Method and control unit for operating a motor vehicle's powertrain

Abstract

Method for operating a motor vehicle drivetrain comprising a drive unit (1) and a transmission (3) with multiple shift elements (4) connected between the drive unit (1) and an output (2), wherein, when driving in a frictionally engaged gear with a gear-dependent, defined target gear ratio, the transmission capacity of a shift element (4) of the transmission (3) closed in the respective engaged gear is reduced by lowering a pressure control thereof, and it is monitored whether the actual gear ratio in the engaged gear deviates from the target gear ratio by more than a limit value, wherein, if it is determined that the actual gear ratio deviates from the target gear ratio by more than the limit value, a flare state of the transmission (3) forced by the reduction of the transmission capacity is detected.wherein, upon or after detection of the flare condition of the transmission (3), the following steps are performed: Determining a reduced transmission torque for the switching element (4) of the transmission (3) for which the transmission capacity has been reduced; Determining a pressure reduction for a system pressure of the transmission (3) depending on the determined reduced transmission torque.

Description

The invention relates to a method for operating a drive train of a motor vehicle. The invention furthermore relates to a control device for carrying out the method.Drive trains of motor vehicles having a transmission connected between a drive unit and an output are sufficiently known from practice. It is also known that a transmission comprises a plurality of shift elements, which can be designed as frictional shift elements or also as form-locking shift elements. Then, when a force-locking gear is engaged in a transmission, a first number of the shift elements of the transmission is closed and a second number of the shift elements of the transmission is opened. When a force-locking gear is engaged in the transmission, the gear is driven with a defined gear-dependent transmission ratio. Depending on this gear-dependent transmission ratio, rotational speeds and torques are converted and thus the traction force supply of the drive unit is provided by the transmission at the output.It is known from practice that the shift elements of the transmission are actuated with a defined pressure for opening and closing. The pressure with which a shifting element is actuated for closing determines the transmission capacity of the shifting element, wherein this transmission capacity is also referred to as torque transmission capacity. The actuation of a shift element with a pressure and the torque transmission capability that is formed in this case is also dependent on a so-called system pressure of the transmission. The system pressure is selected to be so high that the transmission capacity of the shift elements is not too low over all transmission tolerances and over maximum transmission wear. Accordingly, the system pressure in a transmission is always set too high, whereby, on the one hand, a consumption penalty is accepted. In addition to the consumption penalty, too high a system pressure results in a load on the assemblies of the transmission, which then causes wear.There is therefore a need to choose the system pressure of the transmission not to be too high in order to keep a consumption penalty and a component load as low as possible. It is therefore desirable to lower the system pressure compared to conventional transmissions.US 2005 / 0187070 A1 discloses a method for adapting the system pressure of a transmission. To adapt the system pressure, a shift element that is open in an engaged gear is actuated for closing and a system reaction is detected and evaluated. The system response is a change in turbine speed that is detected with a turbine rotation sensor.Proceeding from this, the object of the invention is to create a novel method and a control device of a drive train of a motor vehicle.This object is achieved by a method for operating a drive train of a motor vehicle according to patent claim 1. According to the invention, when, in the case of a force-locking gear engaged in the transmission, driving is carried out with a gear-dependent, defined setpoint transmission ratio, the transmission capacity of a shift element of the transmission that is closed in the respective engaged gear is reduced by lowering a pressure control of the transmission and it is monitored whether an actual transmission ratio deviates from the setpoint transmission ratio by more than a limit value in the engaged gear. If it is determined that the actual transmission ratio deviates from the desired transmission ratio by more than the limit value, a flare state of the transmission forced by the reduction of the transmission capacity is detected, wherein the following steps are carried out when or after the detection of the flare state of the transmission: ascertaining a low transmission capacity torque for the shifting element of the transmission for which the transmission capacity has been reduced. determining a pressure decrease for a system pressure of the transmission as a function of the determined torque of low transmission capability.The invention proposes that the pressure control be reduced in a defined manner at a defined shifting element of the transmission, which is closed in an engaged, force-locking gear, and thus force a flare state in the transmission, i.e. a state in which this shifting element can no longer completely transmit a torque to be transmitted, so that the torque that can no longer be transmitted then causes the drive unit to run up and thus leads to the actual transmission ratio deviating from the desired transmission ratio. Then, when the flare state of the transmission is detected, at least the low transmission capability torque is determined and the shift element for which the transmission capability has been reduced is controlled as a function of the determined low transmission capability torque. Depending on the torque of the low transmission capability, the pressure reduction for the system pressure can be determined. With the invention, it is possible to reduce the system pressure in a defined manner, in order to realize a consumption advantage and to reduce a component load of the transmission.According to an advantageous development, for that shifting element of the transmission for which the transmission capacity is or has been reduced, a first dynamic torque is determined as a function of a temporal actual gradient and setpoint gradient of the transmission input rotational speed and a mass inertia related to the drive unit, from which the low transmission capacity torque is determined. The torque of reduced transmission capability is preferably determined from this first dynamic torque as a function of a gear-dependent and / or shifting element-dependent support factor. The torque of reduced transmission capability can thereby be determined particularly advantageously.According to an advantageous development, in addition to the low transmission capacity torque, a compensation torque for the shift element of the transmission for which the transmission capacity has been reduced is determined. The control of the shift element of the transmission for which the transmission capacity has been reduced then takes place as a function of the determined low transmission capacity torque and as a function of the determined compensation torque, in order to leave the flare state of the transmission and, as a function thereof, to determine the pressure reduction for the system pressure of the transmission.According to an advantageous development, for that shifting element of the transmission for which the transmission capacity is or has been reduced, a second dynamic torque is determined as a function of a temporal setpoint compensation gradient of the transmission input rotational speed and a mass inertia related to the drive unit, from which the compensation torque is determined. The compensation torque is preferably determined from this second dynamic torque as a function of a gear-dependent and / or shifting element-dependent support factor. The compensation torque can thereby be determined particularly advantageously.The pressure reduction for the system pressure of the transmission is preferably determined as a function of a difference between the pressure control of the shift element for which the transmission capacity has been reduced, before the pressure control is reduced and when the flare state is detected, and as a function of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque of the torque. Alternatively, the pressure reduction for the system pressure of the transmission is determined as a function of a difference between the pressure control of the shift element for which the transmission capacity has been reduced, before the pressure control is reduced and after the flare state has been exited, wherein the pressure control after the flare state has been exited is a function of the low transmission capacity torque and the compensation torque. With both alternatives, the pressure reduction for the system pressure of the transmission can be advantageously determined.The control device according to the invention is defined in claim 11.Preferred refinements emerge from the dependent claims and the following description. Exemplary embodiments of the invention are explained in more detail with reference to the drawings, without being restricted thereto. The following shows: FIG. 1 shows a block diagram of a drive train of a motor vehicle; FIG. 2 shows a signal flow diagram to illustrate the method according to the invention; FIG. 3 shows a time diagram for further clarification of the method according to the invention.The invention relates to a method and a control device for operating a drive train of a motor vehicle.FIG. 1 shows a highly schematic block diagram of a drive train of a motor vehicle, wherein the drive train comprises a drive unit 1 and a transmission 3 connected between the drive unit 1 and an output 2. The transmission 3 is an automated or automatic transmission, which converts rotational speeds and torques and thus provides the traction force available to the drive unit 1 at the output 2. The transmission 3 comprises a plurality of shift elements 4, wherein only one shift element 4 is shown in FIG. 1. The shift elements 4 of the transmission 3 are frictional shift elements and, if appropriate, form-locking shift elements, wherein in each engaged, force-locking gear of the transmission 3 a first number of the shift elements 4 is closed and a second number of the shift elements 4 is opened. To carry out a gear change in the transmission 3, a previously closed shifting element is opened and a previously opened shifting element is closed. According to FIG. 1, a separating clutch 5 is connected between the drive unit 1 and the transmission 3.The operation of the drive unit 1 is controlled and / or regulated by an engine control unit 6. The operation of the transmission 3 is controlled and / or regulated by a transmission control unit 7. For this purpose, according to the double arrows shown, the engine control unit 6 exchanges data with the drive unit 1 and the transmission control unit 7 exchanges data with the transmission 3. Furthermore, engine control unit 6 and transmission control unit 7 exchange data with one another.When, in the drive train shown in FIG. 1, a force-locking gear is firmly engaged in the transmission 3 and is driven with a defined nominal transmission ratio dependent on the force-locking gear, the transmission capacity of a defined shift element of the transmission that is closed in the respective engaged gear is reduced by lowering a pressure control of the transmission.In this case, it is monitored whether an actual transmission ratio deviates from the setpoint transmission ratio by more than a limit value in the engaged gear. If this is the case, it is concluded that a so-called flare state exists for the transmission 3, which was forced by the reduction of the transmission capacity of the defined shift element of the transmission.Upon or after detection of such a forced flare state in the transmission 3, at least the following steps are carried out:determining a low transmission capability torque for the shift element 4 of the transmission 3 for which the transmission capability has been reduced.determining a pressure reduction for a system pressure of the transmission 3 as a function of the determined torque of low transmission capability. Preferably, that shifting element 4 of the transmission 3 for which the transmission capacity has been reduced is controlled as a function of the low transmission capacity torque determined.The pressure reduction for the system pressure of the transmission 3 is preferably determined as a function of a difference between the pressure control of the shift element 4 for which the transmission capacity has been reduced, before the pressure control is reduced and when the flare state is detected and as a function of the torque of the low transmission capacity.Preferably, in addition to the low transmission capacity torque, a compensation torque for the shift element of the transmission for which the transmission capacity has been reduced is determined. The control of the shift element of the transmission for which the transmission capacity has been reduced takes place as a function of the determined low transmission capacity torque and as a function of the determined compensation torque, in order to leave the flare state of the transmission and, as a function thereof, to determine the pressure drop for the system pressure of the transmission.The pressure reduction for the system pressure of the transmission is then preferably determined as a function of a difference between the pressure control of the shift element for which the transmission capacity has been reduced, before the pressure control is reduced and after the flare state has been left.Further details of the invention are described below with reference to the signal flow diagram of FIG. 2.In a block 8, when a force-locking gear is firmly engaged in the transmission 3 and is driven with a defined nominal transmission ratio dependent on the force-locking gear, the transmission capacity of a defined shift element 4 of the transmission 3 that is closed in the respective engaged gear is reduced by lowering a pressure control of the same. In a block 12, it is checked whether a flare condition is forced in the transmission 3 in the firmly engaged, force-locking gear.A plurality of input variables 9, 10 and 11 are provided to block 8, namely, the setpoint transmission ratio of the firmly engaged, force-locking gear of transmission 3 with input variable 9, the actual rotational speed of the transmission output and the actual rotational speed of the transmission input with input variable 9, and the current transmission input torque with input variable 11.In block 8, the transmitting capacity of a defined shift element 4 of transmission 3, which shift element is closed in the respective engaged gear, is reduced. Furthermore, in block 8, an actual transmission ratio is calculated as a function of the actual rotational speed of the transmission output and the actual rotational speed of the transmission input, which is compared with the desired transmission ratio in block 12. In a block 12, it is thus checked whether the actual transmission ratio deviates from the desired transmission ratio by more than a limit value.If this is not the case, the system branches back from block 12 to block 8.If, on the other hand, it is determined in block 12 that the actual transmission ratio deviates from the setpoint transmission ratio by more than the limit value, a flare state of the transmission 3 forced by the reduction of the transmission capacity of the defined shift element 4 is recognized and, starting from block 12, the system branches to block 13.In block 13, the torque that the shift element 4 of the transmission 3 for which the transmission capacity has been reduced transmits is determined upon detection of the flare state. This is the current transmission input torque.In a subsequent block 14, a low transmission capability torque is determined for that shifting element of the transmission 3 for which the transmission capability has been reduced. For this purpose, block 14 is provided, on the one hand, as input variable 15 with an actual time gradient of the transmission input rotational speed and, as a further input variable 16, with a mass inertia relating to drive unit 1, as a function of which a first dynamic torque is ascertained for shift element 4 for which the transmission capacity has been reduced. The mass inertia, which is provided to block 14 as input variable 16, is that mass inertia of the drive train which it has between drive unit 1 and shift element 4 with the reduced transmission capacity. The first dynamic torque, which is determined in block 14, is also dependent on a temporal setpoint gradient of the transmission input rotational speed. This temporal setpoint gradient of the transmission input rotational speed is determined as a function of the setpoint transmission input rotational speed, which corresponds to the product of the setpoint transmission ratio and the actual output rotational speed at the output 2. The first dynamic torque corresponds to the product of the mass inertia and the difference between the actual gradient and the setpoint gradient of the transmission input rotational speed. The first dynamic torque is calculated with a support factor of the transmission 3 in order to calculate the torque with which the transmission is less than this first dynamic torque, wherein the support factor of the transmission 3 is dependent on the engaged force-locking gear and on the shift element which has the transmission is less than this.The block 14 outputs the torque of low transmission capability as an output variable.In a subsequent block 17, a check is again carried out, analogously to block 12, as to whether the flare state of the transmission 3 is present.Subsequently, in a block 18, the compensation torque is determined for that shifting element 4 of the transmission 3 for which the transmission capacity has been reduced. This compensation torque 18 is determined as a function of input variables, namely, as a function of the input variables 19 and 16. The input variable 16 is in turn the mass inertia of the output train starting from the drive unit 1 in the direction of the shifting element 4 with the reduced transmission capacity. The input variable 19 is a temporal setpoint compensation gradient of the transmission input rotational speed.From the product of these input variables 16 and 19, a second dynamic torque is determined, which is initially related to the drive and is converted into the compensation torque by calculation with the support factor of the transmission 3, wherein the support factor of the transmission 3 is again gear-dependent. Block 18 accordingly outputs the compensation torque as an output variable.In a block 20, the shift element 4 of the transmission 3 for which the transmission capacity has been reduced is subsequently activated as a function of the low transmission capacity torque determined in block 14 and as a function of the compensation torque determined in block 18, in order to bring the actual transmission ratio closer to the desired transmission ratio as a result of this activation and thus to leave the flare state at the transmission 3.Consequently, in block 17 a continuous monitoring is carried out as to whether a flare condition is present at the transmission 3, wherein if a flare condition is still present at the transmission 3, the compensation torque is continuously recomputed in block 18 in order to adapt the actuation of the shift element 4.The adaptation of the compensation torque in block 18 is dependent on the setpoint compensation gradient 19 for the transmission input rotational speed, which is determined in block 21 of FIG. 2.The setpoint compensation gradient 19 of the transmission input rotational speed is ascertained in block 21 as a function of the input variables 22 and / or 23 and / or 24 and / or 25, the input variable 22 being a current actual differential rotational speed at the shift element 4 for which the transmission capacity has been reduced, the input variable 23 being a current temperature of the shift element 4 for which the transmission capacity has been reduced, the input variable 24 being a current transmitted torque at the shift element 4 for which the transmission capacity has been reduced, and the input variable 25 being a current torque of the transmission input. The setpoint compensation gradient 19 of the transmission input rotational speed is preferably determined as a function of these four input variables 22, 23, 24 and 25. However, it is also possible to take into account only some of these input variables for the determination of the setpoint compensation gradient 19 of the transmission input rotational speed or to add further input variables.In a block 26 of the signal flow diagram of FIG. 2, the pressure reduction for the system pressure of the transmission is determined as a function of the difference between the pressure control of the shift element 4 of the transmission 3, for which the transmission capacity has been reduced, before the pressure control is reduced and after the flare state has been left. This difference corresponds to a maximum admissible pressure reduction for the system pressure of the transmission 3, wherein an offset is preferably subtracted from this maximum admissible pressure reduction in order to prevent the pressure reduction from being too great for safety reasons.The adaptation value stored in block 26 for the pressure reduction of the system pressure can subsequently be used to actually reduce the system pressure in the transmission 3.The method described above can be carried out at different operating points, i.e. at different rotational speeds, torques and force-locking gears, in order to determine the pressure reduction for the system pressure of the transmission 3 as accurately as possible.When using the method described above, the temporal signal curves shown in FIG. 3 can form. Thus, three time signal curves are shown in FIG. 3 over time t, namely with a signal curve 27 a curve of a setpoint transmission input rotational speed of the transmission 3, which corresponds to a product of the setpoint transmission ratio of the force-locking gear engaged in the transmission 3 and a measured actual output rotational speed at the output 2. A curve profile 28 visualizes an actual profile of the transmission input rotational speed. A curve profile 29 visualizes a pressure control for a shift element 4 of the transmission 3 that is closed in a force-locking gear.In FIG. 3, according to the curve profile 29, the pressure control for this switching element 4 is continuously reduced starting from the time t 0.By reducing the pressure control, which begins at the time t0, the transmission capacity of this switching element 4 is likewise continuously reduced, wherein, beginning at the time t1, the curve profile 28 begins to be released from the curve profile 27. Beginning at the time t 1, the pressure reduction 29 for the shifting element 4 of the transmission 3 has accordingly been reduced to such an extent that the shifting element 4 can no longer completely transmit the torque offered by the drive unit 1, wherein the torque that can no longer be transmitted leads to the drive unit 1 running up and thus to the transmission input rotational speed being increased according to the curve profile 28.At the time t 2, it is then recognized that the actual transmission ratio deviates from the setpoint transmission ratio by more than a limit value, so that it is concluded at the time t 2 that a flare state is present in the transmission 3.Beginning at the time t2, the torque of low transmission capacity for the shifting element 4 of the transmission 3, for which the transmission capacity or pressure control has been reduced, is then determined in the sense of the block 14, this torque of low transmission capacity being visualized in FIG. 3 by the double arrow 30.Depending on the difference between the pressure control at time t 0 and the pressure control at time t 2 and depending on the torque of low transmission capability 30, the pressure reduction for the system pressure can then be determined already at and immediately after time t 2. This pressure reduction of the system pressure is visualized in FIG. 3 by the double bearing 31.When the flare state is detected at the time t 2, the pressure control 29 for the shifting element 4 of the transmission 3, for which the pressure control has been reduced until the time t 2, is kept constant until the time t 3, so that a constant gradient of the transmission input rotational speed 28 is then formed according to the curve profile 28.Not only is the low transmission capability torque 30 calculated in block 14, but additionally also the compensation torque is calculated in block 18, wherein in FIG. 3, beginning at the time t 3, the pressure control 29 is adapted as a function of the low transmission capability torque calculated in block 14 and as a function of the compensation torque determined in block 18, in order to adapt the actual transmission ratio again to the desired transmission ratio, wherein the flare state of the transmission 3 was successfully exited at the time t 4.In FIG. 3, the double arrow 31' visualizes a maximum permissible pressure drop of the system pressure, which corresponds to the difference between the pressure control at the time t0 and the pressure control at the time t4, i.e. the pressure control of the switching element 4 for which the transmission capacity has been reduced, before the pressure control is lowered and after leaving the flare state.The pressure drop 31, 31' can be calculated with an offset. The pressure drop 31, 31' can be reduced for safety reasons by means of the offset.With the invention, it is possible to reduce the system pressure in a defined manner via the pressure reduction 31 or 31', in order to realize a consumption advantage and to reduce a component load of the transmission 3.The invention further relates to a control device for operating the drive train of the motor vehicle, wherein the control device executes the method described above on the control side. The control device according to the invention is in particular the transmission control device 7. When a force-locking gear engaged in the transmission 3 is driven in a gear-dependent, defined nominal transmission ratio, the control device 7 reduces the transmission capacity of a shift element 4 of the transmission 3 that is closed in the respective engaged gear by lowering the pressure control of the same. In this case, the control device 7 then monitors whether an actual transmission ratio deviates from the desired transmission ratio by more than a limit value in the engaged gear ratio. If this is the case, the control device 7 concludes that the transmission 3 has a flare state forced by the reduction of the transmission capacity. Subsequently, the control device 7 then executes at least the following steps: ascertaining the low transmission capacity torque for the shift element 4 for which the transmission capacity has been reduced. determining a pressure reduction for a system pressure of the transmission 3 as a function of the determined torque of low transmission capability.The transmission control unit 7 has hardware-side means and software-side means for carrying out the method on the control side, the hardware-side means being data interfaces, a processor and a memory. The data interfaces serve to exchange data with the modules involved in carrying out the method according to the invention. The processor is used for data processing and the memory is used for data storage. Software-side means include program modules for carrying out the method according to the invention.Reference numerals denote reference numerals1 Drive unit 2 Transmission 3 Output 4 Shifting element 5 Separating clutch 6 Engine control unit 7 Transmission control unit 8 Block Setpoint transmission ratio ascertainment 9 Input variable Setpoint transmission ratio 10 Input variable Actual rotational speed Transmission output and Transmission input 11 Input variable Transmission input torque 12 Block Query Flare state 13 Block Shifting element ascertainment 14 Block Low transmission capacity torque ascertainment 15 Input variable Actual gradient Transmission input rotational speed 16 Input variable Mass inertia 17 Block Query Flare state 18 Block Compensation torque ascertainment 19 Input variable Setpoint compensation gradient Transmission input rotational speed 20 Block Shifting element control 21 Block Setpoint compensation gradient ascertainment 22 Input variable Actual differential rotational speed Shifting element 23 Input variable Temperature Shifting element 24 Input variable Torque Shifting element 25 Input variable Torque Transmission input 26 Block Pressure reduction determination 27 Setpoint curve profile Transmission input rotational speed 28 Actual curve profile Transmission input rotational speed 29 Curve profile Torque 30 Low transmission capability torque 31, 31' Pressure reduction

Claims

Method for operating a drive train of a motor vehicle, having a drive unit (1) and a transmission (3) which is connected between the drive unit (1) and an output (2) and has a plurality of shift elements (4), wherein, when driving with a gear-dependent, defined desired transmission ratio when a force-locking gear is engaged in the transmission (3), the transmission capacity of a shift element (4) of the transmission (3) which is closed in the respective engaged gear is reduced by lowering a pressure control of the same and it is monitored whether an actual transmission ratio deviates from the desired transmission ratio by more than a limit value in the engaged gear, wherein, if it is determined that the actual transmission ratio deviates from the desired transmission ratio by more than the limit value, a flare state of the transmission (3) which is forced by the reduction of the transmission capacity is detected, wherein, upon or after detecting the flare condition of the transmission (3), the following steps are carried out: determining a low transmission capacity torque for the shifting element (4) of the transmission (3) for which the transmission capacity has been reduced, determining a pressure drop for a system pressure of the transmission (3) as a function of the determined low transmission capacity torque.Method according to Claim 1, characterized in that, for the shift element (4) of the transmission (3) for which the transmission capacity has been reduced, a dynamic torque is determined as a function of a temporal actual gradient of a transmission input rotational speed, a temporal setpoint gradient of the transmission input rotational speed and a mass inertia relating to the drive unit (1), from which dynamic torque the low transmission capacity torque is determined.Method according to Claim 2, characterized in that the torque with which the low transmission capability is able to be determined from this dynamic torque as a function of a gear-dependent and / or shifting-element-dependent supporting factor.Method according to Claim 2 or 3, characterized in that the setpoint gradient of the transmission input rotational speed is determined from the setpoint transmission input rotational speed which corresponds to the product of the setpoint transmission ratio and the output rotational speed.Method according to one of Claims 1 to 4, characterized by ascertaining a compensation torque for the shift element (4) of the transmission (3) for which the transmission capacity has been reduced, activating the shift element (4) of the transmission (3) for which the transmission capacity has been reduced, as a function of the ascertained low transmission capacity torque and as a function of the ascertained compensation torque, in order to leave the flare state of the transmission and, as a function thereof, to ascertain the pressure reduction for the system pressure of the transmission.Method according to Claim 5, characterized in that, for the shift element (4) of the transmission (3) for which the transmission capacity has been reduced, a dynamic torque is determined as a function of a temporal setpoint compensation gradient of the transmission input rotational speed and a mass inertia relating to the drive unit (1), from which dynamic torque the compensation torque is determined.Method according to Claim 6, characterized in that the compensation torque is determined from this dynamic torque as a function of a gear-dependent and / or shift-element-dependent supporting factor.Method according to Claim 6 or 7, characterized in that the temporal setpoint compensation gradient of the transmission input rotational speed is determined as a function of a temperature of the shift element (4) for which the transmission capacity has been reduced, and / or as a function of a torque of the shift element (4) for which the transmission capacity has been reduced, and / or as a function of a differential rotational speed of the shift element (4) for which the transmission capacity has been reduced, and / or as a function of a transmission input torque.Method according to one of Claims 1 to 8, characterized in that the pressure drop for the system pressure is determined as a function of a difference between the pressure control of the shift element for which the transmission capacity has been reduced, before the pressure control is lowered and when the flare state is detected and as a function of the torque with which the transmission capacity has been reduced.Method according to one of Claims 5 to 8, characterized in that the pressure drop for the system pressure is determined as a function of a difference between the pressure control of the shift element for which the transmission capacity has been reduced, before the pressure control is lowered and after the flare state has been left, wherein the pressure control is dependent on the low transmission capacity torque and the compensation torque after the flare state has been left.Control device (7) for operating a drive train of a motor vehicle, having a drive unit (1) and a transmission (3) which is connected between the drive unit (1) and an output (2) and has a plurality of shift elements (4), wherein, when a force-locking gear engaged in the transmission (3) is driven with a gear-dependent, defined desired transmission ratio, the control device (7) reduces the transmission capacity of a shift element (4) of the transmission (3) which is closed in the respective engaged gear by lowering a pressure control of the same and monitors whether an actual transmission ratio in the engaged gear deviates from the desired transmission ratio by more than a limit value, wherein, when the same establishes that the actual transmission ratio deviates from the desired transmission ratio by more than the limit value, the control device (7) determines that the actual transmission ratio deviates from the desired transmission ratio by more than the limit value, upon or after the sensing of the flare state of the transmission (3), a low transmission capability torque for the shifting element (4) of the transmission (3) for which the transmission capability has been reduced is determined and a pressure reduction for a system pressure of the transmission (3) is determined as a function of the determined low transmission capability torque.Control device according to Claim 11, characterized in that it carries out the method according to one of Claims 1 to 10 on the control side.

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