This application has relation with U.S. patent applications Ser. Nos. 07/795,989 (filed Nov. 22, 1991) now U.S. Pat. No. 5,191,815, 07/850,283 (filed Mar. 12, 1992) now U.S. Pat. No. 5,203,235, 07/878,469 (filed May 5, 1992) now U.S. Pat. No. 5,233,889, 07/939,600 (filed Sep. 2, 1992) now U.S. Pat. No. 5,305,665 and 07/969,072 (filed Oct. 30, 1992) now U.S. Pat. No. 5,249,483.
1. Field of the Invention
The present invention relates in general to automotive automatic transmissions, and more particularly to a control system for the automotive automatic transmissions.
2. Description of the Prior art
In automotive automatic transmissions, there has been proposed a control system, in which a so-called "inertial phase keeping time" for which the transmission gear ratio is being varied is measured, and the line pressure applied to a friction element, such as, clutch or the like, is controlled to a lower level during the gear change operation so that the inertial phase keeping time can take a desired value. However, in case wherein the line pressure is quite low, the gear change can not be completed within the time for which the line pressure is controlled low. When, under this condition, the line pressure is raised thereafter, the friction element is engaged suddenly, which causes a marked shift shock.
In order to solve such drawback, a measure has been proposed, which is shown in Japanese Patent First Provisional Publication 2-21059. That is, in the measure, a so-called "inertial phase starting time" elapsed from issuance of a gear change instruction to the time when the inertial phase actually starts is measured, and when the measured time exceeds a predetermined time (that is, when, due to lowering in the line pressure, the starting of the inertial phase is delayed), the line pressure is raised irrespective of the inertial phase keeping time.
When, for example, a select position switch is out of order, and thus when, irrespective of D-range signal being fed to a control unit, the transmission is kept in N-range, operation of a shift solenoid effected by the gear change instruction does not induce the change of the gear ratio.
However, due to its inherent construction, the above-mentioned measure has the following drawback.
That is, even under the above-mentioned condition, the line pressure is raised by judging that the inertial phase starting time is too long. Accordingly, when thereafter the gear change operation actually starts, the gear change is carried out with the higher line pressure, which causes a marked shift shock.