This invention relates to a control apparatus for a hydraulically operated vehicular transmission comprising first to fourth forward speed transmission trains which are so arranged as to be established by the engagement of first- to fourth-speed hydraulic clutches.
There is a known control apparatus of this kind as disclosed in Japanese Patent Laid-open No. 84450/1986. This known apparatus includes a hydraulic circuit for controlling the supplying and discharging of oil to and from first- to fourth-speed hydraulic clutches. The hydraulic circuit is provided with a first shift valve connected to a hydraulic power source via a manual valve, a second shift valve located on the downstream side of the first shift valve, and a third shift valve located on the downstream side of the second shift valve. The first shift valve is constructed to be switchable to a first speed position at which the supplying of the oil to the first speed hydraulic clutch and the discharging of the oil from the second speed hydraulic clutch are carried out and to a second speed position at which the supplying of the oil to the second shift valve is carried out. The second shift valve is constructed to be switchable to a second speed position at which the supplying of the oil, which is supplied from the first shift valve, to the second speed hydraulic clutch and the discharging of the oil from the third speed hydraulic clutch end carried out and to a third-speed position at which the discharging of the oil from the second-speed hydraulic clutch and the supplying of the oil to the third shift valve are carried out. The third shift valve is constructed to be switchable to a third-speed position at which the supplying of the oil, which is supplied from the second shift valve, to the third speed hydraulic clutch and the discharging of the oil from the fourth speed hydraulic clutch are carried out and to a fourth speed position at which the discharging of the oil from the third speed hydraulic clutch and the supplying of the oil to the fourth-speed clutch are carried out. In this manner, there can be obtained first-speed travelling with the first shift valve set to the first-speed position, second speed travelling with the first and second shift valves set to the second speed positions, third-speed travelling with the second and third shift valves set to the third speed positions while leaving the first shift valve set to the second speed position, and fourth speed travelling with the third shift valve alone switched over to the fourth speed position while leaving the first and second shift valves as they are during third-speed travelling. A throttle pressure according to the degree of opening of a throttle valve in the engine and a governor pressure according to the vehicle speed can be applied to each shift valve in the direction opposite to each other to carry out the switching of these shift valves in accordance with the travelling condition.
The number of control apparatuses using an electronic control circuit to switch and control the shift valves has been increasing lately because such a control circuit enables a finer speed change control operation to be carried out and because the speed change characteristics can be changed easily in accordance with the type of vehicle.
In such a case, one possibility is to arrange the apparatus such that there are provided four shift valves in total, one for each hydraulic clutch, and disposed in parallel with one another with respect to the hydraulic power source. Electromagnetic valves could be connected respectively to the shift valve switching pilot oil passages, these electromagnetic valves being opened and closed by the electronic control circuit so as to carry out switching over of the shift valves. However, this would require an increased number of the valves, and makes it necessary to carry out switching of the shift valve for the engaged hydraulic clutch to the oil supplying position and the switching of the shift valve for the disengaged hydraulic clutch to the oil discharging position simultaneously. Consequently, the control operation would become complicated.
Compared with the above possible arrangement, a control apparatus according to the prior art discussed in the foregoing is more advantageous in that it needs only three shift valves and in that, by having the second shift valve set to the second speed position when in first speed travelling, and the third shift valve set to the third speed position when in second speed travelling, any speed change can be carried out by switching only one shift valve at a time.
In this case, it is possible to arrange the apparatus such that the first to third shift valves may be controlled for switchover by means of electromagnetic valves corresponding thereto which are adapted to be opened and closed by an electronic control circuit. However, in order to further reduce the number of parts, it is also desired that the number of these electromagnetic valves be reduced.