The present invention relates to a hydraulic control system for controlling the oil pressure of frictional engagement means participating in a shift of an automatic transmission, by an electrically controllable valve mechanism.
In case a shift is to be executed by the automatic transmission, the working oil pressure to be fed to the frictional engagement means such as a clutch or brake participating in the shift is so electrically controlled (or regulated) as to establish predetermined hydraulic characteristics by using a duty solenoid valve or linear solenoid valve thereby to improve the shifting characteristics. Generally speaking, the control characteristics are set while estimating the viscosity of the oil at an ordinary oil temperature. In case, however, the oil has a low temperature, its viscosity is high so that the pressure regulating characteristics can not be achieved, as expected. Even if the oil pressure target value is commanded along a solid curve, as illustrated in FIG. 8, the actual oil pressure so responds as is plotted by a broken curve. Specifically, the actual oil pressure delays in response from the oil pressure target value. In the characteristics, only the oil pressure target value advances, but the actual oil pressure rises with a delay.
The characteristics of this actual oil pressure will be specifically described in the following. The target oil pressure is gradually changed at the initial stage of a shift but is raised at the final stage of the shift, so that the difference between the actual oil pressure and the target value increases to cause the change in the actual oil pressure abruptly. At the final stage of the shift, the actual oil pressure abruptly increases to give a torque capacity abruptly to the frictional engagement means. When the frictional engagement means thus acquires the torque capacity abruptly, the output torque abruptly rises, as plotted by a broken curve. This abrupt rise of the output torque invites a disadvantage that the shifting shock is caused.
In order to eliminate such disadvantage, the occurrence of the shifting shock is to be suppressed in the system, as disclosed in Japanese Patent Laid-Open No. 35153/1989, by changing the oil pressure target value according to the oil temperature. In other words, this system raises the oil pressure target value to eliminate the response delay of the oil pressure at a low temperature by increasing the output of the actual oil pressure. According to this system, the applying oil pressure rises while a packing clearance is being reduced by compressing the return spring of the frictional engagement means by the oil pressure. Even after the packing clearance has been reduced, however, the control with the high output continues so that the high oil pressure is further fed with the reduced packing clearance. As a result, the applying oil pressure may abruptly rise to cause the shifting shock, as in the case of the description made with reference to FIG. 8.