controller which sets an oil pressure command value for the oil pressure control mechanism. The controller functions to compute a lower limiting oil pressure which is possible in the present vehicle running state, compare the real oil pressure with the lower limiting oil pressure, compare the real oil pressure with the oil pressure command value, and when the real oil pressure is less than the lower limiting oil pressure and when a pressure difference between the real oil pressure and oil pressure command value exceeds a reference value, determine that there is an abnormal oil pressure reduction due to a fault in the oil pressure control mechanism. 
   The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing an abnormal oil pressure reduction determination device for a vehicle transmission. 
       FIG. 2  is a schematic view of an oil pressure control mechanism for a vehicle transmission, and a CVT controller. 
       FIG. 3  is a block diagram showing the control performed by the CVT controller of the abnormal oil pressure reduction determination device. 
       FIG. 4  is a flowchart showing a control routine performed by the CVT controller of the abnormal oil pressure reduction determination device. 
       FIG. 5  is a flowchart specifically showing a subroutine of a Step S 1  of  FIG. 4 . 
       FIG. 6  is a graph describing a determination of a continuous abnormal oil pressure reduction.  FIG. 6A  shows a time variation of a difference D 1  between an oil pressure command value and a real oil pressure.  FIG. 6B  shows a time variation of an accelerator pedal stroke (throttle valve opening) (TVO).  FIG. 6C  shows a time variation of a vehicle speed (Vsp).  FIG. 6D  shows a time variation of a timer value for measuring the duration of an abnormal oil pressure reduction. 
       FIG. 7  is a graph describing an abnormal oil pressure reduction determination. 
       FIG. 8  is a graph describing the reproducibility determination for the abnormal oil pressure reduction, which expands  FIG. 7  to earlier and later timings. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows one embodiment of a determination device for determining an abnormal oil pressure reduction of a vehicle transmission according to this invention. 
   A torque converter  2 , forward/reverse change-over mechanism  3  and a transmission  10  form a transmission system. In this embodiment, a belt type continuously variable transmission is used as the transmission  10 . The transmission  10  is provided with a primary pulley  11 , secondary pulley  12 , V belt  13 , CVT controller  20  and oil pressure control mechanism  30 . 
   The primary pulley  11  is an input shaft pulley which inputs the rotation of the engine  1  to the transmission  10 . The primary pulley  11  is provided with a fixed conical plate  11   b  which rotates together with the input shaft  11   d , and a movable conical plate  11   a  which can displace in an axial direction. The movable conical plate  11   a  faces the fixed conical plate  11   b  to form a V-shaped pulley groove, and is displaced in the axial direction by an oil pressure (primary pressure) which acts on a primary pulley cylinder chamber  11   c . The primary pulley  11  is connected to the engine  1  via the forward/reverse change-over mechanism  3  and the torque converter  2  provided with a lock-up clutch, and inputs the rotation of the engine  1 . The rotation speed N 1  of the primary pulley  11  is detected by a primary pulley rotation speed sensor  26 . The rotation speed Ne of the engine  1  is detected by an engine rotation speed sensor  37 . 
   The V belt  13  is wound around the primary pulley  11  and the secondary pulley  12 , and transmits the rotation of the primary pulley  11  to the secondary pulley  12 . The secondary pulley  12  outputs the rotation transmitted by the V belt  13  to a differential  4 . The secondary pulley  12  is provided with a fixed conical plate  12   b  which rotates together with an output shaft  12   d , and comprises a movable conical plate  12   a  which can be displaced. The movable conical plate  12   a  faces the fixed conical plate  12   b  to form a V-shaped pulley groove, and is displaced in an axial direction by the oil pressure (secondary pressure) which acts on a secondary pulley cylinder chamber  12   c . The pressure-receiving surface area of the secondary pulley cylinder chamber  12   c  is set approximately equal to the pressure-receiving surface area of the primary pulley cylinder chamber  11   c.    
   The secondary pulley  12  is connected to the differential  4  via an idler gear  14  and an idler shaft, and transmits rotation to the differential  4 . The rotation speed N 2  of the secondary pulley  12  is detected by a secondary pulley rotation speed sensor  27 . A vehicle speed can be computed from the rotation speed N 2  of the secondary pulley  12 . The forward/reverse change-over mechanism  3  disposed between the engine and a CVT speed change part is a common type comprising a planetary gear, a forward clutch, and a reverse clutch (or reverse brake). The clutches change a power transfer path. The forward clutch and reverse clutch are engaged/released by supply/non-supply of oil pressure. When the vehicle advances, due to the oil pressure (forward clutch pressure) supplied to a forward clutch piston chamber, the forward clutch engages with the planetary gear and the fixed conical plate  11   b  rotates clockwise. 
   On the other hand, when the vehicle reverses, due to the oil pressure (reverse clutch pressure) supplied to a reverse clutch piston chamber, the reverse clutch engages with the planetary gear and the fixed conical plate  11   b  rotates counterclockwise. In the neutral state (neutral range and parking range), both the forward clutch and reverse clutch are released. 
   Signals from an inhibitor switch  23 , accelerator stroke amount sensor  24 , oil temperature sensor  25 , primary pulley rotation speed sensor  26 , secondary pulley rotation speed sensor  27 , oil pressure sensor  28 , vehicle-dynamics-control (VDC) unit  29 , engine rotation speed sensor  37  and an input torque signal (engine torque signal) from an engine controller  21  are inputted into the CVT controller  20 . Based on these signals, the CVT controller  20  determines a speed ratio and contact frictional force. Herein, the speed ratio is a value obtained by dividing the effective radius of the secondary pulley  12  by the effective radius of the primary pulley  11 , and is identical to the pulley ratio. The CVT controller  20  transmits a command to the oil pressure control mechanism  30  to control the transmission  10 , and determines whether or not the oil pressure of the transmission dropped abnormally. The specific details of this determination are described later. 
   The VDC unit  29  and the engine controller  21  are microcomputer-based controllers. Also, the CVT controller  20  included in the abnormal oil pressure reduction determination device comprises a microcomputer which is provided with a central processing unit (CPU) which executes programs, read-only memory (ROM) which stores programs and data, random access memory (RAM) which stores the computation result-of the CPU and acquired data temporarily, one or more timers for measuring time, and an input/output interface (I/O interface). 
   The inhibitor switch  23  is a sensor which detects the position of a shift lever (not shown) used so that a driver may select the operating range of the transmission system. The shift lever position and the operating range of the transmission system have a 1:1 correspondence. The inhibitor switch  23  sends out a range signal Rs showing the position of the shift lever to the CVT controller  20 . The accelerator stroke amount sensor  24  detects the stroke of an accelerator pedal. The oil temperature sensor  25  detects the oil temperature supplied to the pulley cylinder chambers of the transmission  10 . The oil pressure sensor  28  detects the oil pressure acting on the cylinder chamber  12   c  of the secondary pulley  12 . 
   The oil pressure control mechanism  30  operates according to a command from the CVT controller  20 . The oil pressure control mechanism  30  supplies oil pressure to the primary pulley  11  and the secondary pulley  12 , and causes the movable conical plate  11   a  and the movable conical plate  12   a  to move to and fro in the respective rotation axis direction. The movement of the movable conical plate  11   a  and movable conical plate  12   a  varies the pulley groove width, so the V belt  13  displaces on the primary pulley  11  and the secondary pulley  12 . Due to this, the contact radius of the V belt  13  with the primary pulley  11  and the secondary pulley  12  varies, and the speed ratio and contact frictional force received by the V belt  13  are controlled. 
   The rotation of the engine  1  is inputted into the transmission  10  via the torque converter  2  and the forward/reverse change-over mechanism  3 , and is transmitted from the primary pulley  11  to the differential  4  via the V belt  13  and the secondary pulley  12 . The rotation speed of the engine  1  detected by the engine rotation speed sensor  37  is inputted into the CVT controller  20  via the engine controller  21 , or it is directly inputted into the CVT controller  20 . 
   If the accelerator pedal is depressed or the operating range of the transmission system is changed in the manual mode, the movable conical plate  11   a  of the primary pulley  11  and movable conical plate  12   a  of the secondary pulley  12  will displace in the respective axial direction. Due to the variation of the contact radius with the V belt  13 , the speed ratio varies continuously. 
     FIG. 2  is a conceptual diagram of the oil pressure control mechanism  30  and of the CVT controller  20  of the transmission according to this invention. The oil pressure control mechanism  30  is provided with a regulator valve  31 , a speed change control valve  32  and a pressure reduction valve  33 . It controls the oil pressure supplied from an oil pump  34 , and supplies it to the primary pulley  11  and the secondary pulley  12 . 
   The regulator valve  31  is an escape valve comprising a solenoid, and regulates the pressure of the oil supplied from an oil pump  34  to a predetermined line pressure PL according to a running state of the vehicle according to commands (for example, a duty signal etc.) from the CVT controller  20 . 
   The speed change control valve  32  controls the oil pressure (henceforth “primary pressure”) of the primary pulley cylinder chamber  11   c  to coincide with a primary pulley target oil pressure (i.e. primary oil pressure command value) described later. The speed change control valve  32  is connected with a servo link  50  forming a mechanical feedback mechanism, and is driven by a step motor  40  connected to the end of the servo link  50 . The groove width, i.e., real speed ratio, is fed back from the movable conical plate  11   a  of the primary pulley  11  connected to the other end of the servo link  50 . The speed ratio control valve  32  extracts or inputs oil pressure from or to the primary pulley cylinder chamber  11   c  according to the displacement of the spool  32   a . The primary pressure is thereby adjusted so that the target speed ratio specified by the rotation position of a step motor  40  is attained, and after an actual speed change-over is completed, the spool  32   a  is held in the closed valve position in response to the displacement from the servo link  50 . 
   The pressure reducing valve  33  is provided with a solenoid, and controls the pressure (henceforth “secondary pressure”) supplied to the secondary pulley cylinder chamber  12   c , to a secondary pulley target oil pressure (i.e. secondary oil pressure command value) described later. The line pressure PL supplied from the oil pump  34 , which was regulated by the regulator valve  31 , is supplied to the speed change control valve  32  and pressure reduction valve  33 , respectively. 
   The speed ratio of the primary pulley  11  and the secondary pulley  12  is controlled by the step motor  40  driven according to a speed change command signal from the CVT controller  20 . The spool  32   a  of the control valve  32  is driven according to the displacement of the servo link  50  in response to the step motor  40 . Due to this, the line pressure PL supplied to the speed change control valve  32  is adjusted, a primary pressure is supplied to the primary pulley  11 , the groove width is controlled, and a predetermined speed ratio is attained. 
   The CVT controller  20  reads the shift lever position from the inhibitor switch  23 , the accelerator pedal stroke from the accelerator stroke sensor  24 , the oil temperature of the transmission  10  from the oil temperature sensor  25 , and signals from the primary pulley speed sensor  26 , the secondary pulley speed sensor  27 , the oil pressure sensor  28  and the vehicle-dynamics-control (VDC) unit  29 . By reading this data, the speed ratio and the contact frictional force of the V belt  13  are controlled, and it is determined whether an abnormal reduction in oil pressure occurred as described later. The VDC unit  29 , in order to prevent lateral instability of the vehicle, controls the engine  1 , braking system and the oil pressure system. 
   The CVT controller  20  controls the present speed ratio to a target speed ratio by determining a target speed gear ratio according to the vehicle speed or throttle opening, and driving the step motor  40 . The CVT controller  20  also controls the thrust of the primary pulley  11  and secondary pulley  12 , i.e., the oil pressure, according to the input torque to the transmission  10 , speed ratio of the transmission  10 , oil temperature and target speed change rate. The input torque to the transmission  10  corresponds to the output torque from the torque converter  2  and is computed by CVT controller  20  based on the engine torque from the engine controller  21  and the ratio between the output rotation speed and input rotation speed of the torque converter  2 . 
   Referring to the block diagram of  FIG. 3 , the abnormal oil pressure reduction determination of the CVT controller  20  will now be described. In  FIG. 3 , each section is an imaginary section showing a function performed by the microcomputer of the CVT controller  20 . 
   A control region determination unit  211  determines whether or not the engine rotation speed and secondary pressure are in a region in which feedback control of secondary pressure is possible. Specifically, the control region determination unit  211  calculates a difference D 1  of the real oil pressure of the secondary pulley and a target oil pressure for the secondary pulley (i.e. secondary oil pressure command value), and determines whether or not the secondary pressure is in a region where the secondary oil pressure command value can be attained. Herein, the case where feedback control of secondary pressure cannot be performed is for example the case that the engine rotation speed is low, or the case that the oil pressure sensor  28  has a fault. As the oil pump  34  which supplies secondary pressure is usually driven by the engine  1 , if the engine rotation speed is lower than a predetermined rotation speed, the oil pump cannot generate sufficient oil pressure. Therefore, when the engine does not reach the predetermined rotation speed, feedback control of secondary pressure is not performed. When the oil pressure sensor  28  has a fault, a precise real oil pressure cannot be obtained for the secondary pressure, and a detection error may occur. Therefore, feedback control is not performed in this case, either. Hence, the control region determination unit  211  determines whether the vehicle is in a state where feedback control of the secondary pressure cannot be performed, based on signals corresponding to the engine rotation speed and the real secondary oil pressure. 
   An oil pressure control mode determination unit  212  determines whether or not an oil pressure control mode is an ordinary mode. Herein, the ordinary mode is a mode in which the oil pressure command values are set based on the input torque to the transmission  10  and speed ratio of the transmission  10 . A non-ordinary mode is a mode in which the oil pressure command values are set regardless of the balance between the oil amount supplied from the pump  34  and the oil amount used, for example in the case where the oil temperature is in a very low temperature region, or the case where the operating range of the transmission system is changing. The very low temperature region may be a region below a predetermined temperature (for example, −30° C.). In other words, if the temperature is very low, the viscosity of the oil is high, so the oil pressure is taken as the maximum pressure which can possibly be generated. Also, when the operating range of the transmission system is changing, in order to ensure the oil pressure required to engage the clutch of the forward/reverse change-over mechanism  3 , the oil pressure (line pressure) supplied to the pulley is limited. As a result, the target oil pressure is not determined based on the input torque and speed ratio, and an abnormal oil pressure reduction in the oil pressure control mechanism  30  cannot be determined by the difference D 1  between the secondary oil pressure command value and real secondary pressure. To detect this situation, the oil pressure control mode determination unit  212  determines whether or not the vehicle is in the aforesaid ordinary mode based on an oil temperature signal from the oil temperature sensor  25  and a range signal Rs from the inhibitor switch  23 . 
   A spin recovery correction determination unit  213  determines whether or not the VDC unit  29  is performing spin recovery correction. Spin recovery correction means that, when vehicle sideslip prevention control is being performed in order to prevent the vehicle from running unstably when the tires slip, the oil pressure is increased to a limit determined by the balance between the supply oil amount and used oil amount, and further the engine torque is limited. During spin recovery correction, when the dynamic frictional coefficient between the vehicle tires and road surface increases, a torque acts from the vehicle wheels on the transmission  1  in the reverse direction, and a correction must be performed for this reverse torque. Therefore, during spin recovery correction, since the extent of the torque input in the reverse direction is unknown, the oil pressure is increased to a limit and further the engine torque is limited. As the engine torque is limited, the engine rotation speed is suppressed low, and thereby the oil pressure which can be generated by the oil pump  34  is low. Even if the reduction of oil pressure is determined in this state, the real oil pressure of the secondary pulley cannot be increased to the secondary oil pressure command value, and thus the determination of abnormal oil pressure reduction will be incorrect. Therefore, the determination is not performed at this time. To avoid performing the determination of abnormal oil pressure reduction during spin recovery correction, by the spin recovery correction determination unit  213 , it is determined whether or not spin recovery correction is being performed based on a control signal from the VDC unit  29  and a vehicle speed signal. 
   A brake SW determination unit  214  determines whether or not a brake switch  41  is OFF. Even if the brake switch  41  is ON, a torque is input from the engine to the transmission  1 , and a torque is input from the vehicle wheels to the transmission  1  in the reverse direction, so a correction must be made for this reverse torque. When the brake switch  41  is ON, the torque input in the reverse direction is normally larger than the torque input from the engine. As a result, the input torque to the transmission  10  is corrected, taking into account the torque input in the reverse direction, and the target oil pressure (i.e. oil pressure command value) is set based on the corrected input torque. Due to this, the secondary oil pressure command value becomes sufficiently large. Also, when the brake switch  41  is ON, the engine rotation speed is suppressed low, so a fairly large oil pressure cannot be generated. However, the secondary oil pressure command value is high because of the correction for the reverse torque. If oil pressure reduction is determined in this state, since the real oil pressure of the secondary pulley cannot be increased to the secondary oil pressure command value, the oil pressure reduction will be incorrectly determined. 
   Therefore, when the brake switch  41  is ON, an oil pressure reduction determination is not performed. In order to avoid performing the oil pressure reduction determination when the brake switch  41  is ON, a signal is input from the brake switch  41 , and ON/OFF of the brake switch  41  is determined in the brake SW determination unit  214 . 
   In a shift range determination unit  215 , it is determined whether or not the shift range position is in a position other than the N range position, based on a range signal Rs. When the shift range position is the N range position, the command value of the line pressure is set to the oil pressure which can really be generated (i.e. the oil pressure above a lower limit), and it is further set to a value lower than an upper limit below which oil pump noise is sufficiently low. Therefore, in this case also, there is a possibility that the real oil pressure for the secondary pulley cannot be increased to the secondary oil pressure command value, so it is determined in the shift range determination unit  215  whether or not the shift range position is outside the N range position. 
   A re-determination prohibition state determination unit  216  determines whether or not re-determination of oil pressure reduction can be performed based on a signal from a re-determination prohibition timer  242 , described later. Specifically, to verify that the oil pressure reduction is reproducible and to avoid incorrect determination, a re-determination is permitted by the re-determination prohibition state determination unit  216  after the unit  216  determines that a second predetermined time period has elapsed in the re-determination prohibition timer  242 . 
   An oil pressure difference determination unit  221  determines whether or not the secondary oil pressure command value and the real secondary pressure effectively coincide. Specifically, if the oil pressure difference D 1  between a secondary oil pressure command value issued to the pressure reduction valve  33  and the real secondary pressure of the oil pressure sensor  28  is less than a reference value, the real secondary pressure effectively coincides with the secondary oil pressure command value, so it is determined that an abnormal oil pressure reduction is absent. If the oil pressure difference D 1  is more than the reference value, it is determined that an abnormal oil pressure is present. Hence, in the oil pressure determination unit  221 , it is determined whether or not the real oil pressure of the secondary pulley has effectively reached the secondary oil pressure command value. Herein, the reference value of the pressure difference D 1  between the secondary oil pressure command value and real secondary pressure is set taking account of the detection precision of the oil pressure sensor and variations in the performance of the oil pressure control mechanism  30 . 
   A pressure comparing unit  222  determines whether or not the real oil pressure is less than a lower limiting oil pressure. The oil pump  34  cannot generate an oil pressure below the lower limiting oil pressure under the present operating conditions of the oil pressure control mechanism  30 . Herein, the oil pump  34  is driven by the engine  1 , so the lower limiting oil pressure depends mainly on the engine rotation speed. However, the lower limiting oil pressure is also affected by the oil temperature and variations (e.g. variations in initial performance or temporal variation in performance due to deterioration) in the condition of the components of the oil pressure control mechanism  30  (in particular, the oil pump). Therefore, the lower limiting oil pressure is computed also taking account of these factors. Thus, when the pressure is less than the lower limiting oil pressure, it is determined that there is a fault of some kind in the oil pressure control mechanism  30 . 
   A running state determination unit  223  determines whether or not the vehicle is running steadily, based on the throttle opening and vehicle speed variation. The throttle opening is normally equivalent to an accelerator pedal stroke. Herein, the steady state means a state which is not a transient state. A transient state is the rapid acceleration state or rapid deceleration state of the vehicle. When the vehicle is in a transient state, there is a large deviation between the secondary oil pressure command value and real secondary pressure, and there is a high possibility of incorrect determination of the absence/existence of an abnormal oil pressure reduction. Therefore, in order to perform an oil pressure reduction determination only in the steady state, the running state determination unit  223  determines whether or not the vehicle is in a steady running state. Herein, when there is a throttle opening variation larger than about ±0.5/8, it may determined that the vehicle is in a transient state. (Here, the maximum throttle opening is 8.) This is because even when the vehicle is running at a fixed speed, the throttle opening normally fluctuates to this extent of ±0.5/8. 
   A continuity determination unit  231  determines whether or not an abnormal drop of the oil pressure has continued for more than a first predetermined time period. This eliminates noise, and prevents incorrect determination of abnormal oil pressure reduction. The continuity determination unit  231  comprises a timer for measuring the duration of the abnormal oil pressure reduction. 
   When a state where the oil pressure difference D 1  between the secondary oil pressure command value and real secondary pressure is larger than the reference value with the vehicle in a steady running state, and the real oil pressure is less than the lower limiting oil pressure, continues for more than the first predetermined time period, the continuity determination unit  231  finally determines that the oil pressure has dropped abnormally due to a fault in the oil pressure control mechanism  30 . 
   A reproducibility determination counter  241  increases by unity when the continuity determination unit  231  determines an abnormal reduction of the oil pressure due to a fault in the oil pressure control mechanism  30 . Specifically, the reproducibility determination counter  241  counts the number of occasions when the abnormal oil pressure reduction is determined in the continuity determination unit  231 . 
   A re-determination prohibition timer  242  is a timer which prohibits re-determination within the second predetermined time period after the reproducibility determination counter  241  has increased by unity. After the second predetermined time period has elapsed, a re-determination prohibition state determination unit  216  permits re-determination. Hence, by re-determining abnormal oil pressure reduction after the second predetermined time period has elapsed, temporarily detected oil pressure reductions are eliminated, and abnormal oil pressure reductions due to a fault in the oil pressure control mechanism  30  are more reliably determined. 
   An abnormal reduction determination unit  251  determines whether or not the value of the reproducibility determination counter  241  is equal to or more than a predetermined counter value. If this determination is positive, it means that the abnormal reduction is reproducible, and occurs repeatedly. In this way, incorrect determination can be prevented. If the value of the reproducibility determination counter  241  is equal to or more than the predetermined counter value, a countermeasure control flag is set to unity. When the countermeasure control flag is set to unity, control for performing a countermeasure against abnormal pressure reduction is performed later. 
   The flowchart of  FIG. 4  shows the control routine executed by the CVT controller  20  in the abnormal oil pressure reduction determining device of the vehicle transmission according to this invention. This control routine is implemented as a program executed by a microcomputer. 
   In a Step S 1 , it is determined whether or not oil pressure reduction determination permission conditions are satisfied. When the oil pressure reduction determination permission conditions are satisfied, the determination permission flag is set to unity. When the oil pressure reduction determination permission conditions are not satisfied, the determination permission flag is set to zero. The details of a subroutine of the Step S 1  will be described later. 
   In a Step S 2 , it is determined whether or not the determination permission flag is set to unity. If the determination permission flag is set to unity (i.e. the oil pressure reduction determining permission conditions are satisfied), the routine proceeds to a Step S 3 , and if the determination permission flag is set to zero, the routine proceeds to a Step S 12 . 
   In a Step S 3 , by the oil pressure difference determination unit  221 , it is determined whether or not the difference D 1  between the secondary oil pressure command value and real secondary pressure is equal to or more than a reference value. When the difference D 1  is equal to or more than the reference value, the routine proceeds to a Step S 4 , and when the difference D 1  is not equal to or more than the reference value, the routine proceeds to the Step S 12 . 
   In the Step S 4 , by the pressure comparing unit  222 , it is determined whether or not the real oil pressure is less than the lower limiting oil pressure under the present running conditions. When it is less than the lower limiting oil pressure, the routine proceeds to a Step S 5 , and when it is equal to or more than the lower limiting oil pressure, the routine proceeds to the Step S 12 . 
   In the Step S 5 , by the running state determination unit  223 , it is determined whether or not the running state of the vehicle is the steady running state. When it is the steady running state, the routine proceeds to a Step S 6 , and when it is a transient running state, the routine proceeds to the Step S 12 . 
   In a Step S 6 , it is determined that there has already been an abnormal oil pressure reduction, so the timer value for measuring the duration of the abnormal oil pressure reduction is decreased by unity. 
   In a Step S 7 , by the continuity determination unit  231 , it is determined whether or not the abnormal oil pressure reduction has continued for a first predetermined time period or longer, i.e., it is determined whether or not the timer value for measuring the duration of the abnormal oil pressure reduction is zero. When the abnormal oil pressure reduction has continued for the first predetermined time period or longer, i.e. the timer value is zero, the routine proceeds to a Step S 8 , and when it has not continued for the first predetermined time period, the routine returns to the Step S 1 . 
   In the Step S 8 , the counter value of the reproducibility determination counter  241  is incremented by unity. In a Step S 9 , it is determined whether or not the counter value of the reproducibility determination counter  241  is equal to or more than a predetermined counter value. When it is equal to or more than the predetermined counter value, the routine proceeds to a Step S 10 , and if it is less than the predetermined counter value, the routine proceeds to a Step S 11 . 
   In the Step S 10 , it is determined that an abnormal oil pressure reduction occurred due to a fault in the oil pressure control mechanism  30 , so by the abnormal reduction determination unit  251 , the countermeasure control flag is set to unity. 
   In the Step S 1 , by the re-determination prohibition timer  242 , after incrementing the abnormal determining counter value by unity in the Step S 8 , the routine waits for the second predetermined time period to elapse, and the routine then returns to the Step S 1 . 
   In the Step S 12 , the timer for measuring the duration of the abnormal reduction in the continuity determination unit  231  is reset to its initial value which corresponds to the first predetermined time period. 
   The aforesaid control routine is repeatedly performed with a timer interrupt processing in a very short interval (e.g., ten milliseconds). However, taking account of the case that the control routine has not terminated within the very short interval due to the waiting time in the Step S 11 , interrupt during execution of the control routine is prohibited. 
   The flowchart of  FIG. 5  shows the details of the subroutine of the Step S 1  of  FIG. 4 . 
   In a Step S 101 , by the control region determination unit  211 , it is determined whether or not the vehicle is in a state where feedback control of the secondary pressure is possible. If the vehicle is in a state where feedback control is possible, the routine proceeds to a Step S 102 , and if the vehicle is a state wherein feedback control is impossible, the routine proceeds to a Step S 107 . Specifically, it is determined whether the engine rotation speed is higher than a predetermined rotation speed, or whether there is a fault in the oil pressure sensor  28 . For example, when the signal from the oil pressure sensor  28  is interrupted, it is determined that there is a fault in the oil pressure sensor  28 . 
   In the Step S 102 , by the oil pressure control mode determination unit  212 , it is determined whether or not the oil pressure control mode is the ordinary mode. In the case of the ordinary mode, the routine proceeds to a Step S 103 , and in the case of a non-ordinary mode, the routine proceeds to the Step S 107 . Specifically, it is determined whether the oil temperature is a very low temperature, or whether the operating range of the transmission system is changing, based on the range signal Rs. 
   In the Step S 103 , by the spin recovery correction determination unit  213 , it is determined whether or not spin recovery correction is being performed. If spin recovery correction is being performed by the VDC unit  29 , the routine proceeds to a Step S 104 , and if spin recovery correction is not being performed, the routine proceeds to the Step S 107 . 
   In the Step S 104 , by the brake SW determination unit  214 , it is determined whether the brake switch  41  is OFF. If the brake switch  41  is OFF, the routine proceeds to a Step S 105 , and if the brake switch  41  is ON, the routine proceeds to the Step S 107 . 
   In the Step S 105 , by the shift range determination unit  215 , it is determined whether or not the shift range position is outside the N range position based on the range signal Rs. If it is outside the N range, the routine proceeds to a Step S 106 , and if it is the N range, the routine proceeds to the Step S 107 . 
   In the Step S 106 , it is determined that an oil pressure reduction determination permission condition is satisfied, and the determination permission flag is set to unity. In the Step S 107 , it is determined that the oil pressure reduction determination permission condition is not satisfied, and the determination permission flag is set to zero. 
   Next, referring to the graph of  FIG. 6 , the determination for the continuity of abnormal oil pressure reduction will be described. 
   As shown in  FIG. 6A , at a time t 1 , if the pressure difference D 1  between the secondary oil pressure command value and real secondary pressure is more than a reference value, it is determined that oil pressure reduction has occurred (Step S 3 ). In this state, it is determined whether or not the vehicle is in the steady running state (Step S 5 ), as shown in  FIGS. 6B and 6C . In other words, it is determined whether or not the accelerator pedal stroke (TVO) and vehicle speed (Vsp) have remained within the permitted range continuously for the first predetermined time period or longer, taking the accelerator pedal stroke (TVO) and the vehicle speed (Vsp) at the time t 1  as references. The permitted range of the accelerator pedal stroke (TVO) is a range centered on the accelerator pedal stroke (TVO) at the time t 1 , and is bounded by a permitted upper limit and permitted lower limit. The permitted range of the vehicle speed (Vsp) is a range centered on the vehicle speed (Vsp) at the time t 1 , and is also bounded by a permitted upper limit and permitted lower limit. 
   If the steady running state collapses before the timer value for measuring the duration of the abnormal oil pressure reduction becomes zero, the timer is reset (Step S 12 ). For example, in  FIG. 6C , this is the case where the vehicle speed at a time t 2  is more than the permitted upper limit. Next, taking the accelerator opening (TVO) and vehicle speed (Vsp) at this time t 2  as new reference values, it is determined whether or not a state wherein the accelerator opening (TVO) and vehicle speed (Vsp) are within the permitted range relative to the new reference values, continues for the first predetermined time period. 
   Next, in  FIG. 6D , if the timer value is zero at a time t 3 , it is determined that there was a fault in the oil pressure control mechanism  30 . In this case, the reproducibility determination counter is incremented by unity (Step S 8 ). 
   Referring to the graph of  FIG. 7 , the determination of abnormal oil pressure reduction will now be described. In  FIG. 7 , the horizontal axis is the time axis. The vertical axis with an arbitrary unit shows oil pressure, vehicle speed (Vsp), accelerator pedal stroke (TVO) and engine rotation speed (EngRev). 
   In  FIG. 7 , the process of the Step S 101  determines a secondary feedback control permission region wherein the engine rotation speed is higher than the predetermined rotation speed and the oil pressure sensor  28  does not have a fault. The process of the Step S 102  determines an ordinary mode wherein the operating range of the transmission is not changing and the oil temperature is not a very low temperature. The process of the Step S 103  determines that spin recovery correction is not being performed. The process of the Step S 104  determines that the brake switch  41  switches from ON to OFF at a time t 0 . The process of the Step S 105  determines that the shift lever position is the D range position. In the above situation, oil pressure reduction determination conditions are satisfied (Step S 106 ). Hence, as shown by the shading in  FIG. 7 , the oil pressure difference D 1  (secondary oil pressure command value—real secondary oil pressure) at the time t 1  is more than a reference value A 1  (Step S 2 ). In the process of the Step S 4 , the real oil pressure is less than the lower limiting oil pressure. In the case of the steady running state wherein variations of the accelerator pedal stroke (TVO) and the vehicle speed (Vsp) are small (Step S 5 ), it is determined in the Step S 6  that there is an abnormal oil pressure reduction due to a fault in the oil pressure control mechanism  30 , and the continuation time (duration) of the abnormal oil pressure reduction is measured from the time t 1 . If this abnormal oil pressure reduction continues beyond the time t 3  for a period longer than the first predetermined time period t 4  (Step S 7 ), an abnormal oil pressure is determined for the first time, and in a Step S 8 , the reproducibility determination counter is incremented by unity. 
   Referring to the graph of  FIG. 8 , the determination of reproducibility of abnormal oil pressure reduction will now be described. The graph of  FIG. 8  is a graph which enlarges  FIG. 7  with respect to time. 
   When the second predetermined time period has elapsed (Step S 11 ) since the last increment of the reproducibility determination counter (Step S 8 ), it is again determined whether or not there is a fault in the oil pressure control mechanism  30  as described above (Steps  1 – 7 ). As shown by the shading in  FIG. 8 , after a second predetermined time period t 5  has elapsed from the time t 3 , during an interval from a time t 6  to the time t 7 , it is again determined whether there was a fault in the oil pressure system. When a fault is determined in the oil pressure system  30  in this way, the reproducibility determination counter is incremented by unity in the Step S 8 . In the determination of the Step S 9 , if the value of the reproducibility determination counter is equal to or more than a predetermined counter value, it can be determined for certain that there is a fault in the oil pressure control mechanism  30 , and that the oil pressure has abnormally decreased due to this reason. This predetermined counter value of the reproducibility determination counter may conveniently be determined based on system reliability and the required precision, and may be 2, 3 or a higher number. 
   The effect of this embodiment will now be described. If the state wherein the pressure difference D 1  between the command pressure and real pressure continues for the first predetermined time period t 4  or longer, it is determined that there is an abnormal pressure reduction. As a result, noise is eliminated, and an incorrect determination is prevented. Also, if the real oil pressure is less than the lower limiting oil pressure which naturally arises in the vehicle running state, it is determined that there is an abnormal oil pressure reduction. Consequently, a precise determination can be made. 
   By providing the reproducibility determination counter, it is determined whether or not an abnormal reduction has occurred on multiple occasions. As a result, incorrect determination is prevented, and an abnormal oil pressure reduction due to a fault in the oil pressure control mechanism can be more precisely determined. By providing the re-determination prohibition timer, a re-determination is performed after the second predetermined time period t 5  has elapsed. Due to this, temporary reductions are eliminated, and an abnormal oil pressure reduction due to a fault in the oil pressure control mechanism  30  can be more precisely determined. 
   When the secondary pressure cannot be feedback controlled (e.g., when the engine rotation speed is low or there is a fault in the oil pressure sensor  28 ), when the oil pressure control mode is not the ordinary mode (e.g., when the oil temperature is a very low temperature, or the operating range of the transmission system is changing), when spin recovery correction is being performed, when the brake switch  41  is ON, when the selected range is a range outside the N range, or when the vehicle running state is the transient state (e.g., rapid acceleration or rapid deceleration), oil pressure reduction determination is inhibited. As a result, incorrect determination of oil pressure reduction due to a fault in the oil pressure control mechanism  30  is definitively prevented. 
   The entire contents of Japanese Patent Application P2002-291891 (filed Oct. 4, 2002) are incorporated herein by reference. 
   Although the invention has been described above by reference to a certain embodiment of the invention, the invention is not limited to the embodiment described above. 
   In the above embodiment, the case is described where the CVT speed change part is controlled by oil pressure, but also in the case of an abnormal reduction of oil pressure in the mechanism which controls the forward/reverse change-over mechanism by oil pressure, abnormal reduction of the oil pressure can be determined by an identical construction. Further, although in the above embodiment, a difference D 1  between the real oil pressure of the secondary pulley and the oil pressure command value for the secondary pulley is calculated, a difference D 1  between the real oil pressure of the first pulley and the oil pressure command value for the first pulley may be calculated to detect an abnormal reduction of oil pressure in the oil pressure control mechanism  30 . Also, this invention may likewise be applied to the case where the transmission is a stepwise transmission having planetary gears. 
   Modifications and variations of the embodiment described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.