Control Apparatus for Vehicular Automatic Transmission

1. Field of the Invention

This invention relates to a control apparatus for a vehicular automatic transmission which has transmission trains of five forward speeds or more.

2. Description of the Related Art

Conventionally, in a control apparatus for a transmission which has a plurality of transmission trains, a plurality of shift valves are connected in series in an oil supply passage which is communicated with a manual valve. In an upshift position of an upstream shift valve, the oil is supplied to a downstream shift valve. In a downshift position of the oil-supplied shift valve, the oil is supplied to a hydraulic engaging element of a corresponding transmission train to thereby establish the transmission train. For example, in a transmission of five forward speeds, there are connected in series, from an upstream side downwards, a first-second (1-2) shift valve, a second-third (2-3) shift valve, a third-fourth (3-4) shift valve and a fourth-fifth (4-5) shift valve.

If the shift valves are connected in series as described above, all shift valves on the downstream side will be affected by an abnormality of a shift valve on an upstream side, with the result that the transmission by these shift valves cannot be effected normally. Therefore, it is necessary to provide shift valves on the upstream side with a fail-safe function. However, with five or more transmission trains, the shift valves to be provided with the fail-safe function increase in number, resulting in a complicated circuit construction for the fail-safe function.

On the other hand, a solenoid valve has been recently proposed which interposed in an oil circuit for spool control of each shift valve so that each shift valve can be shift-controlled, or controlled for changeover, by an electronic control circuit via each solenoid valve. In such an arrangement, the same number of solenoid valves are required as that of the shift valves. With the increase in the number of transmission trains, the number of solenoid valves also increases to thereby bring about a complicated hydraulic oil circuits and higher costs.

In view of the above-described disadvantages, the present invention has an object of providing a control apparatus for a vehicular automatic transmission in which the hydraulic oil circuit for the fail-safe function can be simplified and, further, in which the number of solenoid valves can be reduced by shift-controlling two shift valves by a common solenoid valve.

In order to attain the above-described object, this invention provides a control apparatus for a vehicular automatic transmission having transmission trains of five forward speeds or more, characterized in: that the transmission trains are divided into two groups of a low-speed system comprising relatively low-speed transmission trains and a high-speed system comprising relatively high-speed transmission trains; that a main shift valve is provided for selectively supplying pressurized oil supplied from a manual valve through a first oil supply passage to an oil passage for the low-speed system and to an oil passage for the high-speed system; that a shift valve unit for the low-speed system, which unit comprises one or a plurality of shift valves for selectively establishing transmission trains belonging to the low-speed system, is interposed in the oil passage for the low-speed system; and that a shift valve unit for the high-speed system, which unit comprises one or a plurality of shift valves for selectively establishing transmission trains belonging to the high-speed system, is interposed in the oil passage for the high-speed system.

When the main shift valve is shifted, or changed over, to the low-speed position in which the oil is supplied to the oil passage for the low-speed system, speed changing is effected among the transmission trains which belong to the low-speed system through the change over operation of the shift valve unit for the low-speed system. When the main shift valve is changed over to the high-speed position in which the oil is supplied to the oil passage for the high-speed system, speed changing is effected among the transmission trains which belong to the high-speed system through the change over operation of the shift valve unit for the high-speed system.

Even if an abnormality happens in each of the shift valves which belong to each of the shift valve units, by providing the main shift valve with a fail-safe function, it is possible to carry out starting and acceleration of the vehicle in any of the transmission trains which belong to the low-speed system as well as cruising in any of the transmission trains which belong to the high-speed system. For example, as a fail-safe measure against the locking of the main shift valve at the high-speed position, the following arrangement can be made, i.e., the pressurized oil which is supplied from the manual valve through a second oil supply passage at a predetermined changeover position of the manual valve, e.g., at a low-speed hold position, is supplied to the oil passage for the low-speed system through the main shift valve which is in the high-speed position. Then, even if the main shift valve is locked at the high-speed position, a transmission train which belongs to the low-speed system can be established to start and accelerate the vehicle, by changing over the manual shift valve to the low-speed hold position.

During high-speed cruising in a relatively high-speed transmission train among the transmission trains which belong to the high-speed system, e.g., in the fifth-speed train, if the main shift valve is changed over, due to an abnormality or the like, to establish the first-speed or second-speed transmission train which belongs to the low-speed system, sudden engine brake will be applied, resulting in an excessive force to the engine and the transmission. In order to prevent this kind of problem, it is preferable to provide a means of hydraulically restraining the main shift valve to the high-speed position at the time when the high-speed transmission train has been established. In order to prevent the downshifting from the fifth-speed stage to the first- or second-speed stage in an apparatus, like in a conventional apparatus, in which the shift valves are all connected in series, it is necessary to provide a restraining means for restraining or holding each of the 1-2 shift valve and the 2-3 shift valve to the upshift position. On the contrary, according to this invention, it is sufficient to provide the restraining means only to the main shift valve and, therefore, the construction of the circuits becomes simpler due to the omission of other restraining means.

When the shift valves are shift-controlled by means of solenoid valves, according to this invention, only that shift valve which is interposed in the oil passage to which the oil is supplied by the main shift valve contributes to the transmission or speed change operation, even if any of the shift valves which belong to the shift valve unit for the low-speed system and any of the shift valves which belong to the shift valve unit for the high-speed system are changed over simultaneously. The transmission or the speed change operation is therefore not hindered by the shift valves which are interposed in the other oil passage. It follows that it becomes possible to shift-control a predetermined shift valve in each of the shift valve units by a common solenoid valve.

For example, in case of five-speed transmission trains, the shift valve unit for the low-speed system is made up of a 1-2 shift valve and the shift valve unit for the high-speed system is made up of a 3-4 shift valve and a 4-5 shift valve. The main shift valve, the 1-2 shift valve and the 3-4 shift valve are shift-controlled by first through third solenoid valves, respectively. The 4-5 shift valve is shift-controlled by the second solenoid valve which is for shifting the 1-2 shift valve. The number of solenoid valves can thus be made smaller than that of the shift valves.

In this case, if the combination of opening and closing of the first through third solenoid valves when the first-speed transmission train is established is arranged to be opposite to the combination of opening and closing of the first through third solenoid valves when the fifth-speed train is established, there will occur no sudden downshifting from the fifth-speed stage to the first-speed stage unless abnormalities occur to all of the solenoid valves during running in the fifth-speed stage.

The above and other objects and the attendant advantages of this invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagram of one example of a transmission to which this invention is applied;

FIG. 2 is a hydraulic oil circuit diagram of one embodiment of this invention;

FIG. 3 is an enlarged hydraulic oil circuit diagram shown in FIG. 2; and

FIG. 4 is a diagram showing the characteristics of control of line pressure by a pressure control valve.

Referring to FIG. 1, numeral 1 denotes a transmission for effecting the changing or shifting of a vehicle speed to five forward speeds and one reverse speed. The transmission 1 comprises: an input shaft 1a which is connected to an engine 2 via a fluid torque converter 3; an output shaft 1b which is connected to driving wheels 4 of the vehicle via a differential gear 5; and first- to fifth-speed forward transmission trains G1, G2, G3, G4, G5 and one reverse transmission train GR, all transmission trains being disposed between the input shaft 1a and the output shaft 1b. In each of the forward transmission trains, there is interposed hydraulic engaging means in the form of a hydraulic clutch C1, C2, C3, C4, C5 respectively so that, through the engagement of each hydraulic clutch, each of the transmission trains can be selectively established. The reverse transmission train GR is arranged to use the fourth-speed clutch C4 in common with the fourth-speed transmission train G4. The fourth-speed transmission train G4 or the reverse transmission train GR is selectively established by changing over a selector gear 6 to the forward position in the left-hand side in FIG. 1 and the reverse position in the right-hand side thereof.

The supply and discharge of hydraulic oil to and from each of the hydraulic clutches C1, C2, C3, C4, C5 are controlled by the hydraulic oil circuit shown in FIG. 2. This will now be described in more detail. The hydraulic oil circuit comprises: a hydraulic power source 7; a manual valve 8 which can be changed over among a total of seven positions, i.e., a parking position P, a reverse position R, a neutral position N (the position as illustrated), an automatic transmission or speed change position D.sub.5 for changing the speed among first-speed through fifth-speed, an automatic speed change position D.sub.3 for changing the speed among first-speed through third-speed, a second-speed hold position 2 and a first-speed hold position 1; a main shift valve 9 for selecting a low-speed system consisting of the first-speed transmission train G1 and the second-speed transmission train G2 and a high-speed system consisting of the third-speed transmission train G3, the fourth-speed transmission train G4 and the fifth-speed transmission train G5; a 1-2 shift valve 10 which is connected to the downstream of the main shift valve 9; a 3-4 shift valve 11 which is connected to the downstream of the main shift valve 9 in parallel with the 1-2 shift valve 10; a 4-5 shift valve 12 which is connected to the downstream of the 3-4 shift valve 11; and a servo valve 13 which is for changing over between forward transmission and reverse transmission and is connected with a fork 6a to be engaged with the selector gear 6. These valves are shown in detail in FIG. 3.

When the manual valve 8 is in the D.sub.5 range, No. 1 oil passage L1 which is connected to the hydraulic power source 7 is connected via an annular groove 8a of the manual valve 8 to No. 2 oil passage L2 which is communicated with the main shift valve 9. (This No. 2 oil passage L2 constitutes a first oil supply passage.) The oil which is adjusted by a regulator valve 14 to a predetermined line pressure is thus supplied from No. 1 oil passage L1 to No. 2 oil passage L2. When the main shift valve 9 is changed over to the low-speed position (the position as illustrated) in which the low-speed system is selected, No. 2 oil passage L2 is connected via an annular groove 9a of the main shift valve 9 to No. 3 oil passage L3 which is communicated with the 1-2 shift valve 10. When the 1-2 shift valve 10 is changed over to the right-hand first-speed position, which is the downshift position, No. 3 oil passage L3 is connected via an annular groove 10a of the shift valve 10 to No. 4 oil passage L4 which is communicated with the first-speed hydraulic clutch C1, with the result that the first-speed transmission train G1 is established. When the 1-2 shift valve 10 is changed over to the left-hand second-speed position (the position as illustrated), No. 3 oil passage L3 is connected via an annular groove 10b of the 1- 2 shift valve 10 to No. 5 oil passage L5 which is communicated with the second-speed hydraulic clutch C2, with the result that the second-speed transmission train G2 is established. When the main shift valve 9 is changed over to the left-hand high-speed position in which the high-speed system is selected, No. 2 oil passage L2 is connected via an annular groove 9b of the main shift valve 9 to No. 6 oil passage L6 which is communicated with the 3-4 shift valve 11. When the 3-4 shift valve 11 is changed over to the left-hand third-speed position, which is the downshift position (the position as illustrated), No. 6 oil passage L6 is connected via an annular groove 11a of the 3-4 shift valve 11 to No. 7 oil passage L7 which is communicated with the third-speed hydraulic clutch C3, with the result that the third-speed transmission train G3 is established. When the 3-4 shift valve 11 is changed over to the right-hand fourth-speed position, which is the upshift position, No. 6 oil passage L6 is connected via an annular groove 11b of the 3-4 shift valve 11 to No. 8 oil passage L8 which is communicated with the 4-5 shift valve 12. When the 4-5 shift valve 12 is changed over to the right-side fourth-speed position, which is downshift position, No. 8 oil passage L8 is connected via an annular groove 12a of the 4-5 shift valve 12 to No. 9 oil passage L9. In the D.sub.5 range of the manual valve 8, the oil is supplied to the fourth-speed hydraulic clutch C4 through No. 10 oil passage L10 which is connected to No. 9 oil passage L9 via an annular groove 8b of the manual valve 8, with the result that the fourth-speed transmission train G4 is established. When the 4-5 shift valve 12 is changed over to the left-hand fifth-speed position (the position as illustrated), which is the upshift position, No. 8 oil passage L8 is connected via an annular groove 12b of the 4-5 shift valve 12 to No. 11 oil passage L11 which is communicated with the fifth-speed hydraulic clutch C5, with the result that the fifth-speed transmission train G5 is established.

The main shift valve 9, the 1-2 shift valve 10 and the 3-4 shift valve 11 are shift-controlled by No. 1 through No. 3 solenoid valves 15.sub.1, 15.sub.2, 15.sub.3, respectively, which are opened and closed by a non-illustrated electronic control circuit. This will now be described in more detail. A modulator pressure (i.e., a predetermined pressure which is lower than the line pressure) from a modulator valve 16 which is connected to No. 1 oil passage L1 is arranged to be input: to a right-side oil chamber 9c of the main shift valve 9 through No. 13 passage L13 which is connected via an orifice 17.sub.1 to No. 12 oil passage L12 on the output side of the modulator valve 16; to a left-side oil chamber 10c of the 1-2 shift valve 10 through No. 14 oil passage L14 which is connected via another orifice 17.sub.2 to No. 12 oil passage L12; and to a left-side oil chamber 11c of the 3-4 shift valve 11 through No 15 oil passage L15 which is connected via still another orifice 17.sub.3 to No. 12 oil passage L12. To each of these No. 13 through No. 15 oil passages there are connected No. 1 through No. 3 normally-closed type solenoid valves 15.sub.1, 15.sub.2, 15.sub.3 for opening each of the oil passages to atmosphere. In addition, No. 14 oil passage L14 is connected to a left-side oil chamber 12c of the 4-5 shift valve 12. No. 2 solenoid valve 15.sub.2 is thus made to be a common solenoid valve for changing over both the 1-2 shift valve 10 and the 4-5 shift valve 12. The transmission or speed change operation among the first-speed through the fifth-speed is effected by opening and closing No. 1 through No. 3 solenoid valves 15.sub.1, 15.sub.2, 15.sub.3 in the manner as described below.

Namely, at the first speed, No. 1 solenoid valve 15.sub.1 is opened and No. 2 solenoid valve 15.sub.2 is closed. As a result of this operation, the input of the modulator pressure to the oil chamber 9c of the main shift valve 9 is cut off, so that the main shift valve 9 is changed over to the right-hand low-speed position by a spring 9d. At the same time, the 1-2 shift valve 10 is changed over, due to the input of the modulator pressure to the oil chamber 10c, to the right-hand first-speed position against a spring 10d, with the result that the first-speed transmission train G1 is established as described above. When the 3-4 shift valve 11 is in the right-hand fourth-speed position, as will be described in detail hereinafter, the line pressure is input from No. 1 oil passage L1 to a right-end oil chamber 10e of the 1-2 shift valve 10 through No. 18 oil passage L18, whereby the 1 -2 shift valve 10 is restrained to the left-hand second-speed position. Therefore, at the time of the first speed, No. 3 solenoid valve 15.sub.3 is opened to change over the 3-4 shift valve 11 to the left-hand third-speed position.

At the second speed, No. 1 solenoid valve 15.sub.1 is opened and No. 2 solenoid valve 15.sub.2 is opened. As a result of this operation, the main shift valve 9 is in the low-speed position like in the first speed. On the other hand, the input of the modulator pressure to the oil chamber 10c of the 1-2 shift valve 10 is cut off, so that the 1-2 shift valve 10 is changed over to the left-hand second-speed position by a spring 10d, with the result that the second speed train G2 is established as described above. In this case, No. 3 solenoid valve 15.sub.3 may be left open or closed.

At the third speed, No. 1 solenoid valve 15.sub.1 is closed and No. 3 solenoid valve 15.sub.3 is opened. As a result of this operation, the main shift valve 9 is changed over, due to the input of the modulator pressure to the oil chamber 9c of the main shift valve 9, to the left-hand high-speed position against the spring 9d. At the same time, since the input of the modulator pressure to the oil chamber 11c of the 3-4 shift valve 11 is cut off, the 3-4 shift valve 11 is changed over to the left-hand third-speed position by a spring 11d, with the result that the third-speed transmission train G3 is established as described above. In this case, No. 2 solenoid valve 15.sub.2 may be left open or closed.

At the fourth speed, No. 1 through No. 3 solenoid valves 15.sub.1, 15.sub.2, 15.sub.3 are all closed. As a result of this operation, the main shift valve 9 is in the high-speed position like at the third speed. On the other hand, the 3-4 shift valve 11 is changed over to the right-hand fourth-speed position. At the same time, the 4-5 shift valve 12 is changed over, due to the input of the modulator pressure to the oil chamber 12c of the 4-5 shift valve 12, to the right-hand fourth-speed position against a spring 12d, with the result that the fourth speed transmission train G4 is established as described above.

At the fifth speed, No. 1 and No. 3 solenoid valves 15.sub.1, 15.sub.3 are closed and No. 2 solenoid valve 15.sub.2 is opened. As a result of this operation, the main shift valve 9 and the 3-4 shift valve 11 are held in the high-speed position and the fourth-speed position, respectively, like at the fourth speed. On the other hand, the input of the modulator pressure to the oil chamber 12c of the 4-5 shift valve 12 is cut off, the 4-5 shift valve 12 is changed over to the left-hand fifth-speed position by a spring 12d, with the result that the fifth-speed transmission train G5 is established.

The above-described combination of No. 1 through No. 3 solenoid valves 15.sub.1, 15.sub.2, 15.sub.3 at each of the transmission trains can be summarized as shown in Table 1. These solenoid valves are opened or closed according to a speed change map which is contained in the electronic control circuit, whereby transmission or speed change operation among the first speed through fifth speed is effected.

When the vehicle is running at the fifth speed, No. 2 solenoid valve 15.sub.2 is open and the 1-2 shift valve 10 is in the second-speed position Even if the main shift valve 9 is changed over to the low-speed position due to some abnormality or the like during running at the fifth speed, there will occur no downshifting from the fifth speed to the first speed. However, if the main shift valve 9 is changed over to the low-speed position, there exists a possibility of downshifting from the fifth speed to the second speed and from the fourth speed to the first speed. In order to prevent this kind of downshifting, it is so arranged that the main shift valve 9 is hydraulically restrained to the left-hand high-speed position during running at the fourth and fifth speeds. In other words, there is formed a differential pressure groove 9e, which has a difference in area, on the right side of the main shift valve 9. At the high-speed position of the main shift valve 9, the differential pressure groove 9e is isolated from the drain port so as to be connected to No. 16 oil passage L16 which is communicated with the 4-5 shift valve 12. At the D.sub.5 range of the manual valve 8, No. 17 oil passage L17, which is connected to No. 1 oil passage L1 via an annular groove 8a of the manual valve 8, is connected to No. 16 oil passage L16 via an annular groove 12e of the 4-5 shift valve 12 at its right-hand fourth-speed position. At the right-hand fourth-speed position of the 3-4 shift valve 11, No. 18 oil passage L18, which is connected to No. 1 oil passage L1 via an annular groove 11e of the 3- 4 shift valve 11 is connected to No. 16 oil passage L16 via the annular groove 12e of the 4-5 shift valve 12 at its left-hand fifth-speed position. In this arrangement, at the time of the fourth-speed and the fifth-speed, the line pressure is input to the differential pressure groove 9e through No. 16 oil passage L16, whereby the main shift valve 9 is urged towards the left-hand side due to the differential pressure corresponding to the difference in area between the left end and the right end of the groove 9e, with the result that the main shift valve 9 is restrained to the left-hand high-speed position.

In FIG. 2, numerals A1, A2, A3, A4 denote accumulators which are provided to absorb sudden pressure changes during supplying and discharging of the oil to each of the hydraulic clutches C1, C2, C3, C4. There are provided a first electromagnetic pressure control valve 18.sub.1 for the line pressure and a second electromagnetic pressure control valve 18.sub.2 for the back pressure of the accumulators. No. 1 oil passage L1 is connected to the input side of both pressure control valves 18.sub.1, 18.sub.2. No. 19 oil passage L19 on the output side of the first pressure control valve 18.sub.1 is connected to that oil chamber 14a of the regulator valve 14 which operates to increase the pressure, and No. 20 oil passage L20 on the output side of the second pressure control valve 18.sub.2 is connected to the back pressure chamber of each of the accumulators A1, A2, A3, A4.

Each of the pressure control valves 18.sub.1, 18.sub.2 is so arranged that the output pressure varies in inverse proportion to the value of the electric current to be charged to each solenoid 18a. The line pressure varies, as shown in FIG. 4, between a predetermined upper limit pressure PH and a predetermined lower limit pressure PL depending on the value of the electric current to be charged to the solenoid 18a of the first pressure control valve 18.sub.1. During transmission or speed changing, P.sub.1 which is defined to be the output pressure of the first pressure control valve 18.sub.1 and P.sub.2 which is defined to be the output pressure of the second pressure control valve 18.sub.2 are caused to be varied within the conditions of P.sub.2 .ltoreq.P.sub.1 in accordance with the speed changes or the like of the output shaft of the transmission in order to absorb the speed change shocks. During no speed change operation, both P.sub.1 and P.sub.2 are normally made to be maximum pressures. During running at above a predetermined vehicle speed at a small throttle opening degree, the engine torque is small and, consequently, the clutch engaging force may also be small; therefore, P.sub.1 and P.sub.2 are made to be zero.

If the first pressure control valve 18.sub.1 is locked to a condition in which no oil pressure is output due to some abnormality, such as sticking, the line pressure will become low and slipping of the clutches will occur, thereby influencing the running of the vehicle. Therefore, in the embodiment of this invention, No. 20 oil passage L20 is connected to the drain port 18b of the first pressure control valve 18.sub.1. Thus, even if the first pressure control valve 18.sub.1 is locked to a condition in which no oil pressure is output (in this condition the drain port 18b is communicated with No. 19 oil passage L19), the output pressure P.sub.2 from the second pressure control valve 18.sub.2 is arranged to be input to No. 19 oil passage L19 so as to prevent the dropping of the line pressure. Under normal conditions, the arrangement is made such that P.sub.2 .ltoreq.P.sub.1 and, therefore, no abnormality occurs to the control of the line pressure due to such an occurrence as the input of P.sub.2 to No. 19 oil passage L19. When P.sub.1 and P.sub.2 are zero, No. 19 oil passage L19 is communicated from the drain port 18b of the first pressure control valve 18.sub.1 with the drain port 18b of the second pressure control valve 18.sub.2 via No. 20 oil passage L20.

Explanation has so far been made about the construction or constitution of the hydraulic oil circuit in the D.sub.5 range of the manual valve 8. There will be the same kind of constitution also in the D.sub.3 range. However, in the D.sub.3 range, the first-speed through the third-speed transmission or speed change operation is effected by changing the speed change map to that one for D.sub.3 which is stored in the electronic control circuit. If the third-speed transmission train G3 is promptly established after changing over to the D.sub.3 range while running at the fifth speed, overrunning of the engine will occur. Therefore, the fourth-speed transmission train G4 is established until the vehicle speed becomes lower than a predetermined value right after the change over.

In the 2 range of the manual valve 8, No. 1 oil passage L1 is connected, via the annular groove 8a of the manual valve 8, to No. 2 oil passage L2 and No. 17 oil passage L17 and, at the same time, to No. 21 oil passage L21 which is communicated with a left-end oil chamber 12f of the 4-5 shift valve 12. At the left-hand high-speed position of the main shift valve 9, No. 22 oil passage L22 which is connected to No. 3 oil passage L3 via the annular groove 9a of the shift valve 9, is connected to No. 9 oil passage L9 via the annular groove 8b of the manual valve 8. Further, at the left-hand third-speed position of the 3-4 shift valve 11, No. 23 oil passage L23 which is connected to No. 18 oil passage L18 via the annular groove 11e of the 3-4 shift valve 11 is connected to No. 1 oil passage L1 via an annular groove 8c of the manual valve 8 and a branched port portion of No. 21 oil passage L21. As a result, in the 2 range, the 4-5 shift valve 12 is restrained to the right-hand fourth-speed position by the input of the line pressure to the oil chamber 12f of the 4-5 shift valve 12 through No. 21 oil passage L21. Therefore, No. 8 oil passage L8 is constantly connected to No. 22 oil passage L22 through No. 9 oil passage L9. Further, the line pressure is input to No. 18 oil passage L18 from No. 23 oil passage L23 at the third-speed position of the 3-4 shift valve 11 and from No. 1 oil passage L1 at the fourth-speed position thereof. Thus, the line pressure is constantly input to the right-end oil chamber 10e of the 1-2 shift valve 10 through a branch of No. 18 oil passage L18, with the result that the 1-2 shift valve 10 is restrained to the left-hand second-speed position. (No. 22 oil passage L22 functions as a second oil supply passage.)

Right after changing over to the 2 range, when the vehicle speed V is equal to or above a predetermined high vehicle speed VH (e.g., 100 km/h), No. 1 solenoid valve 15.sub.1 is closed and No. 2 and No. 3 solenoid valves 15.sub.2, 15.sub.3 are opened. The main shift valve 9 is thus positioned at the left-hand high-speed position and the 3-4 shift valve 11 is positioned at the left-hand third-speed position, whereby the third-speed transmission train G3 is established. In this case, even if No. 1 solenoid valve is opened by mistake while running at the third speed, the main shift valve 9 is restrained to the high-speed position by the input of the line pressure to the differential pressure groove 9e of the main shift valve 9 through No. 17 oil passage L17 and No. 16 oil passage L16, whereby downshifting to the second speed or below is prevented. If No. 3 solenoid valve 15.sub.3 is closed by mistake while running at the third speed to cause the 3-4 shift valve 11 to be changed over to the right-hand fourth-speed position, pressurized oil is supplied from No. 8 oil passage L8 to No. 3 oil passage L3 through No. 9 oil passage L9, No. 22 oil passage L22, and the annular groove 9a of the main shift valve 9 which is at the high-speed position. Here, since the 1-2 shift valve 10 is being restrained to the second-speed position as described above, the second-speed transmission train G2 is established and, therefore, downshifting to the first-speed is prevented.

When the vehicle speed V is below VH and is equal to or above a predetermined low vehicle speed VL (e.g., 20 km/h), No. 1 and No. 3 solenoid valves 15.sub.1, 15.sub.3 are closed to shift the main shift valve 9 to the high-speed position and the 3-4 shift valve 11 to the fourth-speed position. The oil is supplied to the second-speed hydraulic clutch C2 through the same route as described above, thereby establishing the second-speed transmission train G2. When V<VL, No. 1 solenoid valve 15.sub.1 is opened to change over the main shift valve 9 to the low-speed position. The hydraulic oil is supplied from No. 2 oil passage L2 to No. 3 oil passage L3 in the same manner as in the D.sub.5 range, thereby establishing the second-speed transmission train G2. The reason why the second-speed transmission train G2 is established at a relatively high vehicle speed by supplying the oil to the second-speed hydraulic clutch C2 in the route via the 3-4 shift valve 11 is as follows. Namely, if the 1-2shift valve 10 is locked to the first-speed position by sticking or the like, the 3-4 shift valve 11 is forcibly changed over by the hydraulic pressure to the third-speed position, thereby stopping the supply of the oil to No. 3 oil passage L3, to prevent the downshifting to the first speed at a high vehicle speed. In other words, the line pressure is input to the right-end oil chamber 11f of the 3-4 shift valve 11 from No. 21 oil passage L21 through: No. 24 oil passage L24 which is connected to No. 21 oil passage L21 via an annular groove 10f of the 1-2 shift valve 10 at the right-hand first-speed position thereof; No. 25 oil passage L25 which is connected to No. 24 oil passage L24 via a notched groove 13a of the servo valve 13 at the left-hand "forward" position (i.e, illustrated position) thereof; and No. 26 oil passage L26 which is connected to No. 25 oil passage L25 via an annular groove 19a of a servo control valve 19, which will be described in more detail hereinafter, at the left-hand position to allow for forward transmission or forward travelling. In this manner, the 3-4 shift valve 11 is arranged to be forcibly shifted to the left-hand third-speed position.

The above-described combination of No. 1 through No. 3 solenoid valves 151, 152, 153 in the 2 range can be summarized as shown in Table 2.

In the 1 range of the manual valve 8, No. 17 oil passage L17 and No. 23 oil passage L23 are opened to the atmosphere, and the other oil passages are the same as in the 2 range.

Also in the 1 range of the manual valve 8, in order to prevent overrunning of the engine right after change over operation, No. 1 and No. 3 solenoid valves 15.sub.1, 15.sub.3 are closed and No. 2 solenoid valve 15.sub.2 is opened when the vehicle speed is equal to or above a predetermined vehicle speed VM (e.g., 50 km/h). In the same manner as in establishing the second-speed transmission train G2 in the high-speed range in the 2 range, the oil is supplied from No. 2 oil passage L2 to No. 3 oil passage L3 through: No. 6 oil passage L6 via the main shift valve 9 which is in the left-hand high-speed position; No. 8 oil passage L8 via the 3-4 shift valve 11 which is in the right-hand fourth-speed position; No. 9 oil passage L9 via the 4-5 shift valve 12 which is being restrained to the right-hand fourth-speed position by the line pressure from No. 21 oil passage L21; and No. 22 oil passage L22 which is connected to No. 9 oil passage L9. In this manner, the second-speed transmission train G2 is established while restraining the 1-2 shift valve 10 to the left-hand second-speed position by the line pressure to be input from No. 1 oil passage L1 through No. 18 oil passage L18. In this case, if the 1-2 shift valve 10 is locked to the first-speed position, the 3-4 shift valve 11 is changed over, like in the above-described case, to the left-hand third-speed position by the input of the line pressure through No. 26 oil passage L26. The third-speed transmission train G3 is thus established and the downshifting to the first speed is prevented.

If the vehicle speed V is below VM, No. 1 and No. 3 solenoid valves 15.sub.1, 15.sub.3 are opened and No. 2 solenoid valve 15.sub.2 is closed. In this case, since No. 17 oil passage L17 is opened to the atmosphere in the 1 range, the line pressure is not input to the differential pressure groove 9e of the main shift valve 9. The main shift valve 9 is therefore changed over from the high-speed position to the right-hand low-speed position by the opening of No. 1 solenoid valve 15.sub.1. In addition, when the 3-4 shift valve 1 is changed over to the left-hand third-speed position as a result of opening of No. 1 solenoid valve 15.sub.1, No. 18 oil passage L18 is opened to the atmosphere through No. 23 oil passage L23 which is connected thereto, whereby the restraining of the 1-2 shift valve 10 to the second-speed position is released. Therefore, by closing No. 2 solenoid valve 15.sub.2 the 1-2 shift valve 10 is changed over to the right-hand first-speed position, and the first-speed transmission train G1 is established.

The above-described combination of No. 1 through No. 3 solenoid valves 15.sub.1, 15.sub.2, 15.sub.3 in the 1 range can be summarized as shown in Table 3.

In the R range of the manual valve 8, the supply of the oil to No. 2 oil passage L2 is stopped, and No. 27 oil passage L27 for reverse transmission train is connected to No. 1 oil passage L1. The line pressure is input to a left-hand oil chamber 13b of the servo valve 13 via No. 28 oil passage L28 which is connected to No. 27 oil passage L27 via an annular groove 10g of the 1-2 shift valve 10 at the left-hand second-speed position thereof. As a result, the servo valve 13 is urged to the right-hand reverse transmission position, and the selector gear 6 is changed over to the right-hand reverse transmission side. At the same time, at the reverse transmission position, No. 28 oil passage L28 is connected to No. 29 oil passage L29 which is communicated with the manual valve 8 via that axial opening 13c of the servo valve 13 which is communicated with the left-hand oil chamber 13b. No. 29 oil passage L29 is connected to No. 10 oil passage L10 for the fourth-speed hydraulic clutch C4 via the annular groove 8b of the manual valve 8 in the R range thereof. The reverse transmission train GR is thus established by changing over the selector gear 6 to the reverse transmission side and by supplying oil to the fourth-speed hydraulic clutch C4.

In the R range, No. 2 solenoid valve 15.sub.2 is opened and the third solenoid valve 15.sub.3 is closed to shift the 1-2 shift valve 10 to the second-speed position and shift the 3-4 shift valve 11 to the right-hand fourth-speed position. Thus, the line pressure is input from No. 1 oil passage L1 to the oil chamber 10e of the 1-2 shift valve 10 through No. 18 oil passage L18. It is thus so arranged that, even if No. 2 solenoid valve 15.sub.2 is closed by mistake, the 1-2 shift valve 10 is restrained to the second-speed position so that the reverse transmission train GR can be established.

Further, when the manual valve 8 is changed over to the R range while running forward at or above a predetermined vehicle speed VI (e.g., 10 km/h), No. 2 solenoid valve 15.sub.2 is closed and No. 3 solenoid valve 15.sub.3 is opened. The 3-4 shift valve 11 is thus changed over to the left-hand third-speed position to thereby shut off the input of the line pressure to the 1-2 shift valve 10 through No. 18 oil passage L18 (No. 23 oil passage L23 is opened to the atmosphere in the R range), and the 1-2 shift valve 10 is changed over to the right-hand first-speed position to shut off the connection between No. 27 oil passage L27 and No. 28 oil passage L28, whereby the reverse transmission train GR is prevented from being established.

The above-described combination of No. 1 through No. 3 solenoid valves 15.sub.1, 15.sub.2, 15.sub.3 in the R range can be summarized as shown in Table 4.

When the manual valve 8 is changed over from the R range to D.sub.5, D.sub.3, 2 or 1 range, the line pressure is input from No. 2 oil passage L2 to an oil chamber 13d which is formed in the servo valve 13 opposite to the above-described oil chamber 13b, via the servo control valve 19 and No. 30 oil passage L30. The servo valve 13 is thus urged towards the left-hand forward transmission or forward travelling position to change over the selector gear 6 to the left-hand forward transmission side. Here, the servo control valve 19 is being urged by a spring 19b towards the right-hand position (i.e., illustrated position) for prohibiting forward transmission, in which position the connection between No. 2 oil passage L2 and No. 30 oil passage L30 is shut off. A right-hand oil chamber 19c of the servo control valve 19 is connected to the downstream side of an orifice 20 which is interposed in No. 2 oil passage L2, through No. 31 oil passage L31. When the oil pressure on the downstream side of the orifice 20 has been raised to a predetermined value after changing over the manual valve 8 from the R range, the servo control valve 19 is changed over to the left-hand position which allows for the forward transmission or forward travelling. It is thus so arranged that No. 30 oil passage L30 is connected to No. 2 oil passage L2 via an annular groove 19d of the servo control valve 19, thereby changing over the servo valve 13 to the forward transmission position. By this arrangement, when the servo valve 13 is changed over to the forward transmission position, the torque transmission has been effected in the direction of forward rotation by the engagement force of the first-speed hydraulic clutch C1 (second-speed hydraulic clutch C2 in the case of the 2 range), which is supplied with the oil right after the change over operation, due to the oil pressure increase on the downstream side of the orifice 20. As a result, even if the manual valve 8 is changed over from the R range to D.sub.5, D.sub.3, 2 or 1 range while the accelerator pedal is depressed, there occurs a condition in which the reverse rotation of the output shaft 1b is prevented by said torque transmission and, therefore, the selector gear 6 will come into a smooth engagement with the side of the fourth-speed transmission train G4, thereby preventing the wear of the gears. In the R range, the servo control valve 19 is securely changed over to the right-hand position in which the forward transmission is prohibited, by the line pressure which is input to a left-end oil chamber 19e through No. 28 oil passage L28.

If the servo control valve 19 is locked to the position in which the forward transmission is prohibited due to sticking or the like, or if the servo valve 13 is locked to the reverse transmission position even if the servo control valve 19 has been changed over to the position which allows for forward transmission, the selector gear 6 will remain in the reverse transmission position even if the manual valve 8 is changed over from the R range to the D.sub.5, D, 2 or 1 range. It follows that, if the oil is supplied to the fourth-speed hydraulic clutch C4, the rear transmission train GR will be established.

Therefore, the following arrangement has been made. Namely, No. 26 oil passage L26 which is communicated with the right-hand oil chamber 11f of the 3-4 shift valve 11 is connected to No. 2 oil passage L2 at the right-hand position of the servo control valve 19, in which position the forward transmission is prohibited, via the annular groove 19a thereof; and to No. 25 oil passage L25 at the left-hand position thereof in which the forward transmission is allowed. At the reverse transmission position of the servo valve 13, No. 25 oil passage L25 is connected to the oil chamber 13d for forward transmission via a notched groove 13e of the servo valve 13. When the above-described abnormality has occurred, the line pressure is input to the oil chamber 11f of the 3-4 shift valve 11 through No. 26 oil passage L26, so that the 3-4 shift valve 11 is restrained to the left-hand third-speed position, whereby the oil is prevented from being supplied to the fourth-speed hydraulic clutch C4. In FIG. 2, numeral 13f denotes a click member for engaging the servo valve 13 to the forward transmission position and the reverse transmission position, respectively.

In the N and P ranges of the manual valve 8, the electric charging to all of the No. 1 through No. 3 solenoid valves 15.sub.1, 15.sub.2, 15.sub.3 is cut off to make them closed for the purpose of energy saving. However, since No. 32 oil passage L32 which is communicated with a left-end oil chamber 9f of the main shift valve 9 is connected to No. 1 oil passage L1 via the annular groove 8a of the manual valve 8, the main shift valve 9 is retained to the right-hand first-speed position. In addition, since the line pressure is input from No. 1 oil passage L1 to the oil chamber 10e of the 1-2 shift valve 10 through No. 18 oil passage L18 as a result of changing over of the 3-4 shift valve 11 to the right-hand fourth-speed position, the 1-2 shift valve 10 is maintained in the left-hand second-speed position even if No. 2 solenoid valve 15.sub.2 is being closed In the P range, No. 27 oil passage L27 is connected to No. 1 oil passage L1 and the line pressure is input from No. 28 oil passage L28 to the oil chamber 13b for the reverse transmission of the servo valve 13. As a result, the servo valve 13 is changed over to and maintained in the reverse transmission position. When the vehicle speed is above VI, No. 3 solenoid valve 15.sub.3 is opened to change over the 3-4 shift valve 11 to the left-hand third-speed position. The input of the line pressure to the oil chamber 10e of the 1-2 shift valve 10 through No. 18 oil passage L18 is shut off to change over the shift valve 10 to the right-hand first-speed position. The interconnection between No. 27 oil passage L27 and No. 28 oil passage L28 is thus shut off.

In FIG. 2, numeral 21 denotes a relief valve which is connected to the drain side of the regulator valve 14, and numeral 22 denotes a control circuit for a lock-up clutch 3a which is incorporated into the fluid torque converter 3.

As is clear from the above description, according to this invention, the shift valve unit is divided into the unit for the low-speed system and the unit for the high-speed system such that the oil is selectively supplied to both shift valve units via the main shift valve. Therefore, even if there occurs an abnormality to the respective shift valves which belong to each shift valve unit, it is possible to perform forward acceleration or travelling at any of the transmission trains which belong to the low-speed system or cruising at any of the transmission trains which belong to the high-speed system, if the main shift valve is provided with a fail-safe function. Consequently, the oil circuit constitution for the fail-safe function can be simplified. In addition, it becomes possible to shift-control a shift valve which belongs to the shift valve unit for the low-speed system and a shift valve which belongs to the shift valve unit for the high-speed system by a common solenoid valve. The cost can therefore be reduced by reducing the number of solenoid valves to be used. Also, the transmission apparatus can be made smaller by reducing the space required for mounting the solenoid valves.

It is readily apparent that the above-described control apparatus for a vehicular automatic transmission has the advantage of wide commercial utility. It should be understood that the specific form of the invention hereinabove described is intended to be representative only, as certain modifications within the scope of these teachings will be apparent to those skilled in the art.

Accordingly, reference should be made to the following claims in determining the full scope of the invention.