The present invention relates to a shift control system used for a vehicular automatic transmission comprising a shift lever, which is manipulated by a driver. 
A vehicular automatic transmission is designed to perform automatic speed control for a vehicle on the basis of the following conditions: the position of the shift lever, which is manipulated by the driver at the driver""s seat of the vehicle; the movement of the accelerator pedal being stepped down by the driver in relation with the respective drive range determined by the position of the shift lever; the speed of the vehicle, etc.  The drive ranges which can be selected by the manipulation of the shift lever are, for example, a parking range (P range), a reverse drive range (R range), a neutral range (N range), and a forward drive range (e.g., D range, etc.),  and in many cases, the forward drive range comprises, for example, xe2x80x9cD5xe2x80x9d range, xe2x80x9cD4xe2x80x9d range, xe2x80x9c3xe2x80x9d range, xe2x80x9c2xe2x80x9d range and xe2x80x9c1xe2x80x9d range.  For the selection of these ranges, generally, the shift lever is swiveled in a plane so that the shift lever is shifted to appropriate positions predetermined for establishing the respective drive ranges.  In this case, a shift guide path is provided linearly in a form of slot at the driver""s seat, and the shift lever, which extends through the shift guide path, is manipulated along the guide path by the driver for the selection of the drive ranges. 
On the basis of this design, shift control systems have been proposed and designed in variation not only to enable the shift lever to swivel linearly along a linear shift guide path but also to allow various patterns of shift manipulation through appropriate arrangement of shift guide paths.  For example, Japanese Laid-Open Patent Publication No.  H2(1990)-8545 discloses a shift control system with two shift guide paths: a linear first shift guide path (or first shift path) and a second shift guide path provided parallel with this first shift guide path.  The manipulation of the shift lever along the first shift guide path enables the selection of the following drive ranges: xe2x80x9cPxe2x80x9d, xe2x80x9cRxe2x80x9d, xe2x80x9cNxe2x80x9d, xe2x80x9cDxe2x80x9d, xe2x80x9c3xe2x80x9d, xe2x80x9c2xe2x80x9d and xe2x80x9c1xe2x80x9d.  In this system, when the shift lever is at the position for the D range, it can be moved into the second shift guide path.  Then, in the second shift guide path, the shift lever is manipulated for exclusive selection of the D, 3, 2 and 1 ranges. 
Also, Japanese Laid-Open Patent Publication No.  H6(1994)-221417 discloses a shift control system which allows the shift lever to be manipulated not only linearly in a shift guide path for the positions of xe2x80x9cPxe2x80x9d, xe2x80x9cRxe2x80x9d, xe2x80x9cNxe2x80x9d, xe2x80x9cDxe2x80x9d and xe2x80x9cBxe2x80x9d ranges but also perpendicularly to this shift guide path when the shift lever is at the B range position, for a manual upshift and a manual downshift in steps.  Furthermore, this patent publication discloses a shift control system in which the shift lever is manipulated along a linear first shift guide path to positions for xe2x80x9cPxe2x80x9d, xe2x80x9cxe2x80x9d, xe2x80x9cNxe2x80x9d, xe2x80x9cDxe2x80x9d, xe2x80x9cMxe2x80x9d and xe2x80x9cLxe2x80x9d ranges, respectively, and is movable laterally at the position for the xe2x80x9cMxe2x80x9d range into a second shift guide path, where the shift lever is manipulated for a manual upshift and a manual downshift in steps. 
In general, the positions of the shift lever of a shift control system for an automatic transmission are predetermined for establishing, for example, xe2x80x9cPxe2x80x9d, xe2x80x9cRxe2x80x9d, xe2x80x9cD5xe2x80x9d, xe2x80x9cD3xe2x80x9d, xe2x80x9c3xe2x80x9d, xe2x80x9c2xe2x80x9d and xe2x80x9c1xe2x80x9d ranges, respectively, and additionally to satisfy demand for a manual shift operation, the system is usually designed to allow manual selection of speed ratios.  On this background, a various shift control systems have been proposed and produced to make the operation of the shift lever for selecting a desired speed ratio as simple as possible or to make the number of actions taken for shifting the shift lever in selecting a desired speed ratio as small as possible.  For example, for an automatic transmission with five forward drive speed ratios, it is desired that a shift control system be designed to enable a manual downshift in which the vehicle driving at the FIFTH speed ratio or at the FOURTH speed ratio is downshifted to the THIRD speed ratio by a smallest number of actions taken for the operation of the shift lever. 
It is an object of the present invention to provide a shift control system for a vehicular automatic transmission, which system simplifies the operation of the shift lever required for changing drive ranges and minimizes the number of actions taken for the operation of the shift lever in shifting the speed ratios of the transmission manually to a desired speed ratio. 
In order to achieve this objective, the present invention provides a shift control system for a vehicular automatic transmission, which system comprises at least a first shift guide path, a connection shift guide path and a second shift guide path.  The first shift guide path guides a shift lever, which is manipulated by a driver, between a first automatic shift control position and a second automatic shift control position.  The connection shift guide path extends in a direction different from that of the first shift guide path and guides the shift lever from the second automatic shift control position to a manual shift control position, and the second shift guide path guides the shift lever which has been positioned at the manual shift control position, into a direction different from that of the connection shift guide path.  In this arrangement, the position of the shift lever triggers a change in the operation of the transmission in the following way.  When the shift lever is shifted to the first automatic shift control position, the transmission is set into an automatic shift control with forward drive speed ratios up to a highest speed ratio (for example, if the transmission has five forward drive speed ratios, then the highest speed ratio is the FIFTH speed ratio, or if the transmission has four forward drive speed ratios, then the highest speed ratio is the FOURTH speed ratio).  When the shift lever is shifted to the second automatic shift control position, the transmission is set into an automatic shift control with forward drive speed ratios up to a second highest speed ratio which is lower than the highest speed ratio (for example, for the five speed ratio transmission, this second highest speed ratio is the FOURTH speed ratio, and for the four speed ratio transmission, it is the THIRD speed ratio).  When the shift lever is shifted along the connection shift guide path from the second automatic shift control position to the manual shift control position, the transmission is shifted to a speed ratio lower than a speed ratio that has been in effect prior to this shifting of the shift lever.  Then, every time the shift lever at the manual shift control position is swiveled along the second shift guide path, the transmission is shifted to a speed ratio next in order in the forward drive speed ratios. 
With this shift control system, for example, when the shift lever is at the first automatic shift control position, the transmission is operated in an automatic shift control with all the forward drive speed ratios including the highest speed ratio.  In this condition, if the shift lever is shifted to the second automatic shift control position along the first shift guide path while the vehicle is driving at the highest speed ratio, then the transmission is downshifted to the second highest speed ratio, causing an action of engine brake.  If the shift lever is shifted further from the second automatic shift control position to the manual shift control position along the connection shift guide path while the vehicle is driving at the second highest speed ratio, then the transmission is downshifted to an even lower speed ratio, increasing the force of the engine brake.  In this way, every time the shift lever is operated appropriately, the shift control system changes the drive range of the transmission to effect a downshift.  Even when the shift lever is shifted along the connection shift guide path, the shift control system effects a change in the drive range.  Therefore, in this system, the number of actions taken by the driver for the shift manipulation is relatively small, so the shift control system according to the present invention offers a superior operativity. 
Furthermore, in the shift control system according to the present invention, the shift lever at the manual shift control position is swiveled along the second shift guide path for a speed ratio change, i.e., a manual control of speed ratio change.  Therefore, the shift control system offers a good operativity not only for switching the drive ranges but also for selecting an appropriate speed ratio manually. 
The above mentioned second highest speed ratio is predetermined as a speed ratio which is one step immediately lower than the highest speed ratio, and the shift control system can be arranged to function in the following way.  When the shift lever is shifted from the second automatic shift control position to the manual shift control position, the transmission is shifted to a speed ratio which is lower than a speed ratio that has been in effect prior to this shifting of the shift lever.  Thereafter, as long as the shift lever is not operated, the transmission is operated in an automatic shift control with forward drive speed ratios up to a third highest speed ratio which is one step lower than the second highest speed ratio (for example, for the five speed ratio transmission, this third highest speed ratio is the THIRD speed ratio, and for the four speed ratio transmission, it is the SECOND speed ratio).  Alternatively, the shift control system may be arranged to function in the following way.  When the shift lever is shifted from the second automatic shift control position to the manual shift control position, the transmission is shifted to a speed ratio which is lower than a speed ratio that has been in effect prior to this shifting of the shift lever.  Thereafter, the transmission is kept at this newly set speed ratio as long as the shift lever is not operated. 
In this shift control system, when the shift lever is shifted from the first automatic shift control position through the second automatic shift control position to the manual shift control position while the vehicle is driving at the highest speed ratio, the transmission is downshifted in steps, one step from the highest speed ratio to the second highest speed ratio, and another step from the second highest speed ratio to the third highest speed ratio. 
Another embodiment of shift control system according to the present invention also comprises a first shift guide path, a connection shift guide path and a second shift guide path.  The first shift guide path guides a shift lever, which is manipulated by a driver, between a first automatic shift control position and a second automatic shift control position.  The connection shift guide path extends in a direction different from that of the first shift guide path and guides the shift lever from the second automatic shift control position to a manual shift control position, and the second shift guide path guides the shift lever which has been positioned at the manual shift control position, into a direction different from that of the connection shift guide path.  However, in this case, the shift control system is arranged to function in the following way.  When the shift lever is either at the first automatic shift control position or at the second automatic shift control position, the transmission is operated in an automatic shift control which is executed with forward drive speed ratios up to the highest speed ratio.  Yet, the automatic shift control for the second automatic shift control position is executed in accordance with speed ratio shift lines which are mapped more toward higher speeds than those mapped for the automatic shift control that is executed for the first automatic shift control position.  In this arrangement, when the shift lever is shifted from the second automatic shift control position to the manual shift control position along the connection shift guide path, the transmission is shifted to a speed ratio which is lower than a speed ratio that has been in effect prior to this shifting of the shift lever.  Then, the shift lever at the manual shift control position is swiveled along the second shift guide path to shift the speed ratios of the transmission, step by step. 
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter.  However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.