JPH08145166A - Control device for lockup clutch of automatic transmission - Google Patents

Abstract

PURPOSE: To provide a control device for lockup clutch of automatic transmission capable of performing lockup control properly at gear change from n th gear to (n+1)th gear by detecting inertia phase accurately by a hydraulic control device without using any electronic control. CONSTITUTION: An automatic transmission which engages a lockup clutch 6 at n th gear and (n+1)th gear is provided with a lockup selector valve 40 which supplies hydraulic pressure selectively to an engagement side oil chamber 6b and a release side oil chamber 6a of the lockup clutch and an inertia phase detector valve 70 which acts hydraulic pressure generated at (n)th gear and drained at (n+1)th gear on one end side and acts a hydraulic pressure according to the engine load on the other side so as to detect inertia phase by a correlation of these hydraulic pressures. Also an output pressure of the inertia phase detector valve 70 acts on the one end side of the lockup selector valve 40 so as to drain hydraulic pressure in an oil chamber on the engagement side of the lockup clutch temporarily by the lockup selector valve which responds to the starting of inertia phase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lock-up clutch control device for a mechanical automatic transmission, and more particularly to a lock-up clutch control device during a gear shift transition.

[0002]

2. Description of the Related Art Generally, an engine output shaft and an input shaft of an automatic transmission are connected via a torque converter, and it is known that the torque converter has a lock-up clutch built therein. The lock-up clutch mechanically connects the input side and the output side of the torque converter, and selectively engages the hydraulic pressure between the engagement-side oil chamber and the release-side oil chamber of the lock-up clutch to engage or disengage. Can be released. By the way, if gear shifting is performed with the lock-up clutch still engaged, an excessive shift shock will occur. In order to avoid this, the lockup clutch is released for a short period of time during a gear shift transition (Japanese Patent Publication No. 63-21064).

[0003]

FIG. 1 shows the switching of the lockup clutch and the change in engine speed during the transition from the third speed to the fourth speed. When shifting starts at time t ₁ , the vehicle acceleration begins to decrease, and the lockup clutch is released (OFF) at time t ₂ . Then, at time t ₃ , the vehicle acceleration reaches the lower limit, and thereafter, the vehicle acceleration turns to increase. Then, continuing to the vehicle acceleration increases until time t _4, and rapidly decreases from the time t ₄ to a predetermined acceleration,
After that, it is held at a constant value. Further, the engine speed gradually decreases from time t ₃ to time t ₅ , and becomes stable after time t ₅ when the lockup clutch is re-engaged. In this way, the period from the start of gear shift (t ₁ ) to the lower limit of the vehicle acceleration (t ₃ ) is called the torque phase, and the period from the time t ₃ to the upper limit of the vehicle acceleration (t ₄ ) is called the inertia phase. . Conventionally, since the lockup clutch is released (OFF) in the torque phase, there is a problem that a phenomenon of engine uprising occurs and the driver feels uncomfortable.

In order to solve such a problem, Japanese Patent Publication No. 2
As disclosed in Japanese Patent No. 36728, there is a lock-up clutch that prevents the engine from rising by adjusting the timing of switching from the ON (engaged) state to the OFF state to the inertia phase. The time from the start of shifting to the inertia phase varies depending on the piston stroke of the hydraulic servo, the orifice diameter in the oil passage, the oil temperature, the throttle opening, and the like. Therefore, in order to detect the inertia phase with high accuracy, the rotation speed determined by the product of the output shaft rotation speed of the automatic transmission and the gear ratio of the gear before the gear change is compared with the engine rotation speed. It is necessary to determine whether or not the above is reached. As a result, conventionally, the inertia phase can be detected only by electronic control, and an electronic control device for controlling the operation of the lockup mechanism, various sensors, actuators, etc. are required, which is very expensive.

An object of the present invention is to provide a lockup clutch for an automatic transmission, which can accurately detect the inertia phase by a hydraulic control device without using electronic control and can appropriately perform lockup control during gear shifting. It is to provide the control device.

[0006]

In order to achieve the above object, the present invention inputs engine power to an automatic transmission through a torque converter and mechanically connects the input side and the output side of the torque converter. In a case where a lockup clutch to be connected is provided and the lockup clutch is engaged at the nth speed and the n + 1th speed of the automatic transmission, the hydraulic pressure is selectively supplied to the engagement side oil chamber and the release side oil chamber of the lockup clutch. The lock-up switching valve and one end side are actuated to generate hydraulic pressure at the n-th speed and drained at the n + 1-th speed, and the other end is actuated to the hydraulic pressure according to the engine load, and the inertia phase is detected by the correlation of these hydraulic pressures. The inertia phase detection valve is installed and the output pressure of the inertia phase detection valve is applied to one end of the lockup switching valve to Lockup switching valve in response to start is one that was configured to temporarily scavenge the oil pressure of the engagement side oil chamber of the lockup clutch.

[0007]

When the gear is shifted from the nth speed to the n + 1th speed, a torque phase is first generated to reduce the vehicle acceleration, but after the acceleration is reduced to a certain acceleration, the vehicle acceleration starts to increase. This turning point is the end point of the torque phase (start point of the inertia phase). According to the present invention, the oil pressure is generated at one side of the inertia phase detection valve at the nth speed, and the hydraulic pressure of the engaging element is drained at the (n + 1) th speed, and the other end side is subjected to the hydraulic pressure (for example, line pressure) according to the engine load. Let As a result, when the transmission torque of the engagement element matches the engine torque (end point of the torque phase)
Switch the inertia phase detection valve and switch the lockup switching valve to the release side. Therefore, the lock-up clutch can be temporarily disengaged at the start of the inertia phase, and the engine can be prevented from rising due to the disengagement of the lock-up clutch.

[0008]

FIG. 2 shows an example of an automatic transmission having four forward gears and one reverse gear, in which an output shaft 1a of an engine 1 is connected to an input shaft 8 via a torque converter 2. The torque converter 2 includes a lockup clutch 6 as well as a pump impeller 3, a turbine runner 4, and a stator 5.
The lock-up clutch 6 is released by guiding the hydraulic pressure to the disengagement side oil chamber 6a, and the torque converter 2 is activated, and the lock-up clutch 6 is moved to the front side by guiding the hydraulic pressure to the engagement side oil chamber 6b. The cover 7 is closely attached and fastened, and the input side (front cover 7) and the output side (input shaft 8) are mechanically connected. Reference numeral 9 is a damper mechanism of the lockup clutch 6.

The input shaft 8 is a Ravigneaux type planetary gear mechanism 1
0 forward sun gear 10a is connected via a C ₁ clutch, and rear sun gear 10b and input shaft 8 are connected to C
Is connected via _two clutches are connected via a C ₃ clutch and the input shaft 8 and the carrier 10c. The carrier 10c is a one-way clutch OWC that allows only the B ₂ brake and the normal rotation (engine rotation direction) of the carrier 10c.
Is connected to a fixing member 12 such as a casing via. The carrier 10c includes two types of planetary gears 10d,
The forward sun gear 10a meshes with the long pinion 10d having a long axial length, and the rear sun gear 10b meshes with the long pinion 10d via the short pinion 10e having a short axial length. The ring gear 10f, which meshes only with the long pinion 10d, is connected to the reduction gear 15. Servo piston 13 is b
₁ brake is operated, b ₁ brake is C
_The clutch drum 14 connecting the _1- clutch and the forward sun gear 10a is fastened or released to the fixed member 12 such as a casing. The reduction gear 15 is a reduction gear 16a provided at the end of the output shaft 16.
Further, the final reduction gear 16b provided at the start end of the output shaft 16 meshes with the ring gear 18 of the differential gear 17, so that the power is transmitted to the axle 19.

The above-mentioned automatic transmission has the clutches C ₁ , C ₂ ,
C ₃ , brakes b ₁ , B ₂ and one-way clutch O
By the operation of the WC, as shown in Table 1, four forward gears and one reverse gear are realized. In the following table, ○ indicates the operating state. In addition, B ₁ is an operating-side oil chamber of the servo piston 13, and B ₁ ′ is a releasing-side oil chamber of the servo piston 13, and when hydraulic pressure is introduced into both oil chambers B ₁ and B ₁ ′ (3 The b ₁ brake is released at the time of speed.

[0011]

[Table 1]

As described above, only the C ₂ clutch is engaged in the first speed, the C ₂ clutch and the b ₁ brake are engaged in the _second speed, and all the clutches C _{1 to} C ₃ are engaged in the third speed. b _1, B ₂ brake is released, 4
At high speed, the C ₃ clutch and the b ₁ brake are engaged.
When the vehicle is moving backward (R range), the C ₁ clutch and the B ₂ brake are engaged.

FIG. 3 is a shift diagram of the automatic transmission,
A solid line indicates an upshift and a broken line indicates a downshift.
Also, the shaded area indicates the lock-up operation region, the vehicle speed is equal to or higher than the set vehicle speed (for example, 35 km / h), the shift speed is the third speed or the fourth speed, and there is no kickdown (for example, throttle opening <85%. ), The lockup clutch 6 is activated.

The clutches C ₁ , C ₂ , C ₃ and the brakes b ₁ , B ₂ are operated by the hydraulic control device shown in FIG. In FIG. 4, the hydraulic circuit of the lockup clutch 6, which is the main part of the present invention, is omitted. This hydraulic control device generally includes an oil pump 20 and primary regulator valves 21, D, R, N, P that generate a line pressure P _L.
And the like, the manual valve 22 manually operated in each range, the throttle valve 23 that generates the throttle pressure P _T according to the throttle opening, and the clutches C _{1 to} C ₃ depending on the relative relationship between the throttle pressure P _T and the governor pressure P _G. And brakes b ₁ , B
1-2 shift valve 24, 2-3 shift valve 25 and 3-4 shift valve 26 for selectively supplying hydraulic pressure to ₂ , governor valve 27 for generating governor pressure P _G according to vehicle speed, throttle modulator valve 28, 4-2 Timing valve 29, 2-4 Timing valve 30, 3-2 Timing valve 31, 3-4 Timing valve 32,
It includes a servo control valve 33, a b ₁ brake accumulator 34, a C ₂ clutch accumulator 35, a C ₁ clutch accumulator 36, and the like. Since the hydraulic control device is already known, detailed description thereof will be omitted here.

FIG. 5 is a hydraulic circuit diagram (at the third speed) for controlling the lockup clutch 6 which is a main part of the present invention. In the figure, 40 is a lockup switching valve, 50 is a lockup pressure control valve, 60 is a lockup timing valve, 70 is an inertia phase detection valve, and 80 is a lockup signal valve.

The lockup switching valve 40 is a switching valve for selectively supplying hydraulic pressure to the engagement side oil chamber 6b and the disengagement side oil chamber 6a of the lockup clutch 6, and includes an input port 41, a drain port 42, and a first port. , Second output ports 43, 44
Have and. The lubricating pressure is input from the regulator valve 21 to the input port 41, the first output port 43 is connected to the release side oil chamber 6a, and the second output port 44 is connected to the engagement side oil chamber 6b via the lockup pressure control valve 50. Connected with.
At one end of the lockup switching valve 40, the hydraulic pressure P _C3 of the C ₃ clutch that operates in the _3rd and 4th speeds from the lockup timing valve 60 and the hydraulic pressure P _i from the inertia phase detection valve 70 act as signal pressure. ing. The spring pressure of the spring 45 and the two line pressures P _L act from the opposite side of the lockup switching valve 40. The line pressure P _L is constantly acting on one of these hydraulic pressures, and the line pressure P _L is selectively acting on the other from the lockup signal valve 80. Therefore, the switching valve 40 has a hydraulic pressure P of the C ₃ clutch.
_It is switched by the balance between the sum of the loads of _C3 and the hydraulic pressure P _i and the sum of the loads of the spring pressure of the spring 45 and the line pressure P _L.

The lockup signal valve 80 has a governor pressure P _G acting on one end side, and a spring pressure and a kickdown pressure P _K by the spring 83 acting on the other end side. The kickdown pressure P _K is the same hydraulic pressure as the throttle pressure P _T output from the throttle valve 23 only when the throttle opening is opened above a certain opening as in kickdown. Therefore, through the port 81 and 82 communicating at the time of low vehicle speed, the line pressure P _L acts on the two locations with respect to the lockup switching valve 40, ports 81 and 82 is cut off at the time of high vehicle speed, the line pressure P _L Is 1 for the lockup switching valve 40
Acts only on points. In addition, even when the vehicle speed is high, when the throttle opening becomes high as in kickdown, the port 8
1, 82 communicate.

When the sum of the loads of the C ₃ clutch pressure P _C3 and the hydraulic pressure P _i becomes larger than the sum of the loads of the spring pressure of the spring 45 and the line pressure P _L , that is, the vehicle speed is set to a set vehicle speed (for example, 35 km / h). ) As described above, when the shift speed is the third speed or the fourth speed, and in the non-kickdown state (for example, the throttle opening <85%), the lockup switching valve 40 switches to the left and the lubricating pressure becomes the second pressure. It is supplied from the output port 44 to the engagement side oil chamber 6b of the lockup clutch 6 via the lockup pressure control valve 50.

The lock-up pressure control valve 50 is a pressure regulating valve for adjusting the oil pressure in the engagement side oil chamber 6b of the lock-up clutch 6, and the spring pressure by the spring 51 and the throttle pressure P.
_The output pressure is adjusted so that the sum of the load with _T and the output pressure are balanced. The first input port 52 is the lockup switching valve 40.
Connected to the second output port 44 of the
Is connected to the second output port 44 via the orifice 54 and is drained via the oil cooler 55. The output port 56 is connected to the engagement-side oil chamber 6b of the lockup clutch 6, and the output pressure is the orifice 5
The signal pressure is fed back to the right end side via 7. Therefore, the oil pressure in the engagement side oil chamber 6b is adjusted according to the throttle opening, and is adjusted to the minimum necessary oil pressure according to the engine load.

The lock-up timing valve 60 has the third speed →
It is a switching valve for temporarily releasing the lockup clutch 6 at the time of shifting to the fourth speed, and turns ON / OFF the signal pressure introduced to the right end side of the lockup switching valve 40. The C ₃ clutch pressure P _C3 is input to the input port 61, and the port 62 is a drain port. The output port 63 is connected to the signal pressure port on the right end side of the lockup switching valve 40. The lock-up timing valve 60 is biased by a spring 64 from the left side, and an output pressure P _i from an inertia phase detection valve 70 described later acts on the right side. The lock-up timing valve 60 is set so that the spring force of the spring 64 is relatively low so that the lock-up timing valve 60 switches to the right when the output pressure P _i of the inertia phase detection valve 70 significantly decreases. Therefore, the switching of the lockup timing valve 60 to the right is slower than the switching of the lockup switching valve 40 to the right.

The inertia phase detection valve 70 is a switching valve that turns ON / OFF the signal pressure P _i introduced to the right end side of the lockup switching valve 40 and the lockup timing valve 60. The C ₃ clutch pressure P _C3 is input to the input port 71, and the port 72 is a drain port. Output port 7
3 is connected to the signal pressure ports on the right end side of the lockup switching valve 40 and the lockup timing valve 60 via the orifice 74, and the lockup accumulator 7
5 is also connected. The inertia phase detection valve 70 is biased by a spring 76 from the left side, and the output pressure P _{B1 ′} of the servo control valve 33 supplied to the release side oil chamber B ₁ ′ of the servo piston 13 acts as a signal pressure. . The line pressure P _L acts as a signal pressure on the right side of the inertia phase detection valve 70.

As is apparent from Table 1, the output pressure P _{B1 '} is a hydraulic pressure that is generated in the third speed and drained in the fourth speed. Further, the line pressure P _L is regulated by the regulator valve 21, but since the throttle pressure P _T is acting on one end side thereof, the line pressure P _L changes depending on the throttle opening, that is, the engine load. Thus, the hydraulic pressure is generated at one end of the inertia phase detection valve 70 at the third speed.
Since the hydraulic pressure P _{B1 ′} drained at high speed is applied and the hydraulic pressure P _L according to the engine load is applied to the other end side,
When the sum of the load of the spring force and the hydraulic pressure P _{B1 ′} is larger than the hydraulic pressure P _L , the hydraulic pressure P _i is output. Servo piston 13
The fact that the oil pressure P _{B1 ′ in} the release side oil chamber starts to decrease means that the b ₁ brake starts to be engaged.
_The torque phase ends when the following equation is established between the transmission torque t _{B1 of one} brake and the engine torque (turbine torque) t _EN .

[0023]

## EQU1 ## t _EN = kt _{B1 In the} above equation, k is a constant determined by the planetary gear ratio, and k is smaller than 1 when the fourth speed is overdrive. The inertia phase detection valve 70 is b
By setting the release side hydraulic pressure P _{B1 ′ of the one} brake and the line pressure P _L to face each other, the output port 73 is drained by switching to the left side when the torque phase ends.

The servo control valve 33 is a switching valve that turns on and off the hydraulic pressure P _{B1 ′} supplied to the release side oil chamber B ₁ ′ of the servo piston 13. First input port 33a
C ₂ clutch pressure P _C2 is input to the second input port 3
C ₃ clutch pressure P _C3 is input to 3b. Also,
The output port 33c is connected to the release side oil chamber B ₁ ′ and also to the left signal port of the inertia phase detection valve 70. The C ₂ clutch pressure P _C2 acts as a signal pressure on the left side of the servo control valve 33, and the C ₂ clutch pressure P _C2 acts on the right side of the servo control valve 33.
_{3 The} clutch pressure P _C3 acts as a signal pressure. Therefore, in the state where C ₂ clutch pressure P _C2 is applied, it is output C ₃ clutch pressure P _C3, C ₃ clutch pressure P _C3 is in the state where the act, C ₂ clutch pressure P _C2 is output. Therefore, as is clear from Table 1, the output pressure P _{B1 ′} is output only in the third speed.

Next, the operation of the hydraulic circuit, particularly the operation of the lockup clutch 6 during the shift from the 3rd speed to the 4th speed will be described with reference to FIGS. The operation described below assumes that the vehicle speed is equal to or higher than the set vehicle speed and the kickdown is not performed. At the 1st speed and the 2nd speed, the C ₂ clutch pressure P _C2 is operating, but the C ₃ clutch pressure P _C3 is drained, so that the lockup switching valve 40 and the lockup timing valve 60 are guided to the right end side. The signal pressure P _i is off. Therefore, valve 4
Both 0 and 60 are shifted to the right, and the lockup clutch 6 is released.

At the third speed, as shown in FIG. 5, the servo control valve 33 outputs the hydraulic pressure P _{B1 ′,} so that the inertia phase detection valve 70 is shifted to the right by the hydraulic pressure P _{B1 ′} . Therefore, the C ₃ clutch pressure P _C3 is reduced to the ports 71, 73.
To act on the right end side port of the lockup switching valve 40 and the right end side port of the lockup timing valve 60, and the lockup timing valve 60 and the lockup timing valve 60 shift to the left. Therefore, the lubricating pressure is supplied to the engagement side oil chamber 6b of the lockup clutch 6 via the lockup switching valve 40 and the lockup pressure control valve 50, and the lockup clutch 6 is engaged.

FIG. 6 shows a shift transition from the third speed to the fourth speed. That is, when the shift from the third speed to the fourth speed is started, the C ₃ clutch pressure P _C3 is kept high, but the C ₂ clutch pressure P _C2 is drained. Due to the decrease in the C ₂ clutch pressure P _C2 , the servo control valve 33 first switches to the left side, and the output pressure P _{B1 ′} begins to decrease. Then, when the torque phase ends (when the inertia phase starts), the inertia phase detection valve 70 switches to the left side, so that the hydraulic pressure P _i gradually decreases due to the action of the orifice 74 and the accumulator 75. Due to the decrease in the oil pressure P _i, the lockup switching valve 40 first switches to the right side, the oil pressure in the engagement side oil chamber 6b of the lockup clutch 6 is drained, and the release side oil chamber 6a.
Lubrication pressure is supplied to. As a result, the lockup clutch 6 is temporarily released. The lock-up timing valve 60 holds the left side position by the residual pressure of the hydraulic pressure P _i .

FIG. 7 shows a state in which the shift to the fourth speed is completed. That is, when the oil pressure P _i further decreases from the state of FIG. 6, the lockup timing valve 60 is switched to the right by the spring 64. As a result, the C ₃ clutch pressure P _C3 acts on the right side of the lockup switching valve 40 via the lockup timing valve 60, and the lockup switching valve 4
Switch 0 to the left. As a result, the lubricating pressure is supplied to the engagement side oil chamber 6b of the lockup clutch 6 via the lockup pressure control valve 50, and the lockup clutch 6 is reengaged. Period from release of lock-up clutch 6 to re-engagement (lock-up OFF period)
Can be freely set by the orifice 74, the accumulator 75, and the spring 64 of the lockup timing valve 60.

FIG. 8 is a time change diagram of the engine speed and the vehicle acceleration during the transition from the third speed to the fourth speed when the above hydraulic circuit is used. As shown, the inertia phase detection valve 70 detects the end point of the torque phase,
Lock-up clutch 6 at the start of inertia phase
As the engine is released, it is possible to reliably prevent the engine from blowing up.

5 to 7, the operation of temporarily releasing the lockup clutch 6 at the time of shifting from the 3rd speed to the 4th speed has been described, but the lockup clutch 6 remains at the time of shifting from the 4th speed to the 3rd speed. It will be released temporarily. In this case, the lockup timing valve 60 first switches to the left side as the output pressure P _{B1 ′} of the servo control valve 33 increases,
The signal pressure acting on the right end side of the lockup switching valve 40 is drained. As a result, the lockup switching valve 40
Switches to the right side, drains the oil pressure in the engagement side oil chamber 6b of the lockup clutch 6, and releases the oil pressure in the release side oil chamber 6b.
Supply lubrication pressure to a. As a result, the lockup clutch 6 is temporarily released. After that, when the output pressure P _{B1 ′} further increases, the lockup switching valve 40 switches to the left side and the lockup clutch 6 is re-engaged.

In the above embodiments, the automatic transmission having the fourth forward speed has been described, but it goes without saying that the present invention can be applied to the automatic transmission having the third forward speed or the fifth speed or more. Although the lockup clutch is operated in the 3rd and 4th speeds, the present invention is not limited to this.
It may be operated at a higher speed. The same applies to automatic transmissions having other gears.
Not only the hydraulic pressure P _{B1 ′} of the disengagement side oil chamber B ₁ ′ of the servo piston 13 but also the C ₁ clutch pressure is used as the hydraulic pressure generated at the n-th speed acting on one end side of the inertia phase detection valve and drained at the n + 1-th speed. It is also possible to use a C ₂ clutch. As the hydraulic pressure corresponding to the engine load acting on the other end of the inertia phase detecting valve, the line pressure P _L
However, it is possible to use other hydraulic pressure such as the throttle pressure P _T.

[0032]

As is apparent from the above description, according to the present invention, the inertia phase is detected by the inertia phase detection valve during the shift transition from the nth speed to the n + 1th speed, and the inertia phase is locked in response to the start of the inertia phase. Since the up-switching valve is temporarily switched to the disengagement side, it is possible to prevent the engine from rising due to the release of the lock-up clutch at the time of the shift transition, and it is possible to eliminate the driver's discomfort. Further, in the present invention, since the inertia phase is mechanically detected by the inertia phase detection valve, it is not necessary to use an expensive electronic control device or the like, and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a time chart showing changes in engine speed and vehicle acceleration during conventional gear shifting from third speed to fourth speed.

FIG. 2 is a schematic mechanism diagram of an example of an automatic transmission to which the present invention is applied.

FIG. 3 is a shift diagram of the automatic transmission of FIG.

4 is an overall circuit diagram of a hydraulic control device for the automatic transmission of FIG.

FIG. 5 is a hydraulic circuit diagram of the lockup clutch according to the present invention at the third speed.

FIG. 6 is a hydraulic circuit diagram of the lock-up clutch according to the present invention during a speed change from 3rd speed to 4th speed.

FIG. 7 is a hydraulic circuit diagram of the lockup clutch according to the present invention at the 4th speed.

FIG. 8 is a time chart showing changes in engine speed and vehicle acceleration during a shift from the third speed to the fourth speed according to the present invention.

[Explanation of symbols]

 1 engine 2 torque converter 6 lockup clutch 6a release side oil chamber 6b engagement side oil chamber 33 servo control valve 40 lockup switching valve 50 lockup pressure control valve 60 lockup timing valve 70 inertia phase detection valve

Claims (1)

[Claims] Claim: What is claimed is: 1. An engine power is input to an automatic transmission through a torque converter, and the torque converter is provided with a lock-up clutch that mechanically connects the input side and the output side of the automatic transmission. In the case where the lockup clutch is engaged at the (n + 1) th speed, a lockup switching valve that selectively supplies hydraulic pressure to the engagement-side oil chamber and the release-side oil chamber of the lockup clutch, and the hydraulic pressure is generated at one end side at the nth speed. The hydraulic pressure drained at the (n + 1) th speed is applied, the hydraulic pressure according to the engine load is applied to the other end side, and the inertia phase detection valve for detecting the inertia phase based on the correlation of these hydraulic pressures is provided. By applying pressure to one end of the lockup switching valve, the lockup switching valve locks up in response to the start of the inertia phase. Control system for an automatic transmission for a lock-up clutch, characterized by being configured so as to temporarily scavenge the oil pressure of the engagement side oil chamber of the latch.