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In a hydraulic control apparatus for a vehicular automatic transmission, if it is determined that there is a failure that may cause tie-up in an automatic transmission, communication is provided between a first output port and a first drain port in a switching valve. As a result, a hydraulic frictio

In a hydraulic control apparatus for a vehicular automatic transmission, if it is determined that there is a failure that may cause tie-up in an automatic transmission, communication is provided between a first output port and a first drain port in a switching valve. As a result, a hydraulic frictional engagement device, which should be disengaged when there is a failure, is disengaged. Also, a specific shift speed is achieved by controlling the switching valve. Therefore, it is not necessary to select a shift speed that is achieved on the condition that the hydraulic frictional engagement device, which should be disengaged when there is a failure, is engaged. Thus, it is possible to prevent a shift that may impair driveability when there is a failure that may cause tie-up in the automatic transmission.

대표청구항 ▼

What is claimed is: 1. A hydraulic control apparatus for a vehicular automatic transmission in which a plurality of shift speeds having different speed ratios are achieved by selectively engaging and disengaging a plurality of hydraulic frictional engagement devices, the hydraulic control apparatus

What is claimed is: 1. A hydraulic control apparatus for a vehicular automatic transmission in which a plurality of shift speeds having different speed ratios are achieved by selectively engaging and disengaging a plurality of hydraulic frictional engagement devices, the hydraulic control apparatus comprising: a plurality of solenoid valves which respectively correspond to respective hydraulic frictional engagement devices of the plurality of hydraulic frictional engagement devices, which output hydraulic pressure directly to the respective hydraulic frictional engagement devices, and which are controlled to achieve any one of a plurality of shift speeds; and a switching valve device which includes a first output port connected to an input port of a first solenoid valve among the plurality of solenoid valves, wherein the first solenoid valve corresponds to a hydraulic frictional engagement device among the plurality of hydraulic frictional engagement devices, and wherein the first solenoid valve is disengaged when there is a failure that may cause tie-up in the automatic transmission, a first input port to which an original pressure is supplied, a first drain port, wherein when there is no failure that may cause tie-up in the automatic transmission, communication is provided between the first output port and the first input port, and wherein when there is a failure that may cause tie-up, communication is provided between the first output port and the first drain port to achieve a specific shift speed, a second output port connected to a drain port of a second solenoid valve among the plurality of solenoid valves, wherein the second solenoid valve corresponds to a hydraulic frictional engagement device among the plurality of hydraulic frictional engagement devices, and wherein the second solenoid valve is engaged when there is a failure that may cause tie-up, a second input port to which the original pressure is supplied, and a second drain port, wherein when there is no failure that may cause tie-up in the automatic transmission, communication is provided between the second output port and the second drain port, and wherein when there is a failure that may cause tie-up, communication is provided between the second output port and the second input port, wherein the plurality of hydraulic frictional engagement devices include a first hydraulic frictional engagement device that is engaged when a shift speed of the automatic transmission is lower than or equal to a predetermined shift speed, and is disengaged when the shift speed is higher than the predetermined shift speed, and wherein the hydraulic frictional engagement device that is disengaged when there is a failure that may cause tie-up includes the first hydraulic frictional engagement device. 2. The hydraulic control apparatus according to claim 1, wherein the specific shift speed is lower than a highest shift speed of the automatic transmission by one speed level. 3. The hydraulic control apparatus according to claim 1, wherein the switching valve device includes an electromagnetic valve that outputs a hydraulic signal and stops outputting the hydraulic signal according to an electric signal; and a switching valve that selects among passages through which the hydraulic pressure is supplied to the plurality of solenoid valves, according to the hydraulic signal output from the electromagnetic valve. 4. The hydraulic control apparatus according to 1, wherein the hydraulic frictional engagement device that is engaged when there is a failure that may cause tie-up is a hydraulic frictional engagement device that is engaged to achieve a shift speed that is lower than a highest shift speed of the automatic transmission by one speed level. 5. The hydraulic control apparatus according to claim 1, wherein the hydraulic control apparatus further includes a control device that electrically controls the switching valve device, and the control device determines whether there is a failure that may cause tie-up in the automatic transmission, and wherein if the control device determines that there is a failure that may cause tie-up, the control device controls the switching valve device to provide communication between the first output port and the first drain port, and if the control device determines that there is no failure, the control device controls the switching valve device to provide communication between the first output port and the first input port. 6. The hydraulic control apparatus according to claim 3, wherein the hydraulic signal output by the electromagnetic valve is supplied to the electromagnetic valve by a modulator valve that generates a modulator pressure by regulating the original pressure to a constant value. 7. The hydraulic control apparatus according to claim 6, wherein the modulator valve includes a modulator input port, a modulator output port, a modulator drain port, and a spring, wherein the original pressure is supplied to the modulator input port, wherein the modulator output port is connected to the electromagnetic valve, and wherein the spring of the modulator valve regulates the original pressure down to the modulator pressure based on a load of the spring such that the modulator pressure is a constant value regardless of a change in the original pressure.