JPH0429671A - Speed change control device for automatic transmission - Google Patents

Abstract

PURPOSE:To prevent a function of an accumulator from deteriorating, even when a speed change condition is provided so that supply of pressure oil is performed just after a predetermined speed change, by providing such constitution as boosting a back pressure of the accumulator in the speed change final period at the time of the predetermined speed change in which operating oil is discharged from a friction element. CONSTITUTION:In a control unit 50, a speed change is controlled by outputting a signal of controlling a shift valve or the like in accordance with an operational condition, checked by throttle opening, turbine speed, etc., based on a speed change pattern with a shift line predetermined. A back pressure of an accumulator 31 is controlled by controlling a line pressure of a hydraulic control circuit 30 and further controlling a back pressure regulating means 33. Especially a back pressure control means 51, contained in this control unit 50, is constituted such that the back pressure of the accumulator 31 is boosted in the speed change final period at the time of predetermined speed change in which operating oil is discharged from a friction element through an operating oil supplying oil line L4.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gear change control device for an automatic transmission in which an accumulator is provided in a hydraulic control circuit.

C. Prior Art] In general, an automatic transmission consists of a torque converter, a multi-stage transmission mechanism connected to the output side of the torque converter, and friction between various clutches, brakes, etc. that are operated by hydraulic pressure to switch the power transmission path of the multi-stage transmission mechanism. element, and a hydraulic l111m circuit that supplies and discharges hydraulic pressure to and from these friction elements. Then, the hydraulic pressure supply is controlled according to the operating conditions, etc., and the jl! By engaging and releasing the friction elements, the gears are automatically shifted.

In such an automatic transmission, for example, Japanese Patent Application Laid-Open No. 61-1
As shown in Japanese Patent No. 49657, the hydraulic control circuit includes:
Hydraulic pressure control 11f, which is equipped with an accumulator connected to an oil passage for supplying working pressure to a specific*m element, and back pressure adjusting means such as a duty valve that controls the back pressure of this accumulator in accordance with a control signal, is a known hydraulic pressure control 11f. It is being The accumulator is configured to gradually increase the friction element operating pressure for a certain period of time by introducing a portion of the hydraulic oil into the pressure accumulation chamber during a gear shift to switch to a state in which hydraulic pressure is supplied to the friction element. This provides a buffering effect by providing suitable operating pressure change characteristics. In this case, by controlling the back pressure of the accumulator, the operating pressure change characteristic of the accumulator is adjusted in accordance with the operating state and the like.

[Problem to be solved by the invention]

In such a hydraulic system, normally an accumulator provides a buffering effect when the friction element is engaged, but the release is quickly performed during a predetermined speed change when the hydraulic fluid is discharged from the friction element and the friction element is released. is required to be carried out. However, when the hydraulic oil is discharged from the friction element through the working pressure supply oil passage, the hydraulic oil stored in the accumulator's pressure storage chamber also flows into the working pressure supply oil passage, and if the flow rate is large, it will be discharged from the friction element. Discharge of the hydraulic oil is obstructed, causing a delay in the reduction of the operating pressure of the friction element.

Therefore, in order to improve the responsiveness of the motor wI element release.

The flow rate when the hydraulic oil flows out from the pressure accumulation chamber is set at t! to the oil passage that communicates the working pressure supply oil passage with the pressure accumulation chamber of the accumulator. 11 is provided with a restrictor to slow the flow of hydraulic oil from the pressure accumulation chamber during the above-mentioned predetermined speed change, thereby promoting discharge of hydraulic oil from the friction element until the working pressure becomes sufficiently low. is considered.

However, if a throttle is provided to slow the flow of hydraulic oil from the accumulator's pressure storage chamber in this way, the speed change in which the friction element is re-engaged immediately after the predetermined speed change in which the friction element is released will not be possible. When this is done, the problem arises that the function of the accumulator deteriorates. In other words, during a predetermined speed change when the friction WI element is released, the friction element is quickly released due to the action of the throttle, but even after that, the state in which hydraulic oil remains in the pressure accumulation chamber of the friction element continues for a relatively long time. However, if during that time the gear changes to a state in which the frictional elements are to be engaged again and hydraulic pressure is supplied to the frictional elements, the accumulator becomes substantially insufficient in capacity and the 111 function cannot be obtained sufficiently.

In view of these circumstances, the present invention allows the friction element working pressure to be reduced in a responsive manner during a predetermined speed change in which hydraulic oil is discharged from the friction element through the working pressure supply oil passage to which the accumulator is connected. An automatic transmission that can prevent deterioration of the function of the accumulator when a shift is performed in which hydraulic pressure is supplied to the friction element again immediately after such a predetermined shift. The object of the present invention is to provide a speed change control device.

[Means for Solving the Problems] In order to achieve the above objects, the present invention provides a hydraulic control circuit for a friction element incorporated in a multi-stage transmission mechanism, which is connected to an oil passage for supplying working pressure to the friction element. In an automatic transmission having an accumulator, an oil passage communicating between the working pressure supply oil passage and a pressure accumulation chamber of the accumulator is provided with a restriction for restricting the flow rate when the hydraulic oil flows out from the pressure accumulation chamber, and The apparatus is equipped with back pressure control means for increasing the back pressure of the accumulator at the end of a predetermined shift during which the hydraulic fluid is discharged from the friction element through the hydraulic pressure supply passage.

(Function) According to the above configuration, during a predetermined gear shift in which hydraulic oil is discharged from the friction element through the hydraulic pressure supply passage, the back pressure of the accumulator remains relatively low until the end of the gear shift. Since the hydraulic oil flows out slowly from the pressure storage chamber of the accumulator to the working pressure supply oil passage through the restriction, the working pressure is quickly lowered due to the discharge of the working oil from the friction element. At the end of the shift when the pressure is sufficiently low and the switching of the friction elements is completed, the back pressure is increased, increasing the hydraulic oil pushing force and preventing the hydraulic oil from flowing out from the pressure accumulation chamber. promoted.

〔Example〕

Embodiments of the present invention will be described based on the drawings.

FIG. 1 shows the overall configuration of an embodiment of the present invention. In this figure, automatic transmission AT includes a torque converter 2 and a multi-stage transmission mechanism 10 connected to the output side of the torque converter 2.
and a hydraulic control circuit 30. The multi-stage transmission mechanism 10 includes various friction elements (
A hydraulic control circuit 30 controls the engagement and release of various frictional elements (clutches, brakes).

The hydraulic control circuit 30 includes hydraulic supply/discharge control elements such as various shift valves that switch between supplying and discharging hydraulic pressure to each friction element, a line pressure adjustment element (omitted in FIG. 1), and the like. An accumulator 31 is provided which is connected to the oil passage 4 for supplying working pressure to the Wl rubbing element, and the oil passage communicating between the oil passage L4 for supplying working pressure and the pressure accumulation chamber of the accumulator 31 is provided with an oil passage connected to the oil passage 4 for supplying working pressure to the Wl rubbing element. A baffle 32b is provided to limit the flow rate when oil spills, and a back pressure adjusting means 33 is further provided for the accumulator 31.

Reference numeral 50 denotes a control unit (ECU) for the hydraulic control circuit 30, which is composed of a microcomputer and the like. A signal from a turbine rotation speed sensor 62 that detects the turbine rotation speed of the torque converter 2 corresponding to the rotation speed on the input side of the transmission, and the like are input.

The control unit 50 controls the gear shift by outputting a signal to control the shift valve, etc. according to the operating state determined by the throttle opening, turbine rotation speed, etc., based on a shift pattern with a shift line determined in advance. At the same time, the line pressure of the hydraulic control circuit 30 is controlled, and the back pressure of the accumulator 31 is controlled by controlling the back pressure adjusting means 33. In particular, the back pressure control means 51 included in the control unit 50 controls the back pressure of the accumulator 31 at the end of a predetermined shift period when the working pressure is discharged from the friction element through the working pressure supply oil path L4. It is designed to boost the pressure.

FIG. 2 shows an example of the structure of the torque converter 2 and multi-stage transmission mechanism 10 of an automatic transmission.

In this figure, the torque converter 2 includes a pump 3 fixedly installed in a case connected to the output shaft 1 of the engine,
A turbine 4 is arranged to face the pump 3 and is driven by the pump 3 via hydraulic oil, and a fixed shaft is interposed between the pump 3 and the turbine 4 and connected via a one-way clutch 6. The stator 5 is supported on the stator 7.

Furthermore, this torque converter 2 is provided with a lock-up clutch 8 that directly connects its input side and output side.

The transmission mechanism 10 includes an oil pump driving solid shaft 12 whose base end is fixed to the output shaft 1 of the engine and whose distal end is connected to the oil pump 11. It has a hollow turbine shaft 13 whose base end is connected to the turbine 4 of the torque converter 2, and a Rapigno type planetary gear set M14 is provided on the turbine shaft 13. This planetary gear device 14 includes a small diameter sun gear 15, a large diameter sun gear 16, a long pinion gear 17,
It has a short pinion gear 18 and a ring gear 19. The above small diameter sun gear 15 and large diameter sun gear 16
are loosely fitted to the turbine shaft 13, and a plurality of short pinion gears 18 mesh with the small diameter sun gear 15,
A long pinion gear 17 meshes with the short pinion gear 18 and the large diameter sun gear 16, a ring gear 19 meshes with the long pinion gear 17, and the long pinion gear 17 and the short pinion gear 18 are rotatably supported by a carrier 28. . The following various friction elements are incorporated into this planetary gear device 14.

A forward clutch 20 and a coast clutch 21 are arranged in parallel between the turbine shaft 13 and the small diameter sun gear 15. The forward clutch 20 is connected to the turbine shaft 1 via a first one-way clutch 22.
3 to the small diameter sun gear 15, and the coast clutch 21
3 and small-diameter sun gear 15, mutual power transmission is interrupted. A 2-4 brake 23 is arranged radially outward of the coast clutch 21, and includes a brake drum 23a connected to the large diameter sun gear 16 and a brake band 23b hooked to the brake drum 23a. When this 2-4 brake 23 is engaged, the large diameter sun gear 16 is fixed. On the side of this 2-4 brake 23, the brake drum 2
Reverse clutch 2 for backward running that connects and disconnects power transmission between large-diameter sun gear 16 and turbine shaft 13 via 3a
4 is placed. Also, between the carrier 28 of the planetary device 14 and the case of the transmission mechanism 10, there is a low reverse brake 2 for engaging and disengaging the carrier 28 and the case.
5 is arranged, and a second one-way clutch 26 is arranged in parallel thereto.

A 3-4 clutch 27 is disposed between the carrier 28 and the turbine shaft 13 to connect and disconnect power transmission between them. Also, on the side of this 3-4 clutch 27,
An output gear 29 connected to the ring gear 19 is disposed, and this gear 29 is connected to the output shaft 2.
It is attached to 9a.

This transmission mechanism 10 itself has four forward speeds and one reverse speed, and can obtain the desired speed by appropriately operating the clutches 20, 21, 24, 27 and brakes 23, 25. It is possible. Here, Table 1 shows the relationship between each gear stage and the operation of the clutch and brake.

FIG. 3 shows a part of the hydraulic control circuit 3o.
This figure shows a hydraulic supply/discharge system for the 3-4 clutch 27 that is engaged in third and fourth speeds, a back pressure adjustment system for the accumulator 31 connected thereto, and a line pressure adjustment system.

The 3-4 clutch 27 is engaged when hydraulic pressure is supplied to its actuator, and released when the hydraulic pressure is discharged.

In this hydraulic control circuit 30, the pressure (line pressure) of the hydraulic oil discharged from the oil pump 13 to the main line L1 is regulated by a pressure regulator valve 34, which is connected to the first duty solenoid valve 35. controlled by That is, the hydraulic pressure of the hydraulic oil reduced to a predetermined pressure by the reducing valve 36 is applied to the first duty solenoid valve 3.
In other words, the hydraulic pressure is controlled by controlling the duty by adjusting the opening/closing time ratio of the first duty T isolenoid valve 35 and controlling the drain amount by 1IjllllII. This oil pressure is applied as a pilot pressure to the boat 34a at one end of the brake regulator valve 34 via the line L2, so that the line pressure is adjusted in accordance with this pilot pressure. The line pressure is controlled by operating the duty solenoid valve 35 in response to a duty signal from the control unit 50.

The line pressure of the main line L1 is guided to the shift valve 37 through a line L3 when the vehicle is in the forward travel range via a manual valve for range selection (not shown).

Between this shift valve 37 and 34 clutch 27,
3-4 which is an oil passage for supplying operating pressure to the 3-4 clutch 27
A clutch line L4 is provided, and a one-way orifice 38 is interposed in this 3-4 clutch line L4.

Pilot port on one end side of the shift valve 37)-3
A pilot line L5 is connected to 7a, and a solenoid valve 39 is connected to this pilot line L5. This solenoid valve 39 is turned ON and OFF by a control signal from a control unit 50.

In 1st and 2nd gear, solenoid valve 3
9 is turned ON, the pilot pressure is drained, the spool 37b is positioned on the right side in the figure, and the 3-4 clutch line L4 is in communication with the drain boat. Also, in 3rd and 4th gear, the solenoid valve 39 is turned off.
By setting it as FF, a predetermined pilot pressure acts on the pilot boat 37a, and the spool 37b of the shift valve 37 is located on the left side in the figure, and the 3-4 clutch line L
4 is in communication with line 13, and hydraulic pressure is supplied to 3-4 clutch 27 via one-way orifice 38.

An accumulator 31 is connected to a location between the one-way orifice 38 of the 3-4 clutch line L4 and the 3-4 clutch 27 via a one-way orifice 32. This accumulator 31 is on top! ! Il! 3
- Hydraulic pressure is supplied to 4 clutch 27, and 3-4 clutch 2
Adjust the increase in 3-4 clutch pressure (pressure applied to the actuator of 3-4 clutch 27) during 2-3 gear shifting (when shifting from 2nd to 3rd gear) when 7 is engaged! It is used for operation, and includes a piston 31b that expands and contracts the pressure storage chamber 31a.
is biased in the direction of shrinking. Furthermore, the one-way orifice 32 allows the hydraulic fluid to be quickly discharged from the 3-4 clutch 27 during a 3-2 gear shift (when shifting from 3rd gear to 2nd gear) when the 3-4 clutch 27 is released. In order to ensure
When hydraulic oil flows into the accumulator 31, it passes through a check valve 32a, and when it flows out, it passes through a throttle 32b.

The back pressure of the accumulator 31 is controlled by a back pressure adjusting means 33 comprised of an accumulator control valve 41, a second duty solenoid valve 42, and the like. That is, the accumulator control valve 41 is connected to a spool 41a, a boat 41b that communicates with a line [6 from which hydraulic pressure is introduced from a main line, etc.], and a boat 41c that communicates with a back pressure introduction boat 310 of the accumulator 31 via a back pressure supply line L7. , a drain port 41d, a pilot boat 41e communicating with the pilot line L8, and the like. By operating the spool 41a in accordance with the pilot pressure introduced into the pilot port 41e and the pressure on the boat 41C side, pressure corresponding to the pilot pressure is sent to the back pressure supply line L7. Hydraulic pressure adjusted to a constant pressure by a reducing valve 43 is sent to this accumulator control valve 41 to a pilot line L8, and a second duty solenoid valve 42 connected to this pilot line L8 controls this pressure. The pilot pressure guided from the line L8 to the pilot port 41e is duty-controlled. In this way, the back pressure of the accumulator 31 is adjusted independently of the line pressure, and the second duty solenoid valve 42 is actuated by the duty signal from the control unit 50, thereby adjusting the back pressure. It's about to be controlled.

Although Fig. 3 shows a simplified hydraulic supply/discharge system for the 3-4 clutch 27, there is a circuit between the 3-4 clutch 27 and the shift valve 37 for adjusting the engagement timing of the 3-4 clutch. A bypass valve etc. are also provided, and the shift valve 37 is connected to the illustrated oil passages L3 and L4.
The communication state of other oil passages can also be changed. In addition to the illustrated shift valve (2-3 shift valve) 37, the hydraulic control circuit 30 is provided with a 12-shift valve and a 3-4 shift valve, each of which is controlled by a solenoid valve. , supply and discharge of hydraulic pressure to the apply boat and release boat of the actuator of the 2-4 brake 23 (consisting of a servo piston having an apply boat and a release boat), supply and discharge of hydraulic pressure to the coast clutch 21, etc. It is to be carried out accordingly. Also, in Fig. 3, 3-4 clutch line L4
through the one-way orifice 32 to the accumulator
2-37 accumulator) 31 is connected, but an accumulator (1-2 accumulator) is also connected to the oil passage for supplying working pressure to other friction elements, for example, the servo apply line leading to the apply boat of the 2-4 brake. The connection may be made through a one-way orifice similar to the illustrated one-way orifice 32, and the back pressure may be adjusted by the back pressure adjusting means 33. When adjusting the back pressure of a plurality of accumulators in this way, the back pressure supply line [7 may be branched and connected to the back pressure introduction boat of each accumulator.

FIG. 4 is a flowchart showing a specific example of line pressure control and accumulator back pressure control by the control unit 50.

In this flowchart, first, in step S1, the throttle opening degree and the turbine rotation speed are measured by the sensors 61 and 6, respectively.
Enter from step 2. Subsequently, in step S2, it is checked whether there is a predetermined speed change signal indicating a predetermined speed change in which the operating pressure is discharged from the friction element. In other words, as shown in FIG. 3, the 2-3 accumulator 31
If it is connected to the 3-4 clutch line L4 via 2, check whether there is a 3-2 gear shift signal,
In addition, for example, if a 1-2 accumulator is connected to a servo apply line via a one-way orifice, it is checked whether there is a shift signal for a 3-2 shift or a 2-1 shift.

When the determination in step S2 is No, in step S3, a line pressure control value is calculated according to the operating state, etc., for example, the line pressure according to the throttle opening and turbine rotation speed is determined from a map, and the line pressure is calculated in accordance with the throttle opening and turbine rotation speed. The control value (duty rate) of the first duty solenoid valve 35 is calculated, and then the process returns.

If the determination in step S2 is YES, the type of shift such as 3-2 shifting or 2-1 shifting is determined in step S4, and then in step S5, the back pressure of the accumulator is set to the basic control value. do. This basic control value is determined from a map depending on, for example, the type of gear change, throttle opening, and turbine rotational speed, and is set to a relatively low value. By setting this step S5, a duty signal corresponding to the above-mentioned basic control value is given to the second duty solenoid valve 42.

Next, in step S6, it is determined whether or not the shift has been completed based on whether the turbine rotation speed n has reached the shift end rotation speed rz (see FIG. 5). The shift end rotation speed n1 is determined by the turbine rotation speed and the type of shift (gear ratio) before the rotation speed change due to shift starts. The back pressure control state set in step S5 is maintained until the end of the shift, but at the end of the shift, a back pressure correction value ΔP is calculated in accordance with the operating state etc. in step S7. In S8, the duty rate of the second duty solenoid valve 42 is changed so as to increase the back pressure of the accumulator by the upper 2 correction value ΔP. Further, in step S9, a timer t is started, and in step 5111
Then, it is checked whether the elapsed time from the end of the shift measured by the timer t has delayed by a certain time Ta, and within the certain time Ta, the boosted pressure state by the process of step S8 is maintained, and when the certain time Ta has elapsed, the pressure is increased. After canceling the increase in back pressure in step S11, the process returns.

FIG. 5 is a time chart showing the changes in the clutch pressure of the 3-4 clutch 27, the back pressure of the accumulator 31, and the turbine speed during 32-speed shifting when using the device of this embodiment. The operation of the apparatus of this embodiment will be explained based on the following.

In this figure, to varies from 3 to 2 depending on changes in operating conditions.
Indicates the time point at which a shift signal is applied to start shifting. At this shift start time point to, the solenoid valve 39 is turned on, and the corresponding shift valve 37 is operated to connect the servo apply line L4 to the drain boat. As a result, the hydraulic oil is discharged from the 3-4 clutch 27, which had been in the engaged state until then, and the clutch pressure decreases to a predetermined pressure pc.
The 3-4 clutch 27 starts to be released from the time t1 when the turbine rotation speed reaches the inflection point rotation speed no.
The speed increases from 0 to 01 at the end of the shift.

By the way, the accumulator 31 is in the pressure storage chamber 31 in the third gear.
The piston 31b is pushed by the hydraulic pressure introduced into the pressure chamber 31a, and from this state, 3-2
During gear shifting, the hydraulic oil in the pressure accumulation chamber 31a flows out to the 3-4 clutch line L4.

In this case, if the flow rate is not restricted by the throttle 32b, a large amount of hydraulic fluid will flow from the pressure accumulation chamber 31a to the 3-4 clutch line L4 during gear shifting, and the discharge of hydraulic fluid from the 3-4 clutch 27 will become an obstruction. As a result, the reduction in clutch pressure (release of the 3-4 clutch 27) is delayed, as shown by a two-dot chain line 71' in FIG. On the other hand, the aperture 3
2b is provided, and the accumulator 3 during gear change.
When the back pressure of No. 1 is made relatively low, the flow rate of hydraulic oil flowing from the pressure storage chamber 318 of the accumulator 31 through the clutch line 32b to the 3-4 clutch line L4 is reduced, so that the solid line in FIG. 71, the 3-4 clutch pressure quickly decreases to a sufficiently low pressure, and the 3-4 clutch 2
7 is released with good responsiveness.

However, if the flow rate of the hydraulic oil flowing out from the pressure accumulation chamber 318 remains small even after the shift is completed, the state in which hydraulic oil remains in the pressure accumulation chamber 31a of the accumulator 31 will continue, although the clutch pressure is sufficiently low. (Dotted line 72' in FIG. 5). On the other hand, in the device of this embodiment, at the end of the shift, t2, when the turbine rotation speed reaches the shift end rotation speed n1.
However, by increasing the back pressure of the accumulator 31 for a predetermined time Ta, the pushing force applied to the hydraulic fluid via the piston 31b is strengthened. Therefore, after the clutch pressure becomes sufficiently low and the 3-4 clutch 27 is released, the amount of hydraulic oil flowing out from the pressure accumulating chamber 31a is increased, and the flow of hydraulic oil is completed and the accumulator 31 is in the initial state. The FR interval until returning to v1 will be shortened (5th
In fact [172] in the figure, even when the 23rd gear shift is performed a short time after the 3-2 gear shift is performed, the accumulator 3
1 exhibits favorable effects.

〔Effect of the invention〕

As described above, the present invention provides an oil passage that communicates the working pressure supply oil passage with the pressure accumulation chamber of the accumulator, and includes a restriction for restricting the flow rate when the hydraulic oil flows out from the pressure accumulation chamber, and Since the back pressure of the accumulator is increased at the end of the predetermined shift when the hydraulic fluid is discharged from the element, the back pressure of the accumulator is increased during the predetermined shift.
During gear shifting, l! due to the action of the aperture mentioned above. i! It is possible to reduce the working pressure by discharging the hydraulic oil from the friction element with good response, and at the end of the shift, the back pressure increases to promote the outflow of the hydraulic oil from the pressure accumulator, thereby increasing the M of the predetermined shift. This can prevent deterioration of the function of the accumulator even if a gear change state occurs in which oil pressure is supplied to the friction element later.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the overall configuration of an embodiment of the present invention, FIG. 2 is a schematic diagram showing the outline of the structure of an automatic transmission, and FIG. 3 is a hydraulic circuit diagram of the main parts of the hydraulic control circuit. FIG. 4 is a flow chart showing an example of control of line pressure and accumulator back pressure, and FIG. 5 is a time chart showing changes in clutch pressure, accumulator back pressure, and turbine rotational speed during 3-2 gear shifting. AT... automatic transmission, 10... transmission mechanism, 30...
・Hydraulic control circuit, 31...accumulator, 32...
・One-way clutch, 32b...Aperture, 33...
Back pressure adjustment means, 5o... control unit, 51
... Back pressure adjustment means. Patent applicant Mazda Co., Ltd. Agent
Person Patent Attorney Kiyoaki Kotani
Patent Attorney Chief 1) Masamukai Patent Attorney
Takao Ito Figure Figure Figure t. fishing

Claims (1)

[Claims] 1. In an automatic transmission having an accumulator connected to an oil passage for supplying working pressure to the friction element in a hydraulic control circuit for a friction element incorporated in a multi-stage transmission mechanism, the oil passage for supplying working pressure and the accumulator are connected to the oil passage for supplying working pressure to the friction element. A predetermined speed change in which a restriction is provided in the oil passage communicating with the pressure accumulation chamber to limit the flow rate when the hydraulic oil flows out from the pressure accumulation chamber, and the hydraulic oil is discharged from the friction element through the oil passage for supplying working pressure. 1. A shift control device for an automatic transmission, comprising back pressure control means for increasing the back pressure of the accumulator at the end of a shift.