Method of bleeding an accumulator of a variable speed transmission

Abstract

The present disclosure provides a "method of bleeding an accumulator of a transmission." A transmission includes an accumulator for holding one of a plurality of shift elements in an engaged state when the engine is off. The transmission also includes a hydraulic park system that disengages the park range in response to engagement of two shift elements. In some cases, bleeding the accumulator in an uncontrolled manner can cause the park range to be inadvertently disengaged in the presence of a failed valve. To avoid this, the accumulator is bled in a controlled manner. Fluid is first transferred from the accumulator to a shift element apply chamber. The fluid is then drained from the shift element apply chamber.

Description

Technical Field

[0001] This disclosure relates to the field of hydraulic control systems for automatic transmissions in motor vehicles. More specifically, this disclosure relates to a method for safely releasing a hydraulic accumulator without unintentionally disengaging it from the parking mechanism. Background Technology

[0002] Many vehicles operate at a wide range of speeds, including both forward and reverse movement. However, some types of engines can only operate effectively within a narrow range of engine speeds. Therefore, transmissions capable of efficiently transmitting power at various gear ratios are often employed. When the vehicle is at low speeds, the transmission typically operates at a high gear ratio, causing it to multiply engine torque to increase acceleration. At high speeds, operating the transmission at a low gear ratio allows for engine speeds associated with quiet, fuel-efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven via the engine crankshaft, and an output shaft that typically drives the wheels via a differential assembly that allows the left and right wheels to rotate at slightly different speeds when the vehicle is cornering.

[0003] Discrete ratio transmissions can transmit power via various power flow paths, each associated with a different gear ratio. Specific power flow paths are established by engaging specific shift elements (such as clutches or brakes). Shifting from one gear ratio to another involves changing which shift elements are engaged. In many transmissions, the torque capacity of each shift element is controlled by directing fluid to it at controlled pressure. The controller adjusts the pressure by sending electrical signals to the valve body.

[0004] When parked, the transmission can engage a parking pawl, which holds the transmission shaft stationary to prevent the vehicle from rolling. The parking system is designed to remain engaged without consuming any power during extended periods of unattended operation. Typically, the parking pawl engages in response to the driver selecting the Park gear and disengages in response to the driver selecting any other gear, such as Reverse, Neutral, Drive, or a low gear. However, there are situations where the transmission may override the driver's selection.

[0005] To save fuel, some vehicles are programmed to automatically shut off the engine when the vehicle stops in drive, such as while waiting at a red traffic light. The engine then automatically restarts when the driver lifts his or her foot off the brake pedal. It is important that the vehicle begins to accelerate as soon as the driver presses the accelerator pedal. To achieve this, it may be necessary to keep one or more shift elements engaged. Since engine-driven pumps do not provide pressurized fluid when the engine is off, various pressurized fluid sources can be included, such as electrically driven pumps or hydraulic accumulators. Summary of the Invention

[0006] A transmission includes an accumulator valve, a first shift valve, and a controller. The accumulator valve is configured to selectively fluidly connect an accumulator to a line pressure circuit. The first shift valve is configured to alternately fluidly connect a first apply chamber to the line pressure circuit and an outlet. The controller is programmed to respond to a request to discharge the accumulator by issuing a sequence of valve commands. The sequence includes commanding the first shift valve to fluidly connect the first apply chamber to the line pressure circuit, commanding the accumulator valve to fluidly connect the accumulator to the line pressure circuit to partially fill the first apply chamber, and commanding the first shift valve to fluidly connect the first apply chamber to the outlet. The controller may also be configured to measure the pressure in a first clutch apply chamber and repeat the valve command sequence until the pressure no longer increases. The transmission may further include: a second shift valve configured to alternately fluidly connect a second apply chamber to the line pressure circuit and the outlet; and a parking valve configured to disengage a parking mechanism in response to an increase in pressure in the second apply chamber. The controller can also be programmed to, in response to the parking valve not moving, command the accumulator valve to connect the accumulator fluid to the line pressure circuit until the accumulator is released, without operating either shift valve. The transmission may also include a pump configured to supply pressurized fluid to a pump outlet circuit, wherein the pressure in the pump outlet circuit biases the parking valve to prevent disengagement from the parking mechanism.

[0007] A transmission includes an accumulator, a first shift element application circuit, and a controller. The accumulator is selectively fluidly connected to a line pressure circuit. The first shift element application circuit is alternately fluidly connected to the line pressure circuit and an outlet. The controller is programmed to repeatedly discharge fluid from the accumulator by partially filling a first application chamber via the first shift element application circuit and subsequently discharging the first application chamber until no increase in clutch pressure is detected. The controller may also be programmed to partially discharge the accumulator in response to an accumulator release request, provided that the first application circuit is fluidly disconnected from the line pressure circuit; and to fully discharge the accumulator in response to a parking valve not moving, provided that the first application circuit is fluidly disconnected from the line pressure circuit. The transmission may further include a second shift element application circuit, which is alternately fluidly connected to the line pressure circuit and the outlet; and a parking valve configured to disengage a parking mechanism in response to an increase in pressure in the second application circuit.

[0008] A method for venting fluid from an accumulator in a transmission includes repeating a series of steps until no increase in clutch pressure is detected. The series of steps includes operating a shift valve to connect the shift element application chamber fluid to a line pressure circuit; opening an accumulator control valve to connect the accumulator fluid to the line pressure circuit to partially fill the application chamber; and operating the shift valve to drain fluid from the partially filled application chamber. The method may further include opening the accumulator valve in response to an accumulator venting request, provided that the application chamber is disconnected from the line pressure circuit fluid, and keeping the accumulator valve open until the accumulator is completely vented, provided that the parking valve has not moved. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of a transmission system.

[0010] Figure 2 It is used for Figure 1 A schematic diagram of the fluid supply subsystem of the hydraulic control system of the transmission.

[0011] Figure 3 It is used for Figure 1 A schematic diagram of the shift element control subsystem of the hydraulic control system of a transmission.

[0012] Figure 4 It is used for Figure 1 A schematic diagram of the parking subsystem of the hydraulic control system of the transmission.

[0013] Figure 5 In the position corresponding to engaging the parking gear Figure 4 The cross-section of the parking valve assembly of the parking subsystem.

[0014] Figure 6 In the position corresponding to disengaging from parking gear Figure 4 The cross-section of the parking valve assembly of the parking subsystem.

[0015] Figure 7 It is through Figure 3 Filtering subsystem Figure 1 The energy storage device did not accidentally detach. Figures 4 to 6 A flowchart illustrating the risks of parking systems. Detailed Implementation

[0016] Embodiments of this disclosure are described herein. However, it should be understood that the disclosed embodiments are merely examples, and other embodiments may take various and alternative forms. The drawings are not necessarily drawn to scale; some features may be exaggerated or minimized to show detail of particular components. Therefore, the specific structural and functional details disclosed herein should not be construed as limiting, but merely as a representative basis for teaching those skilled in the art to employ the invention in different ways. As those skilled in the art will understand, various features shown and described with reference to any of the drawings can be combined with features shown in one or more other drawings to produce embodiments not explicitly shown or described. Combinations of features shown provide representative embodiments for typical applications. However, various combinations and modifications of features consistent with the teachings of this disclosure may be desired for a particular application or implementation.

[0017] Figure 1 A vehicle transmission is schematically illustrated. Bold solid lines represent mechanical power flow connections. Thin solid lines represent hydraulic fluid flow. Dashed lines represent the flow of information signals. Power, typically from the crankshaft of an internal combustion engine, is supplied at the input shaft 10. A hydraulic coupling 12 includes a pump impeller drivably connected to the input shaft 10 and a turbine drivably connected to a turbine shaft 14. Power is transferred from the pump impeller to the turbine via moving fluid whenever the pump impeller rotates faster than the turbine. The hydraulic coupling 12 may be a torque converter, which also includes a stator that redirects the fluid when the pump impeller rotates much faster than the turbine, such that the turbine torque is multiple times that of the pump impeller torque. The transmission 16 includes gear drive elements and shift elements configured to establish various power flow paths between the turbine shaft 14 and the output shaft 18. Each power flow path can be established by engaging a specified subset of the shift elements. At low vehicle speeds, a power flow path providing torque multiplication and speed reduction can be established between the turbine shaft and the output shaft to optimize vehicle performance. At higher vehicle speeds, a power flow path that provides a significant speed boost can be established to minimize fuel consumption.

[0018] Most shift elements within the transmission 16 are engaged by supplying hydraulic fluid at increased pressure to the clutch application chamber. (The transmission 16 may also include passively engaged one-way clutches or electrically actuated elements.) Each shift element may include a clutch assembly having a friction plate splined to a component, the friction plate being interleaved with a partition splined to a different component. Pressurized fluid pushes a piston to compress the clutch assembly, causing friction between the friction plate and the partition to engage the component. The torque capacity of each shift element varies proportionally to the change in fluid pressure. A pump 20, driven by the input shaft 10, draws fluid from the oil pan 22 and delivers it at increased pressure to the valve body 24. The valve body 24 delivers fluid to the clutch application chamber at a pressure controlled according to a signal from the powertrain controller 26. In addition to the fluid supplied to the clutch application chamber, the valve body also supplies fluid for lubrication and fluid to the torque converter 12. The fluid is eventually discharged from the transmission 16 back to the oil pan 22 at ambient pressure.

[0019] To reduce energy consumption, the internal combustion engine can be shut off when the vehicle is stopped (such as while waiting at a red light). The engine is then restarted when the driver releases the brake pedal. To ensure the powertrain is ready to transmit power upon engine startup, it is desirable to keep some shifting elements engaged when the engine is off. To achieve this, pressurized fluid is allowed to flow into the accumulator 28 while the engine is running, and subsequently, when the engine is off, the pressurized fluid returns to the valve body 24.

[0020] Figure 2 A portion of the valve body 24 constituting the fluid supply subsystem is schematically shown. Similar fluid supply subsystems are discussed in U.S. Patent Application Publications 2013 / 0014498 and 2013 / 0017112, which are incorporated herein by reference in their entirety. A pump 20, driven by the transmission input shaft, draws fluid from the oil pan 22 and delivers the fluid to the pump outlet circuit 32. Pump 20 is a positive displacement pump. Ignoring leakage, a positive displacement pump delivers a certain amount of fluid per revolution of the pump shaft, regardless of the relative pressure at the pump inlet and outlet. The amount of fluid delivered per revolution is called the pump displacement. The displacement of pump 20 varies within predetermined limits based on the pressure in the displacement reduction circuit 34. The torque required to rotate the pump shaft increases with increasing pressure at the pump outlet relative to the inlet pressure and also increases proportionally to the pump displacement.

[0021] During normal operation, the backflow shut-off valve 36 is open, allowing fluid to flow freely from the pump outlet circuit 32 to the line pressure circuit 38, with the pressures in both circuits being substantially equal. The controller 26 adjusts the pressures in these two circuits by sending commands to the line pressure variable force solenoid (VFS) 40. Fluid flows from the pump outlet circuit 32 through orifice 41, through the valve opening in the line pressure VFS 40, and then into the LP control circuit 46. The pressure drop from the pump outlet circuit 32 to the LP control circuit 46 varies depending on the size of the opening in the line pressure VFS 40. The size of the opening in the line pressure VFS 40 varies based on the movement of the slide valve. Current from the controller 26 generates a magnetic force on the slide valve that tends to widen the opening. The fluid in the LP control circuit 46 acts on a certain area of ​​the slide valve to generate a force that tends to decrease the size of the opening. An equilibrium is reached where the pressure in the LP control circuit 46 is proportional to the current.

[0022] The main regulating valve 48 adjusts the displacement of pump 20 to maintain the pressure in pump outlet circuit 32 in proportion to the pressure in LP control circuit 46. The pressure in LP control circuit 46 exerts a force on the spool of the main regulating valve 48. The pressure in pump outlet circuit 32 exerts a force in the opposite direction on the spool. When the pressure in pump outlet circuit 32 exceeds the pressure in LP control circuit 46, the spool moves to allow flow from pump outlet circuit 32 to displacement reduction circuit 34. The pressure in circuit 34 causes a decrease in flow velocity from pump 20 to pump outlet circuit 32. The components supplied by pump outlet circuit 32 and line pressure circuit 38 establish a relationship between pressure and flow velocity in these circuits. Therefore, the decrease in flow velocity causes a decrease in pressure in pump outlet circuit 32 until equilibrium is reached.

[0023] When the vehicle stops, such as when waiting at a traffic light, the powertrain controller 26 can shut off the engine to save fuel. When the driver demands torque again by releasing the brake and pressing the accelerator pedal, the controller restarts the engine. To ensure a quick response after the engine restarts, it is important to keep some clutches engaged. Fluid flow to these clutches is provided by an accumulator 28, which supplies line pressure circuit 38 via accumulator control valve 50. When the engine is running and the pressure in the line pressure circuit exceeds the pressure in the accumulator, the controller 26 opens valve 50 to charge the accumulator. In some embodiments, accumulator charging can be passively achieved via a check valve connected in parallel with valve 50. During engine-off periods, the controller 26 pressurizes line pressure circuit 38 by opening valve 50. During these engine-off periods, the controller 26 also sets the current to line pressure VFS 40 to an intermediate level, which keeps the pressure in LP control circuit 46 at an intermediate level. In response to this reduction in the LP control pressure, the spool of the backflow shut-off valve 36 moves to a position isolated from the pump outlet circuit 32, which reduces the number of components that must be supplied by the accumulator. In the case described below, the controller 26 can set the current to the line pressure VFS 40 to a low level, which moves the spool of the backflow shut-off valve 36 to a position isolated from the pump outlet circuit 32 and fluidly connected to the discharge circuit. In this case, the pressure in the line pressure circuit 38 rapidly drops to ambient pressure.

[0024] Figure 3A portion of the clutch control subsystem is schematically shown. The clutch application chamber of the first shift element 60 is supplied from the pump-out circuit 32 via VFS valve 62. As a result, shift element 60 can only be used when the engine is running. To engage the shift element, the controller commands valve 62 to set the pressure of the clutch application chamber to a value between zero and the pressure in the pump-out circuit 32. The controller adjusts the pressure by regulating the current to the solenoid in valve 62. When the current is set to zero, valve 62 isolates the clutch application chamber from the pump-out circuit 32 and connects the clutch application chamber 60 to the oil pan 22, thereby allowing fluid to escape to release the clutch. Although a single valve is shown for simplicity, the system may also include a lock-up valve or other auxiliary valves. Similarly, the clutch application chambers of the second shift element 64 and the third shift element 66 are supplied from the line pressure circuit 38 via VFS valves 68 and 69, respectively. As a result, shift elements 64 and 66 can be used even when the engine is not running. To engage clutch 2, the controller commands valve 68 to set the pressure in clutch application chamber 64 to a value between zero and the pressure in pressure circuit 38. The controller adjusts the pressure by regulating the current to the solenoid in valve 68. When the current is set to zero, valve 68 isolates clutch application chamber 64 from pump-out circuit 32 and connects clutch application chamber 64 to oil pan 22, thereby allowing fluid to escape to release the clutch. Similarly, additional shifting elements are controlled, some supplied by line pressure circuit 38 and some by pump-out circuit 32.

[0025] Figure 4 The parking control subsystem of valve body 24 is schematically shown. The spool of parking valve 70 is mechanically connected to the parking mechanism, such that movement of the spool in one direction engages the parking mechanism, and movement in the opposite direction disengages it. The structure of parking valve 70 is described in more detail below. A spring within the parking mechanism biases the system towards engagement. Furthermore, pressure in the pump-out circuit 32 acts on a specific area of ​​the spool to bias the system towards parking engagement. When disengaged from parking, parking valve 70 connects disengaged parking circuit 72 to line pressure circuit 38, and discharges disengaged parking circuit 72 when parking is engaged. Hydraulic circuits 72 and 76 bias the parking valve towards the disengaged position. Check valve 78 fluidly connects circuit 74 to the higher pressure of either clutch 1 application circuit 60 or disengaged parking circuit 72. Similarly, check valve 80 fluidly connects circuit 76 to the higher pressure of either clutch 2 application circuit 64 or disengaged parking circuit 72.

[0026] When the parking mechanism is engaged, the pressure in the parking mechanism itself and in the pump-out circuit 32 tends to hold the parking valve 70 in the engaged position. To disengage the parking mechanism, clutches 1 and 2 are engaged by commanding full pressure to application circuits 60 and 64. Check valves 78 and 80 fluidly connect these clutch application circuits to circuits 74 and 76, respectively. The pressure in circuits 74 and 76 pushes the parking valve into the disengaged position. Once in the disengaged position, parking valve 70 fluidly connects the disengaged parking gear circuit 72 to the line pressure circuit 38. As a result, even if clutches 1 and 2 are released later, the parking valve remains in the disengaged position. To re-engage the parking mechanism, the pressure in the pump-out circuit 32 decreases to the level at which the parking mechanism spring pushes the parking valve into the engaged position. To engage the parking gear more quickly, the line pressure can be discharged via the return shut-off valve 36 as described above. With the engine running, the pump-out circuit 32 will have pressure that pushes the spool valve into the engaged position.

[0027] Figure 5 A cross-section of the parking valve 70 in the position corresponding to the parking position is shown. The valve includes a housing 90 having an orifice and several ports. A slide valve 92 slides within the orifice. The slide valve 92 is mechanically connected to the parking mechanism via features such as an orifice 94. The parking mechanism includes a spring that applies a return force 96 to the slide valve 92, tending to push it to the left (pawl engaged position). A pump-out circuit 32 is connected to port 98 to supply fluid to chamber 100. The pressure in chamber 100 deflects the slide valve 92 to the left, which tends to hold the parking mechanism in the engaged position.

[0028] End cap 102 is secured to housing 90. End cap 102, housing 90, and slide valve 92 cooperate to define chambers 104 and 106. Circuits 74 and 76 are connected to ports 108 and 110, respectively, to supply fluid to chambers 104 and 106. The fluid pressure in these chambers acts on separate regions of slide valve 92, causing slide valve 92 to deflect to the right. Ports 112, 114, and 116 are connected to outlet 42, disengagement parking brake circuit 72, and line pressure circuit 38, respectively. When slide valve 92 is in Figure 5 In the position shown, its fluid connection ports 112 and 114 make the pressure in the disengaged parking gear circuit 72 approximately equal to the ambient pressure.

[0029] The retaining pin 118 is shown in the retracted position, where the retaining pin 118 is not engaged with the slide valve 92. The pin 118 can be biased into this position by a spring or other force. When current is supplied to the solenoid 120, it applies a magnetic force to the pin 118, pushing it to engage with the slide valve 92.

[0030] Figure 6 A cross-section of the parking valve 70 is shown in its position corresponding to drive, reverse, or neutral. The valve is opened by engaging clutches 1 and 2. Figure 5Move the position to Figure 6 The clutch application pressure of these shifting elements acts on the area on the left side of the valve 92 greater than the area on the right side of the valve 92 due to the pumping pressure, resulting in a net force to the right. When the line pressure is high enough, this net force overcomes the force of the parking mechanism return spring, thus pushing the valve 92 to its position. Figure 6 The position shown in the image is where the parking pawl disengages.

[0031] With spool valve 92 in this position, port 114 is fluidly connected to port 116, causing line pressure circuit 38 to be fluidly connected to disengaged parking gear circuit 72. Due to the behavior of check valves 78 and 80, line pressure will continue to be supplied to ports 108 and 110 even if clutches 1 and 2 are later released.

[0032] To re-engage the parking gear, the line pressure circuit is discharged via the anti-backflow valve, and then the pressure in the pump-out circuit 32 is used to move the slide valve to the left.

[0033] When the vehicle is parked, it is advantageous to release the accumulator to relieve pressure on the system components. This is especially important if maintenance is to be performed. However, there is a risk that uncontrolled accumulator release in the presence of a malfunction in clutch valve 68 could lead to unintentional release of the parking mechanism. Normally, both shift elements must be engaged to disengage the parking mechanism. However, when the engine is off, the pump-out circuit 32 is not pressurized, so the parking position can be disengaged by pressurizing circuit 64. During uncontrolled accumulator release, the line pressure circuit 38 is pressurized. If valve 68 malfunctions while engaged, the clutch application chamber 64 may also be pressurized.

[0034] Figure 7 A method for controlling the release of pressure from an accumulator is shown. This method is executed in response to a request to release the accumulator. Such a request might occur, for example, whenever a driver turns off the ignition while the transmission is in park. At 130, the controller commands valve 50 to “pulse” or temporarily open to release some fluid from the accumulator. The duration of the pulse is calculated; if valve 68 malfunctions, the fluid volume will be insufficient to disengage the parking mechanism, but if valve 68 malfunctions, the fluid volume will be sufficient to cause some movement of spool valve 92. At 132, the controller monitors the position of spool valve 92. If no movement is detected, the controller opens valve 50 and keeps it open until the accumulator is completely discharged via a leak in line pressure circuit 38.

[0035] If movement of spool valve 92 is detected at 132, the controller continues the controlled release of fluid. At 136, the controller commands engagement of at least one accumulator-supplied clutch by connecting the line pressure circuit 38 to the clutch application chamber 66 via valve 69. At 138, the controller commands valve 50 to perform another short-duration fluid release from the accumulator. The release duration at 138 is not necessarily the same as the release duration at 130. Fluid flows from the accumulator into the clutch application chamber 66, causing a stroke in the piston of the corresponding shift element. During the fluid release, at 140, the controller monitors the pressure sensor for the clutch application circuit 66. If the pressure does not increase, the controller infers that the accumulator must be empty, and the process ends. Otherwise, at 142, the controller commands disengagement of the shift element by venting the clutch application chamber 66 via valve 69. As a result, the clutch return spring pushes fluid from the clutch application chamber to the oil pan. At 144, the controller monitors the pressure in the clutch application chamber until it reaches ambient pressure, indicating that fluid has been released. Then, repeat the process until the accumulator is empty, as shown at 140.

[0036] While exemplary embodiments have been described above, these embodiments are not intended to describe all possible forms covered by the claims. The language used in this specification is descriptive rather than restrictive, and it should be understood that various changes may be made without departing from the spirit and scope of this disclosure. As previously stated, features of various embodiments may be combined to form other embodiments of the invention that are not explicitly described or shown. While various embodiments may have been described as providing advantages or being more preferred than other embodiments or prior art implementations in terms of one or more desired features, those skilled in the art will recognize that trade-offs may be made to one or more features or characteristics to achieve the desired overall system properties, depending on the specific application and implementation. Therefore, embodiments described as less desirable than other embodiments or prior art implementations in terms of one or more features are not outside the scope of this disclosure and may be desirable for a particular application.

Claims

1. A transmission comprising: An accumulator valve, the accumulator valve being configured to selectively fluidly connect an accumulator to a line pressure circuit; A first shift valve is configured to alternately fluidly connect a first application chamber to the pipeline pressure circuit and the outlet. The parking valve is configured to engage or disengage the parking mechanism; as well as A controller configured to respond to a request to discharge the accumulator by issuing a sequence of valve commands, the sequence of valve commands including: The first shift valve is commanded to connect the fluid in the first application chamber to the pipeline pressure circuit. The accumulator valve is commanded to connect the accumulator fluid to the pipeline pressure circuit to partially fill the first application chamber, and The first shift valve is commanded to connect the fluid in the first application chamber to the outlet. The controller is further configured as follows: In response to the parking valve not moving, when the first application chamber is disconnected from the pipeline pressure circuit fluid, the accumulator valve is commanded to connect the accumulator fluid to the pipeline pressure circuit until the accumulator is released, without operating any shift valve.

2. The transmission of claim 1, wherein the controller is further configured to: Measuring the pressure in the first application chamber; and Repeat the valve command sequence until the pressure no longer increases.

3. The transmission as claimed in claim 2, further comprising: A second shift valve is configured to alternately fluidly connect a second application chamber to the pipeline pressure circuit and the outlet. The parking valve is configured to disengage from the parking mechanism in response to an increase in pressure in the second application chamber.

4. The transmission as claimed in claim 3, further comprising: A pump configured to supply pressurized fluid to a pump outlet circuit, wherein the pressure in the pump outlet circuit biases the parking valve to prevent disengagement from the parking mechanism.

5. A transmission comprising: An accumulator, which is selectively fluidly connected to a pipeline pressure loop; The first shift element application circuit is alternately fluidly connected to the pipeline pressure circuit and the outlet. The parking valve is configured to engage or disengage the parking mechanism; as well as A controller configured to repeatedly discharge fluid from the accumulator by partially filling the first application chamber via the first shift element application circuit and subsequently discharging the first application chamber until no increase in pressure in the first application chamber is detected. The controller is further configured as follows: In response to a venting request, the accumulator is partially vented when the first shift element application circuit is disconnected from the pipeline pressure circuit. as well as In response to the parking valve not moving, the accumulator is completely discharged when the first shift element application circuit is fluidly disconnected from the pipeline pressure circuit.

6. The transmission as claimed in claim 5, further comprising: The second shift element application circuit is alternately fluidly connected to the pipeline pressure circuit and the outlet. The parking valve is configured to disengage from the parking mechanism in response to an increase in pressure in the application circuit of the second shift element.

7. The transmission as claimed in claim 6, further comprising: A pump configured to supply pressurized fluid to a pump outlet circuit, wherein the pressure in the pump outlet circuit biases the parking valve to prevent disengagement from the parking mechanism.

8. A method for discharging fluid from an accumulator in a transmission, comprising: Repeat the following operations: Operate the shift valve to connect the shift element application chamber fluid to the pipeline pressure circuit; Open the accumulator valve to connect the accumulator fluid to the pipeline pressure circuit to partially fill the shift element application chamber; as well as Operate the shift valve to discharge fluid from the partially filled shift element application chamber. Until no increase in pressure in the shift element application chamber can be detected, The method further includes: In response to an accumulator release request, the accumulator valve is opened when the fluid in the shift element application chamber is disconnected from the pipeline pressure circuit; and In response to the parking valve not moving, the accumulator valve is kept open until the accumulator is completely discharged when the shift element application chamber is disconnected from the pipeline pressure circuit fluid.

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