Abstract:
A method and apparatus for operating an automatic transmission as a function of the engine rotational speed of a variable speed internal combustion engine. Upon a failure of the electronic control system, a return home of a motor vehicle under its own power is enabled. A delivery system delivers a working medium with which a transmission ratio adjusting device is actuated by a first control device that, in turn, is controlled by a pilot pressure that can be accurately changed by a second control device to actuate the transmission ratio adjusting device. The pilot pressure for controlling the first control device is changed by a third control device as a function of the rotational speed of the internal combustion engine when an adjusting device is switched out of a normal operation position into an emergency operation position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for operating an automatic transmission as a function of, among other things, engine rotational speed. The invention also relates to an automatic transmission for a variable speed internal combustion engine, with a delivery system for a working medium with which at least one adjusting device is acted on by a first control device which, in turn, is controlled by a pilot pressure that can be accurately changed by a second control device to operate the adjusting device. 
     2. Description of the Related Art 
     A similar method and a similar automatic transmission are disclosed in German Patent Publication No. DE 195 46 293 A1, in which is disclosed a belt driven, conical pulley transmission with input side and output side pairs of conical disks. The power transmission between the two pairs of conical disks can be effected by a steel loop or band. Each pair of conical disks includes an axially movable disk and an axially fixed conical disk. The axially displaceable conical disk halves are arranged diagonally opposite each other. By an axial displacement of the movable conical disk halves, the effective lever arms of the pairs of conical disks become larger or smaller in opposite directions. 
     The axial displacement of the movable conical disk halves is achieved with an adjusting device. A first control device actuates the adjusting device. The first control device includes one or two hydraulic valves to which a pilot pressure is applied. The first control device is controlled by an electrically actuated second control device, for example a proportional valve. To adjust the transmission ratio of the belt driven, conical disk transmission, depending upon the demand, the pilot pressure is accurately changed by the second control device, for example by suitable software. 
     In the event of an electronic failure, the adjusting device can no longer be controlled by the first control device because of the failure of the electronically-operated second control device, and the automatic transmission will no longer function. Consequently, continued operation (limp home) of a motor vehicle with that type of automatic transmission is no longer possible by its own power, and it must therefore be towed away. 
     The object of the invention is to provide an appropriate method and an appropriate automatic transmission that enable continued operation by its own power when an electronic failure occurs and/or the second control device fails. 
     SUMMARY OF THE INVENTION 
     The object is attained with a method for operating an automatic transmission as a function of, among other things, the engine rotational speed, in that the transmission ratio is changed to a limp home condition of the automatic transmission after a starting procedure in order to maintain the engine rotational speed at a specific constant value. A transmission ratio of high speed results when the engine rotational speed increases, and a transmission ratio of low speed results when the engine rotational speed decreases. A constant engine rotational speed is maintained through the transmission ratio control in a limp home condition. 
     A preferred embodiment of the method is characterized in that the transmission ratio is adjusted to a high speed ratio, after the starting process, at increasing engine rotational speed, until the minimum possible transmission ratio condition is reached, which is then maintained during further increases in engine rotational speed. After reaching the minimum transmission ratio condition, the engine rotational speed can no longer be maintained constant, and it increases until the engine performance is in equilibrium with the running resistance. 
     Another preferred embodiment of the method is characterized in that the transmission ratio, after the starting process, is adjusted to a low speed ratio at decreasing engine rotational speed, until the largest possible transmission ratio condition is reached, which is then maintained during further decreases in engine rotational speed. After reaching the largest possible transmission ratio condition, the engine rotational speed can no longer be maintained constant and decreases. 
     A further preferred embodiment of the method is characterized in that, when the rotational speed is increased further, the transfer of torque from an input element to an output element is interrupted with the aid of a clutch or converter. The engine is thereby prevented from stalling. 
     A further preferred embodiment of the method is characterized in that the specific value at which the engine rotational speed is maintained constant is greater than the so-called stall speed. The maximum rotational speed at which a clutch or a converter can still slip is designated as the stall speed. The stall speed is therefore the rotational speed at which the converter or the clutch stops the engine with completely applied brakes and full throttle. A certain minimum difference between the stall speed and the transmission ratio control rotational speed is to be maintained for safety reasons. 
     The object is achieved in an automatic transmission for a variable rotational speed internal combustion engine, with a delivery system for a working medium, with which at least one adjusting device is actuated by a first control device, which, in turn, is controlled by a pilot pressure that can be specifically changed by a second control device in order to operate the adjusting device, in that in addition to the second control device a third control device is provided, which is actuated by a working medium transported by the delivery system, with the help of which the pilot pressure for controlling the first control device is changed as a function of the rotational speed of the internal combustion engine, when a switching device is switched from a normal position to a limp home position. 
     The automatic transmission in accordance with the invention is distinguished by an additional third control device, which is hydraulically or pneumatically operated by the working medium transported by the delivery system. As a result, the operation of the adjusting device is ensured even when the second control device fails. In that way an emergency operation of the automatic transmission is ensured, so that continued operation, for example to the nearest service station, is possible. 
     One embodiment of the automatic transmission is characterized in that the delivery system for the working medium is composed of a pump whose delivered volumetric flow depends upon the rotational speed of the internal combustion engine. That can be achieved, for example, by coupling the input of the pump with the camshaft or the crankshaft of the internal combustion engine. The volumetric flow of the pump, which is dependent upon the speed of the internal combustion engine, is utilized in a so-called limp home condition of the automatic transmission for controlling the transmission ratio of the transmission and/or a starting clutch. 
     A further embodiment of the automatic transmission is characterized in that the third control device has an axially-displaceable, spring-biased control plunger with a first and second working surface for the working medium delivered by the delivery system, as well as with a third working surface, which is contacted by the regulated pilot pressure, whereby the first and second working surfaces on the control plunger of the third control device, which are contacted by the working medium, are of the same size. In the normal condition of the transmission, the same working medium pressure acts on the first and second working surfaces on the control plunger of the third control device. The magnitude of the regulated pilot pressure can be adjusted by the spring bias acting on the control plunger. The spring-biased control plunger of the third control device functions as a pressure regulator that maintains the pilot pressure at a constant value. Two control edges can be formed on the control plunger, which ensure that the pilot pressure will decrease when it exceeds a specified value, and which will ensure that the pilot pressure will increase when it drops below the specified value. 
     A further embodiment of the automatic transmission is characterized in that the first and second working surfaces, for the working medium, of the control plunger of the third control device are connected to each other through a connecting conduit, in which an orifice plate is provided, through which flows the working medium that is delivered by the delivery system when the switching device is in the limp home position. In the normal condition of the automatic transmission, the working medium delivered by the delivery system does not flow through the orifice plate. In the limp home condition, the switching device ensures that the working medium delivered by the delivery system flows through the orifice plate. That leads to a different working medium pressure acting on the first working surface of the control plunger of the third control device than on the second working surface. The pressure difference increases with the volumetric flow delivered by the delivery system, which, in turn, is dependent upon the rotational speed of the connected internal combustion engine. The pilot pressure that exists at the third working surface of the control plunger is changed by the pressure difference between the first and second working surfaces for the working medium at the control plunger of the third control device, which is dependent upon the rotational speed of the internal combustion engine. A throttle can be utilized instead of the orifice plate. 
     A particular embodiment of the automatic transmission is characterized in that the switching device includes an axially displaceable, spring-biased switching plunger, which is moved from its normal position into its limp home position during a limp home condition of the automatic transmission by the spring biasing force, whereby a direct connection between the delivery system and the adjusting device is interrupted and a connection between the delivery system and the adjusting device is released by the orifice plate in the connecting conduit between the first and second working surfaces, for the working medium, that are on the control plunger of the third control device. By the spring biasing force acting on the switching plunger, the switching plunger is also automatically shifted in a simple way into its limp home position when there is a total failure of the electronics. Simultaneously, by the displacement of the plunger to its limp home position, the flow through the orifice plate is forced into the connecting conduit between the first and second working surfaces on the control plunger of the third control device. 
     A further particular embodiment of the automatic transmission is characterized in that a device for adjusting the transmission ratio of the automatic transmission and a device for adjusting the starting clutch are coupled with the delivery system as well as with the third control device. That provides the advantage that only one delivery system is required to ensure a sufficient transmission ratio as well as a satisfactory functioning of the starting clutch in a limp home condition of the transmission. If a first control device with an increasing characteristic curve is utilized for the control of the adjusting device of the starting clutch, a reversing slide valve is required to ensure a proper function of the starting clutch during limp home. 
     A further particular embodiment of the automatic transmission is characterized in that in the limp home position of the switching device, between the third pilot pressure working surface on the control plunger of the third control device and a connection to a pressure relief chamber, a control pressure connecting conduit is disconnected, in which there are arranged, starting from the connection to the pressure relief chamber, a first orifice plate, a branch to the adjusting device for the starting clutch, a second orifice plate, and a branch to the transmission ratio adjusting device. The cascade-like arrangement of the orifice plates makes possible agreement between rotational speed and starting pressure, between rotational speed and adjusting pressure, as well as between starting pressure and adjusting pressure. In the limp home position of the switching device, the pilot pressure drops to almost zero at high engine rotational speed. Consequently, the starting clutch is engaged and a transmission ratio of fast (overdrive) results. With decreasing engine rotational speed, the pilot pressure increases and a transmission ratio of slow (underdrive) results. 
     The stepless automatic transmission in accordance with the invention can also be called a CVT (continuously variable transmission). In a CVT, among other things, the starting and transmission ratio adjustment functions must be ensured. The starting function is assured either directly through a starting clutch or through a torque converter, after an accompanying reverse set clutch has been engaged. The starting clutch is ordinarily engaged by pressure. That can occur, for example, with a valve that increases or reverses the pressure without current. A torque converter starts practically by itself, without hydraulic control contact. 
     The transmission ratio of the automatic transmission must always be capable of being adjusted, because without pressure and without additional hydraulic transmission ratio adjustment, a transmission ratio of close to 1:1 (somewhere between third and fourth gear) would be set. Therefore, a transmission ratio adjustment is absolutely required in the limp home condition. As a rule, for safety reasons, the transmission ratio adjustment is designed in such a way that when the pilot pressure is close to zero an adjustment of the transmission ratio to fast results. 
     The basic idea of the emergency operation is as follows. The starting clutch is engaged hydraulically or it is started by the torque converter. Only when that process has been completed does the transmission ratio control to a constant engine rotational speed begin. That means that the transmission ratio control operates in the emergency operating mode so that a constant engine rotational speed is regulated. The engine rotational speed can be, for example, 3,500 revolutions/minute. When the engine rotational speed increases, the transmission ratio is automatically set to fast. When the engine rotational speed decreases, the transmission ratio is automatically set to slow. When a further adjustment of the transmission ratio to fast is no longer possible, the longest transmission ratio is maintained, even when the engine rotational speed increases further. When further adjustment of the transmission ratio to slow is no longer possible, the engine rotational speed drops further. It is only then that the starting clutch or the converter disengages. 
     During a power failure, the switching device switches to the limp home position. The pilot pressure of the first control device is uncoupled from the second control device and is controlled by the third control device. On the basis of the production by a metering orifice of a pressure that is proportional to the engine rotational speed, the pilot pressure on the first control device is decreased when the engine rotational speed, and thereby the pressure on the metering orifice plate, increases. That, in turn, causes an adjustment of the transmission ratio to fast, and the opposite to slow. The engine rotational speed that is to be controlled results from a coordination of the size of the metering orifice with the springs of the first control device. The connection between increasing pressure on the metering orifice plate and decreasing pilot pressure can be realized with a separate pressure reducing valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages, features, and details of the invention are provided in the following description, which describes two embodiments of the invention in detail with reference to the drawings, in which: 
     FIG. 1 shows a hydraulic circuit diagram for controlling an automatic transmission in accordance with the invention when it is in the normal condition; 
     FIG. 2 shows the control system of FIG. 1 in the limp home condition; 
     FIG. 3 shows a hydraulic circuit diagram for controlling an automatic transmission in the limp home condition in accordance with another embodiment of the invention; 
     FIG. 4 shows a hydraulic circuit diagram for two conical disk sets of a belt-driven, conical pulley transmission; and 
     FIG. 5 shows a hydraulic circuit diagram for a starting clutch. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a hydraulic circuit diagram for the control of a continuously variable, belt-driven, conical pulley transmission, such as the transmission shown schematically in FIG.  4 . The belt-driven, conical pulley transmission shown in FIG. 4 includes an input-side conical disk pair  101  and an output-side conical disk pair  102 . The transmission of power between the two conical disk pairs  101  and  102  takes place by a steel band  103 . Each conical disk pair  101 ,  102  has an axially-movable conical disk  105 ,  107  and an axially-fixed conical disk  106 ,  108 . The axially-movable conical disks  105 ,  107  are arranged diagonally opposite each other. An axial displacement of conical disks  105 ,  107  causes the effective lever arms of the conical disk pairs  101 ,  102  to become larger or smaller in opposite directions. Axial displacement of the movable conical disks  105 ,  107  is achieved by adjusting devices  1 ,  2 . 
     Adjusting devices  1 ,  2  for changing the transmission ratio of the automatic transmission include two working chambers  3  and  4 . In order to achieve an adjustment of the transmission ratio, working chambers  3  and  4  are alternately filled with a working medium by a pump  5  in order to vary the spacing between the axially-movable conical disks  105 ,  107  and the associated axially-fixed conical disks  106  and  108 . Pump  5  has a mechanical drive that is coupled with the crankshaft or the camshaft of an internal combustion engine of a motor vehicle. In that way the flow delivered by pump  5  increases or decreases in proportion to the rotational speed of the internal combustion engine. 
     The pressure in the working chambers  3  and  4  is controlled by a first control device  6  (see FIG.  1 ). That device includes two pressure recirculation control valves  7  and  8 . Control valves  7  and  8  each include a respective guided, axially-movable spool  9 ,  10 . Spools  9  and  10  are in each case biased in an axial direction by a respective spring  11 ,  12 . On each of control valves  7  and  8  there is provided a respective port  13 ,  14  for the working medium delivered by pump  5 . The working medium is a hydraulic fluid. Ports  13  and  14  are interconnected with each other by a conduit  15 , from which extends a conduit  17 . Conduit  17  is connected with the pressure side of pump  5  by conduits  18  and  19 . 
     Additionally, ports  21  and  22  are provided on control valves  7  and  8  and are connected to an unpressurized chamber, for example a reservoir for the hydraulic fluid. Furthermore, control valves  7  and  8  also have ports  23  and  24 . Port  23  is connected with working chamber  3  (see FIG. 4) by a conduit  25 . Port  24  is connected with working chamber  4  (see FIG. 4) by a conduit  26 . The pressure recirculation of spools  9  and  10  is realized by means of conduits  32  and  33  that extend from respective conduits  25  and  26 . In conduits  32  and  33  there is arranged a respective orifice plate  32   a ,  33   a.    
     Finally, control valves  7  and  8  also have ports  27  and  28 , through which the pilot pressure acts on the end faces of spools  9  and  10  that face away from springs  11  and  12 . Ports  27  and  28  of control valves  7  and  8  are connected with a second control device  34  by conduits  29 ,  30 , and  31 . Second control device  34  is an electrically-controlled proportional valve. 
     In the normal condition of the automatic transmission, a constant pilot pressure exists in conduits  29 ,  30 , and  31 . By the electrically-controlled proportional valve  34 , the prevailing pilot pressure in conduits  29 ,  30 , and  31  can be changed as needed. 
     When the pilot pressure that exists at ports  27  and  28  of control valves  7  and  8  increases, spools  9  and  10  move against the respective opposed spring bias forces. Two control edges  56  and  57  are formed on control spool  9 . Two control edges  96  and  97  are formed on control spool  10 . When spool  9  moves toward spring  11  because of increased pilot pressure at port  27 , control edge  56  opens a connection between ports  13  and  23  on control valve  7 , and the working medium delivered by pump  5  reaches working chamber  3  through conduit  25 . At the same time, a connection is opened on control valve  8  between ports  24  and  22 , and the working medium existing in working chamber  4  can flow out into the unpressurized chamber through conduit  26 . An increase in the pilot pressure therefore leads to an increase in the pressure in working chamber  3  and a decrease in the pressure in working chamber  4 . 
     When the pilot pressure at ports  27  and  28  of control valves  7  and  8  decreases, the result is that spools  9  and  10  move in the direction of the spring force exerted by the biasing forces imposed by springs  11  and  12 . Consequently, control edge  57  opens a connection between ports  23  and  21  on control valve  7 . As a result, the working medium that exists in working chamber  3  can flow out into the unpressurized chamber through conduit  25 . At the same time, control edge  96  on spool  10  ensures that a connection between ports  14  and  24  on control valve  8  is opened. By that connection, the working medium delivered by pump  5  reaches working chamber  4  through conduit  26 . Consequently, the pressure in working chamber  4  increases and the pressure in working chamber  3  decreases. That, in turn, effects an adjustment in the transmission ratio of the automatic transmission to fast (overdrive). 
     The hydraulic circuit diagram shown in FIG. 5 represents a second adjusting device  37 . Second adjusting device  37  serves to actuate a starting clutch  39 . Adjusting device  37  for starting clutch  39  has a cylinder  36  in which a piston  38  is movably received for back and forth movement. Piston  38  is, for example, the pressure plate of a clutch, which interacts with the friction linings of a clutch disk. 
     The interior of cylinder  36  is connected with a first control device  41  (see FIG. 1) for the starting clutch by a conduit  40 . First control device  41  is a pressure recirculation control valve. In control valve  41 , a spool  42  is movably received for back and forth movement against the biasing force of a spring  43 . Control valve  41  is connected by a conduit  44  to conduit  18 , which, in turn, is connected by conduit  19  to the pressure side of pump  5 . Pilot pressure is applied through conduit  46  to the end face of spool  42  of control valve  41  that faces away from pressure-biased spring  43 . The end face of spool  42  of the control valve  41  that faces away from the biasing spring  43  is acted on by a pilot pressure through a conduit  46 . Conduit  46  is connected with a second control device  50  by a conduit  47 . Second control device  50  is an electrically-controlled proportional valve. 
     In the normal condition of the automatic transmission, the prevailing pilot pressure in conduits  46  and  47  can be varied by electrically-controlled proportional valve  50  to operate the starting clutch (not shown). On control spool  42  of control valve  41  two control edges  84  and  85  are formed. Additionally, two ports  48  and  49  to a pressure relief chamber are provided on control valve  41 . 
     When the pilot pressure in conduit  46  increases, the result is that spool  42  of control valve  41  is moved against the biasing force of spring  43 . In such a displacement of spool  42 , control edge  85  opens a connection from the interior of cylinder  36  through conduit  40  to port  48 , which is connected to the unpressurized chamber. Consequently, the pressure in the interior of cylinder  36  decreases, whereby the starting clutch engages, since the clutch is engaged when there is no pressure. 
     When the pilot pressure in conduit  46  decreases, spool  42  is moved in the opposite direction by the biasing force of spring  43  in such a way that a connection between conduit  44  and conduit  40  is opened. The result is the working medium that is delivered by pump  5  is delivered to the interior of cylinder  36 . The associated pressure increase in the interior of cylinder  36  causes the starting clutch to be disengaged. 
     Pilot pressure conduits  31  and  47  are connected with a conduit  53  by conduits  51  and  52 , in each of which an orifice plate  51   a ,  52   a , respectively, is provided. A conduit  54  extends from conduit  53  to a third control device that includes a hydraulically-operated control valve  58 . 
     A control plunger  59  is received in control valve  58  for back and forth movement against the biasing force of a spring  60 . Two control edges  88  and  89  are provided on control plunger  59 . Additionally, a first working surface  61  and a second working surface  62  are formed on control plunger  59  for the working medium delivered by pump  5 . First working surface  61  has the same size for the working medium as second working surface  62 . Beyond that, a third working surface  63  is formed on control plunger  59 , against which the regulated pilot pressure acts. 
     Two ports  64  and  65  are provided on control valve  58  that are connected to the pressure relief chamber. Additionally, a port  66  for conduit  54  is provided on control valve  58 . Moreover, control valve  58  is provided with a port  67  to which a conduit  68  is connected. Conduit  68  runs to conduit  54 . Finally, two ports  81  and  83  are provided on control valve  58 , to which conduits  80  and  82  are connected, the latter of which are connected to each other. From the junction of conduits  80  and  82  there extends a conduit  79  that runs to conduit  19 . 
     In FIG. 1 the automatic transmission is in its normal condition. In the normal condition the same working medium pressure that is delivered by pump  5  is applied to first working surface  61  and to second working surface  62 . Pilot pressure acts on third working surface  63  of control plunger  59 . When the pilot pressure at working surface  63  increases, control plunger  59  moves against the biasing force of spring  60 . As a result, control edge  88  opens a conduit between ports  65  and  66  of control valve  58 . That leads to a decrease in the pilot pressure. When the pilot pressure on third working surface  63  of control plunger  59  decreases, control plunger  59  is displaced toward port  67  because of the biasing force of spring  60 . Control edge  89  then opens a conduit between ports  81  and  66  of control valve  58 . As a result the pilot pressure increases. In the normal condition of the automatic transmission, control valve  58  acts as a pressure regulator for maintaining the pilot pressure constant. Orifice plates or throttles in conduits  51  and  52 , as well as in conduit  68 , ensure that the function of second control devices  34  and  50  is not impaired in the normal condition of the automatic transmission. 
     A constant pilot pressure serves as the supply pressure for proportional valves  34 ,  50 . Proportional valves  34 ,  50  can set a pressure of near zero to maximum pilot pressure in order to regulate the transmission ratio change and the function of the starting clutch. 
     Control devices  34 ,  50 , and  58  are connected by a switching device  69  to adjusting devices  1 ,  2  for adjusting the transmission ratio of the automatic transmission and to adjusting device  37  for the starting clutch of the transmission. In switching device  69 , a switching spool  70  is received for back and forth movement against the biasing force of a spring  71 . Switching device  69  is connected to conduit  54  by a conduit  72  and a conduit  73 , in which an orifice plate  73   a  is arranged. Additionally, a  2 / 2  switching valve  74  is connected at the end of conduit  72 . 
     In the normal condition of the automatic transmission the  2 / 2  switching valve  74  is closed. In the event of a failure of the electronic system, the  2 / 2  switching valve  74  releases a connection between conduit  72  and the pressure relief chamber. The pressure relief of conduit  72  causes the pressure at the end face of spool  70  that faces away from spring  71  to drop. Consequently, spool  70  moves upward toward conduit  72  because of the biasing force of spring  71 . Switching device  69  has a port  75  to the pressure relief chamber. Additionally, a conduit  76  is connected to switching device  69  and runs to conduit  54 . Beyond that, a conduit  77  is connected to switching device  69 , from which a conduit  78  leads to first working surface  61  for the working medium that acts on control plunger  59  and which runs to conduit  79 . A metering orifice plate  86  is provided in conduit  77  between the connection points for conduits  78  and  79 . Beyond that, two orifice plates  94  and  95  are incorporated in conduit  54 . Between orifice plates  94  and  95  is a connection point for conduit  76 . A conduit  55  extends from orifice plate  95  to switching device  69 . Adjacent to port  75  to the pressure relief chamber, switching device  69  has ports for conduits  55 ,  47 ,  76 ,  31 ,  53 ,  19 ,  77 ,  72 ,  18 ,  30 , and  46 . 
     In the normal condition of the automatic transmission, spool  70  is in its normal position as shown in FIG.  1 . In the normal position of spool  70 , proportional valve  50  is connected by conduit  47  and conduit  46  to control valve  41  for adjusting device  36 ,  37  of the starting clutch. Likewise, proportional valve  34  is connected by conduits  31 ,  30 , and  29  to control valves  7  and  8  for adjusting devices  1 ,  2  for the transmission ratio of the automatic transmission. Conduits  31  and  47  are connected to control valve  58  by conduits  51  and  52 , which are provided with orifice plates  51   a ,  52   a . The pressure that can be controlled by proportional valves  34 ,  50 , produces different adjusting and driving pressures. The adjustment of adjusting devices  1 ,  2 , and  37  is controlled by control devices  6  and  41  through proportional valves  34  and  50 . 
     In the event of a failure of the electronic system, electrically-controlled proportional valves  34  and  50  can also fail. In such a case, spring-force-actuated  2 / 2  switching valve  74  also opens, and spool  70  moves into the emergency position shown in FIG.  2 . 
     FIG. 2 shows switching device  69  in the emergency position. Equivalent parts are provided with the same reference numerals so that the description of FIG. 1 applies. In the limp home position of spool  70 , conduit  54 , in which both orifice plates  94  and  95  are arranged, is connected with the pressure relief chamber by conduit  55 . The pressure level is decreased because of the metering orifice plates  94 ,  95 . As a result, the starting clutch engages before the transmission ratio control takes hold. 
     Additionally, conduit  76  is connected by conduit  46  with the end face of spool  42  that faces away from spring  43  of control valve  41 . Moreover, conduit  53  is connected by conduits  30  and  29  with ports  27  and  28  of control valves  7  and  8 . Finally, connecting conduit  77 , which is provided between conduits  78  and  79  and which includes metering orifice plate  86 , is connected by conduits  18 ,  17 , and  44  to control valves  7 ,  8 , and  41 . Conduit  77  includes two sections  77   a  and  77   b . Section  77   a  extends from switching device  69  to the connection point of conduit  78 . Section  77   b  extends between the two connection points of conduits  78  and  54 . Metering orifice plate  86  is arranged in section  77   b , through which no medium flows in the normal condition of the automatic transmission, so that the same pressure exists on working surfaces  61  and  62  of control plunger  59 . 
     In the limp home condition of the automatic transmission shown in FIG. 2, the working medium delivered by pump  5  through conduits  19 ,  79 , and  77 , as well as orifice plate  86  and conduit  78 , reaches first working surface  61  for the working medium on control plunger  59  of third control device  58 . Additionally, the working medium delivered by pump  5  through conduits  19 ,  79 , and  82  reaches second working surface  62  for the working medium on control plunger  59  of third control device  58 . 
     When the working medium flows through orifice plate  86  in conduit section  77   b , which is the case in FIG. 2, the result is a drop in pressure. Because of the drop in pressure at metering orifice plate  86 , a lower working medium pressure acts on first working surface  61  of control plunger  59  than on second working surface  62 . Since first working surface  61  is the same size as second working surface  62 , the higher pressure acting on second working surface  62  causes control plunger  59  to be displaced against the biasing force of spring  60  toward conduit  78 —to the left as viewed in FIG.  2 . That results in a decrease in the pilot pressure acting on third working surface  63 . 
     With increasing rotational speed of the internal combustion engine, the volumetric flow delivered by pump  5  increases. That causes the pressure difference across metering orifice plate  86  also to increase. That, in turn, results in a further decrease in the pilot pressure. As a result, it is established that the pilot pressure decreases with increasing rotational speed of the internal combustion engine. Conversely, the pilot pressure increases with decreasing rotational speed. 
     The pilot pressure, which is dependent upon the speed of the internal combustion engine, acts on third working surface  63  of control plunger  59  of third control device  58 . Third working surface  63  for the pilot pressure is in communication with first control device  6  of adjusting device  1 ,  2  for the transmission ratio of the automatic transmission by conduit  68 , conduit  54 , conduit  53 , conduit  30 , and conduit  29 . Additionally, third working surface  63  for the pilot pressure is in communication with first control device  41  for adjusting device  37  for the starting clutch by conduits  68 ,  54 ,  76 , and  46 . A drop in the pilot pressure is achieved through both orifice plates  94  and  95  in conduit  54 , which is connected to the pressure relief chamber. The drop in pressure results in a higher pressure acting on working surface  63  of control plunger  59  and at ports  27 ,  28  than on the end face of spool  42  that faces away from spring  43 . By arranging orifice plate  94  between the intersection of conduits  53 ,  54  and the intersection of conduits  76 ,  54 , the pilot pressure for control device  6  for the transmission ratio is greater than the pilot pressure for control device  41  for the starting clutch because of the arrangement of 94 between 54/30 and 76/46. The pressure drop across orifice plates  94  and  95  only occurs when conduit  55  is relieved into the tank, which represents a pressure relief chamber, through port  75 . 
     Control valve  41  for the starting clutch has a falling characteristic line, that is, a decreasing pilot pressure ensures an increase in the clutch pressure. As described above, the pilot pressure decreases with increasing rotational speed. In the limp home condition, the clutch will therefore be engaged at increasing rotational speed of the internal combustion engine. 
     In the embodiment shown in FIGS. 1 and 2, control valve  41  is arranged in such a way that the starting clutch engages at a decreasing pilot pressure. If that is not the case, a so-called reversing spool is used. The reversing spool serves to produce a relationship between increasing pressure difference across the metering orifice plate and the necessary pilot pressure for operating the starting clutch and the transmission ratio adjustment. Depending upon the connection of working surfaces  61 ,  62 , a direct or indirect relationship is produced. 
     FIG. 3 is a hydraulic circuit diagram of an automatic transmission in accordance with another embodiment of the invention. The control shown in FIG. 3 largely corresponds with the control shown in FIGS. 1 and 2. Equivalent parts are identified by the same reference numerals so that to that extent reference should be made to the description of FIGS. 1 and 2. In the following, only the differences between the two embodiments are pointed out. 
     In the embodiment shown in FIG. 3, adjusting device  37  for the starting clutch is controlled by a control valve  100 . Control valve  100  is a classic pressure reducing valve. With increasing pilot pressure, the output pressure decreases. In order to ensure engagement of the clutch in the limp home condition at an increasing rotational speed of the internal combustion engine, a reversing spool  112  is required. Reversing spool  112  ensures that the change in pilot pressure occurs exactly opposite from that in the embodiment shown in FIGS. 1 and 2. In other respects the function of the arrangement shown in FIG. 3 is identical to the arrangement shown in FIGS. 1 and 2. 
     The claims included in the application are illustrative and are without prejudice to acquiring wider patent protection. The applicant reserves the right to claim additional combinations of features disclosed in the specification and/or drawings. 
     The references contained in the dependent claims point to further developments of the object of the main claim by means of the features of the particular claim; they are not to be construed as renunciation to independent, objective protection for the combinations of features of the related dependent claims. 
     Although the subject matter of the dependent claims can constitute separate and independent inventions in the light of the state of the art on the priority date, the applicants reserve the right to make them the subject of independent claims or separate statements. They can, moreover, also embody independent inventions that can be produced from the independent developments of the subject matter of the included dependent claims. 
     The exemplary embodiments are not to be considered to be limitations of the invention. On the contrary, many changes and variations are possible within the scope of the invention in the existing disclosure, in particular such variants, elements, and combinations and/or materials which, for example, are inventive by combining or modifying single features that are in combination and are described individually in relation to the general specification and embodiments as well as the claims and shown in the drawings, as well as elements or method steps that can be derived by a person skilled in the art in the light of the disclosed solutions of the problem, and which by means of combined features lead to a new object or new method steps or sequences of method steps, as well as manufacturing, testing and operational procedures.