1. Field of the Invention
The present invention relates to an upshift control device for an automatic transmission of a motor vehicle, and more particularly to a technique for controlling a back pressure of an accumulator of an engaging element to be released during upshifting.
2. Description of the Prior Art
There is known a shift control device for an automatic transmission as disclosed in Japanese laid-open Patent Publication No. 5-248526. When the transmission is shifted up from its 3rd-speed gear position to 4th-speed gear position, racing of the engine may occur or the torque of the output shaft of the transmission may be suddenly reduced if the released pressure of a low clutch 50 to be released inappropriately overlaps the applied pressure of the a band brake 28 to be engaged. In view of this problem, the shift control device disclosed in the above-identified publication is provided with a low clutch sequence valve 90 that is switched from a supply position for applying a pressure to a low clutch accumulator 83 of the low clutch 50 to a drain position for draining the accumulator 83, when a difference between the accumulator back pressure and the 4th-speed engaging pressure as operating signal pressures becomes equal to a predetermined value, so that the released pressure and the applied pressure are controlled to the same level in appropriate timing.
If the low clutch sequence valve 90 of the known shift control device as described above is tuned to be switched at a point of time when interlocking occurs, namely, when the applied pressure and released pressure overlap each other, the torque of the output shaft of the transmission is pulled down or drops to a large depth in the initial period of inertia phase, resulting in increased shocks, as shown in FIG. 9.
If the low clutch sequence valve 90 is tuned to be switched at a point of time when the applied pressure and the released pressure hardly overlap each other, so as to alleviate pull-down of the torque of the output shaft and thus reduce shocks, on the other hand, racing of the engine would occur due to an insufficient engaging capacity before or after the start of the inertial phase. In order to prevent racing of the engine while at the same time reducing shocks, therefore, the low clutch sequence valve 90 needs to be switched just before the engine starts racing.
Where the low clutch sequence valve is tuned to be switched at a point of time just before the engine starts racing as shown in FIG. 10, however, racing of the engine may occur since the applied pressure, released pressure and torque of the output shaft vary depending upon shift conditions. In view of this, the sequence valve is actually tuned so that the hydraulic capacity of the engaging elements is closer to the interlocking condition. In this case, however, the torque of the output shaft may be pulled down deeply or the engine suffers from racing due to variations in the oil pressure or the output torque.