Abstract:
The present invention provides a method for approximating the flow rate of hydraulic fluid in an automatic transmission. The method includes estimating a flow rate value for each of a plurality of temperatures. Thereafter, the current transmission temperature is measured. The flow rate corresponding to the current transmission temperature is then learned in the following manner. The process of learning the flow rate initially includes identifying the presence of a predefined shift aberration. If the predefined shift aberration was not identified, the flow rate estimation corresponding to the current transmission temperature is iteratively adjusted. If the predefined shift aberration was identified, the flow rate estimation corresponding to the current transmission temperature is reversed by one iterative step thereby providing the learned flow rate value for the current transmission temperature.

Description:
TECHNICAL FIELD 
     The present invention pertains generally to a method for learning the flow rate of hydraulic fluid in an automatic transmission. 
     BACKGROUND OF THE INVENTION 
     Generally, a motor vehicle automatic transmission includes a number of gear elements coupling its input and output shafts, and a related number of torque establishing devices such as clutches and brakes that are selectively engageable to activate certain gear elements for establishing a desired speed ratio between the input and output shafts. As used herein, the term “torque transmitting device” will be used to refer to brakes as well as clutches. 
     The transmission input shaft is connected to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is connected directly to the vehicle wheels. Shifting from one speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing one or more clutches (off-going) associated with the current or attained speed ratio and applying one or more clutches (on-coming) associated with the desired or commanded speed ratio. 
     The speed ratio is defined as the transmission input speed or turbine speed divided by the output speed. Thus, a low gear range has a high speed ratio and a higher gear range has a lower speed ratio. Shifts from one speed ratio to another require precise timing in order to achieve high quality shifting. The quality of shift depends on the cooperative operation of several functions, such as pressure changes within the clutch apply chambers and the timing of control events. Moreover, manufacturing tolerances in each transmission, changes due to wear, variations in oil quality and temperature, etc., lead to shift quality degradation. 
     SUMMARY OF THE INVENTION 
     The method of the present invention includes estimating a flow rate value for each of a plurality of temperatures. Thereafter, the current transmission temperature is measured. The flow rate corresponding to the current transmission temperature is then learned in the following manner. The process of learning the flow rate initially includes identifying the presence of a predefined shift aberration. If the predefined shift aberration was not identified, the flow rate estimation corresponding to the current transmission temperature is iteratively adjusted. If the predefined shift aberration was identified, the flow rate estimation corresponding to the current transmission temperature is reversed by one iterative step thereby providing the learned flow rate value for the current transmission temperature. 
     The process of learning the flow rate may be performed only after the completion of a shift from which the flow rate is to be learned. 
     The process of learning the flow rate may be performed only if the measured transmission temperature is outside a predefined normal operating temperature range. 
     The process of learning the flow rate may be performed only if the completed shift occurred at the normal shift point. 
     The process of learning the flow rate may be performed only if the maximum engine speed during the shift from which the flow rate is to be learned was less than a predefined engine speed value. 
     The process of learning the flow rate may be performed only if a transmission pump speed is adequate to regulate pressure. 
     The method for approximating the flow rate of hydraulic fluid in an automatic transmission may also include storing the learned flow rate value into a non-volatile memory device. 
     The method for approximating the flow rate of hydraulic fluid in an automatic transmission may also include decreasing the iterative step after the flow rate value has been learned. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a vehicle power train in accordance with the present invention; 
         FIG. 2  is an exemplary table on which estimated flow rate values corresponding to a plurality of temperatures are stored; 
         FIG. 3  is a flow chart illustrating a method of estimating flow rate based on a preceding upshift; and 
         FIG. 4  is a flow chart illustrating a method of estimating flow rate based on a preceding regulated closed throttle downshift or a regulated garage shift. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings wherein like characters represent the same or corresponding parts through the several views, there is shown in  FIG. 1  a schematic illustration of an exemplary vehicle power train  10 . It should be appreciated that the power train  10  is shown for illustrative purposes, and that the present invention is also applicable to alternate power train configurations. The vehicle power train  10  preferably includes an engine  12 , a transmission  14 , and a torque converter  16  providing a fluid coupling between the engine  12  and a transmission input shaft  18 . 
     A torque converter clutch or TCC  19  is selectively engaged under certain conditions to provide a mechanical coupling between engine  12  and transmission input shaft  18 . The transmission output shaft  20  is coupled to the driving wheels of the vehicle in one of several conventional ways. The illustrated embodiment depicts a four-wheel-drive (4WD) application in which the output shaft  20  is connected to a transfer case  21  that is also coupled to a rear drive shaft R and a front drive shaft F. Typically, the transfer case  21  is manually shiftable to selectively establish one of several drive conditions, including various combinations of two-wheel-drive and four-wheel drive, and high or low speed range, with a neutral condition occurring intermediate the two and four wheel drive conditions. 
     The transmission  14  has three inter-connected planetary gear sets, designated generally by the reference numerals  23 ,  24  and  25 . The planetary gear set  23  includes a sun gear member  28 , a ring gear member  29 , and a planet carrier assembly  30 . The planet carrier assembly  30  includes a plurality of pinion gears rotatably mounted on a carrier member and disposed in meshing relationship with both the sun gear member  28  and the ring gear member  29 . The planetary gear set  24  includes a sun gear member  31 , a ring gear member  32 , and a planet carrier assembly  33 . The planet carrier assembly  33  includes a plurality of pinion gears rotatably mounted on a carrier member and disposed in meshing relationship with both the sun gear member  31  and the ring gear member  32 . The planetary gear set  25  includes a sun gear member  34 , a ring gear member  35 , and a planet carrier assembly  36 . The planet carrier assembly  36  includes a plurality of pinion gears rotatably mounted on a carrier member and disposed in meshing relationship with both the sun gear member  34  and the ring gear member  35 . 
     The input shaft  18  continuously drives the sun gear  28  of gear set  23 , selectively drives the sun gears  31 ,  34  of gear sets  24 ,  25  via clutch C 1 , and selectively drives the carrier  33  of gear set  24  via clutch C 2 . The ring gears  29 ,  32 ,  35  of gear sets  23 ,  24 ,  25  are selectively connected to ground  42  via clutches (i.e., brakes) C 3 , C 4  and C 5 , respectively. 
     The state of the clutches C 1 -C 5  (i.e., engaged or disengaged) can be controlled to provide six forward speed ratios (1, 2, 3, 4, 5, 6), a reverse speed ratio (R) or a neutral condition (N). For example, the first forward speed ratio is achieved by engaging clutches C 1  and C 5 . Shifting from one forward speed ratio to another is generally achieved by disengaging one clutch (referred to as the off-going clutch) while engaging another clutch (referred to as the on-coming clutch). For example, the transmission  14  is downshifted from second to first by disengaging clutch C 4  while engaging clutch C 5 . 
     The torque converter clutch  19  and the transmission clutches C 1 -C 5  are controlled by an electro-hydraulic control system, generally designated by reference numeral  44 . The hydraulic portions of the control system  44  include a pump  46  which draws hydraulic fluid from a reservoir  48 , a pressure regulator  50  which returns a portion of the pump output to reservoir  48  to develop a regulated pressure in line  52 , a secondary pressure regulator valve  54 , a manual valve  56  manipulated by the driver of the vehicle, and a number of solenoid-operated fluid control valves  58 ,  60 ,  62  and  64 . 
     The electronic portion of the electro-hydraulic control system  44  is primarily embodied in the transmission control unit  66 , or controller, which is microprocessor-based and conventional in architecture. The transmission control unit  66  controls the solenoid-operated fluid control valves  58 - 64  based on a number of inputs  68  to achieve a desired transmission speed ratio. Such inputs include, for example, signals representing the transmission input speed TIS, a driver torque command TQ, the transmission output speed TOS, and the hydraulic fluid temperature Tsump. Sensors for developing such signals may be conventional in nature, and have been omitted for simplicity. 
     The control lever  82  of manual valve  56  is coupled to a sensor and display module  84  that produces a diagnostic signal on line  86  based on the control lever position; such signal is conventionally referred to as a PRNDL signal, since it indicates which of the transmission ranges (P, R, N, D or L) has been selected by the vehicle driver. Finally, fluid control valves  60  are provided with pressure switches  74 ,  76 ,  78  for supplying diagnostic signals to control unit  66  on lines  80  based on the respective relay valve positions. The control unit  66 , in turn, monitors the various diagnostic signals for the purpose of electrically verifying proper operation of the controlled elements. 
     The solenoid-operated fluid control valves  58 - 64  are generally characterized as being either of the on/off or modulated type. To reduce cost, the electro-hydraulic control system  44  is configured to minimize the number of modulated fluid control valves, as modulated valves are generally more expensive to implement. To this end, fluid control valves  60  are a set of three on/off relay valves, shown in  FIG. 1  as a consolidated block, and are utilized in concert with manual valve  56  to enable controlled engagement and disengagement of each of the clutches C 1 -C 5 . Valves  62 ,  64  are of the modulated type. For any selected ratio, the control unit  66  activates a particular combination of relay valves  60  for coupling one of the modulated valves  62 ,  64  to the on-coming clutch, and the other one of the modulated valves  62 ,  64  to the off-going clutch. 
     The modulated valves  62 ,  64  each comprise a conventional pressure regulator valve biased by a variable pilot pressure that is developed by current controlled force motors (not shown). Fluid control valve  58  is also a modulated valve, and controls the fluid supply path to converter clutch  19  in lines  70 ,  72  for selectively engaging and disengaging the converter clutch  19 . The transmission control unit  66  determines pressure commands for smoothly engaging the on-coming clutch while smoothly disengaging the off-going clutch to shift from one speed ratio to another, develops corresponding force motor current commands, and then supplies current to the respective force motors in accordance with the current commands. Thus, the clutches C 1 -C 5  are responsive to the pressure commands via the valves  58 - 64  and their respective actuating elements (e.g., solenoids, current-controlled force motors). 
     As indicated above, each shift from one speed ratio to another includes a fill or preparation phase during which an apply chamber  91  of the on-coming clutch is filled in preparation for torque transmission. Fluid supplied to the apply chamber compresses an internal return spring (not shown), thereby stroking a piston (not shown). Once the apply chamber is filled, the piston applies a force to the clutch plates, developing torque capacity beyond the initial return spring pressure. Thereafter, the clutch transmits torque in relation to the clutch pressure, and the shift can be completed using various control strategies. A typical control strategy involves commanding a maximum on-coming clutch pressure for an empirically determined clutch fill time. The clutch fill time can be calculated based on the clutch volume and the flow rate according to the equation: clutch fill time=“clutch volume”/“flow rate”. The “clutch volume” is the volume of fluid required to fill a clutch apply chamber and thereby cause the clutch to gain torque capacity. The “flow rate” is the rate at which hydraulic fluid is transferred to the clutch apply chamber. 
     According to the preferred embodiment, the on-coming clutch volume is calculated or “learned” in the manner disclosed in commonly assigned U.S. Pat. No. 6,915,890 issued to Whitton et al., and which is hereby incorporated by reference in its entirety. Advantageously, the “learned” on-coming clutch volume can account for build variations and tolerances, and can also account for variation over time due to wear. For purposes of the present invention, a “learned value” is value that is estimated using an adaptive process. The adaptive process is so named because the process is adaptable or variable to reflect new information and thereby account for changes over time. 
     It has been observed that the flow rate of the hydraulic fluid being transferred to a clutch apply chamber is temperature dependent. Conventionally, the flow rate was measured at a wide variety of temperatures to generate a flow rate curve. Generating flow rate curves requires extensive hot and cold testing such that the flow rate curves are expensive and time consuming to produce. It is therefore an object of the present invention learn the flow rate without reliance on extensive testing. 
     According to the preferred embodiment of the present invention, the flow rate is first roughly estimated in a conventional manner (e.g., based on a nominal flow rate or on previously compiled test data) at a plurality of temperatures, and is thereafter learned at such temperatures to provide a more accurate estimation. The learned flow rate values and their corresponding temperatures are preferably stored as a table in a non-volatile memory device such as the non-volatile random access memory (NOVRAM)  96 . Advantageously, the NOVRAM  96  retains information after losing power such that the flow rate data saved therein is not lost when the vehicle is shut off. 
     Referring to  FIG. 2 , an exemplary flow rate table  98  as stored in the NOVRAM  96  (shown in  FIG. 1 ) is shown. The flow rate data of  FIG. 2  is representative of the initial rough estimates for flow rate at the plurality of different temperatures (i.e., −40, 0, 40, 80 and 120 degrees Celsius). It should be appreciated that table  98  of  FIG. 2  is merely illustrative, and that the estimated flow rate values and/or the listed temperatures may be varied as required to meet the needs of a particular application. 
     Each time the flow rate is learned at one of the temperatures included in table  98 , the learned flow rate value is saved to the table thereby replacing any previously estimated value. The learned flow rate data is retrievable from the table  98  to calculate the clutch fill time of the on-coming clutch for subsequent ratio changes. If a measured temperature falls between two temperatures included in table  98 , the corresponding flow rate can be obtained by interpolation. 
     Referring to  FIG. 3 , a method  100  (also referred to herein as algorithm  100 ) for learning a flow rate during an upshift is shown. More precisely,  FIG. 3  shows a block diagram representing steps performed by a control device such as the control unit  66  (shown in  FIG. 1 ). 
     At step  102 , the algorithm  100  determines whether an upshift from which the flow rate is to be learned is completely finished. This step is implemented to ensure the upshift has been completed before the process of learning from the upshift is initiated. If, at step  102 , the upshift has not yet been completed, the algorithm  100  repeats step  102 . If, at step  102 , the upshift has been completed, the algorithm  100  proceeds to step  104 . 
     At step  104 , the algorithm  100  determines whether the current transmission temperature is outside a predefined normal operating range (e.g., between 70 and 100 degrees Celsius). The current transmission temperature is obtainable using temperature sensors (not shown) configured to measure and transmit temperature data to the control unit  66  (shown in  FIG. 1 ). The flow rate within the normal operating range is preferably estimated based on a nominal hydraulic fluid flow rate value and the method of the present invention is applied to learn the flow rate only when the current transmission temperature is outside this range. Therefore, if the current transmission temperature is within the predefined normal operating range, the algorithm  100  proceeds to step  106  at which the algorithm  100  waits for the next upshift, and thereafter the algorithm  100  returns to step  102 . If the current transmission temperature is outside the predefined normal operating range, the algorithm  100  proceeds to step  108 . 
     At step  108 , the algorithm  100  determines whether the minimum throttle input to the engine  12  (shown in  FIG. 1 ) during the upshift was greater than a predetermined amount. This step is implemented because the method of the present invention learns the flow rate during an upshift in response to an engine flare condition, which is described in detail hereinafter, and such engine flare may not be detectable unless the minimum engine throttle is greater than a predetermined amount. Therefore, if the minimum engine throttle is below the predetermined amount, the algorithm  100  proceeds to step  106  at which the algorithm  100  waits for the next upshift, and thereafter the algorithm  100  returns to step  102 . If the minimum engine throttle is equal to or greater than the predetermined amount, the algorithm  100  proceeds to step  110 . 
     At step  110 , the algorithm  100  determines whether the previous upshift occurred at the normal shift point. This step is implemented because the method of the present invention preferably does not learn from an upshift unless it occurs at the normal shift point. As an example, if the vehicle operator overrides the normally scheduled shift point by manually moving the shift selector (not shown), timing information pertaining to the manual shift is not implemented to learn the flow rate. Therefore, if the previous upshift did not occur at the normal shift point, the algorithm  100  proceeds to step  106  at which the algorithm  100  waits for the next upshift, and thereafter the algorithm  100  returns to step  102 . If the previous upshift did occur at the normal shift point, the algorithm  100  proceeds to step  112 . 
     At step  112 , the algorithm  100  determines whether the maximum engine speed during the upshift was less than a predetermined speed. This step is implemented because the method of the present invention learns flow rate in response to an engine flare condition, which is described in detail hereinafter, and such engine flare may not be detectable if the engine speed is high enough to induce engine output limits such as with a governor (not shown). Therefore, the maximum engine speed during the upshift was equal to or greater than the predetermined speed, the algorithm  100  proceeds to step  106  at which the algorithm  100  waits for the next upshift, and thereafter the algorithm  100  returns to step  102 . If the maximum engine speed during the upshift was less than the predetermined speed, the algorithm  100  proceeds to step  114 . 
     At step  114 , the algorithm  100  determines if engine flare has been identified. Engine flare is a shift aberration wherein the on-coming clutch gains capacity late resulting in a condition similar to the neutral gear speed ratio. Engine flare is preferably identified when the turbine speed or the transmission input speed, which can be measured with a speed sensor, rises more than a predetermined amount (e.g., 50 rpm) above the commanded gear speed. If engine flare has not been identified at step  114 , the algorithm  100  proceeds to step  116 . If engine flare has been identified at step  114 , the algorithm  100  proceeds to step  118 . 
     At step  116 , the algorithm  100  iteratively increases the estimated flow rate value in the table  98  (shown in  FIG. 2 ) corresponding to the current transmission temperature. The “iterative step” is the amount by which the flow rate value is increased, and is configurable to meet the needs of a particular application. According to the preferred embodiment, the iterative step is larger before a flow rate value is learned for the first time, and after a particular flow rate value has been learned the iterative step is reduced. As an example, the iterative step before a flow rate value is learned may be 10 cc/second, and thereafter be reduced to 2 cc/second. If the current transmission temperature falls between two of the temperatures listed in table  98 , a flow rate estimation is obtainable by interpolating or scaling between the flow rate values in table  98  that correspond to the two closest temperatures. 
     At step  118 , the algorithm  100  reduces the estimated flow rate value in the table  98  (shown in  FIG. 2 ) corresponding to the current transmission temperature by one iterative step. As engine flare was identified at step  114 , the estimated flow rate value used to calculate clutch fill time during the previous ratio change is likely to be too high. Therefore, the estimated flow rate is reduced at step  118  by one iterative step to provide a closer approximation of the actual flow rate. The iteratively reduced flow rate is the “learned” flow rate value for the current transmission temperature and is saved into the table  98 . If the current transmission temperature falls between two of the temperatures listed in table  98 , a flow rate estimation is obtainable by interpolating or scaling between the flow rate values in table  98  that correspond to the two closest temperatures. Also at step  118 , after the flow rate value has been learned as described hereinabove, the iterative step for this value is preferably reduced to a minimal value (e.g., 2 cc/second) so that the process of learning can continue throughout the life of the vehicle and thereby account for changes to the system over time. The reduction of the iterative step is optional and is predicated on the assumption that the previously learned flow rate value is close to the actual and therefore any changes to the learned flow rate should be relatively small. 
     Although the present invention has been described only as being applicable to upshifts, other shift types may be envisioned. Referring to  FIG. 4 , a method  130  (also referred to herein as algorithm  130 ) for learning a flow rate during a “regulated closed-throttle downshifts” or a “regulated garage shift” is shown. More precisely,  FIG. 4  shows a block diagram representing steps performed by a control device such as the control unit  66  (shown in  FIG. 1 ). For purposes of the present invention, the term “regulated” refers a shift which takes place while the transmission pump  46  (shown in  FIG. 1 ) is capable of meeting the pressure requirements of the hydraulic system. A non-regulated shift may take place, for instance, when the pump  46  is being driven by the engine  12  (shown in  FIG. 1 ) and the engine  12  is operating at low speeds. A “closed throttle downshift” is a downshift taking place with zero throttle input to the engine  12 . A “garage shift” is a shift from neutral to drive or from neutral to reverse. 
     At step  132 , the algorithm  130  determines whether the “regulated closed-throttle downshift” or the “regulated garage shift” from which the flow rate is to be learned is completely finished. This step is implemented to ensure the shift has been completed before the process of learning from the shift is initiated. If, at step  132 , the shift has not yet been completed, the algorithm  130  repeats step  132 . If, at step  132 , the shift has been completed, the algorithm  130  proceeds to step  134 . 
     At step  134 , the algorithm  130  determines whether the current transmission temperature is outside a predefined normal operating range (e.g., between 70 and 100 degrees Celsius). The current transmission temperature is obtainable using temperature sensors (not shown) configured to measure and transmit temperature data to the control unit  66  (shown in  FIG. 1 ). The flow rate within the normal operating range is preferably estimated based on a nominal hydraulic fluid flow rate value and the method of the present invention is applied to learn the flow rate only when the current transmission temperature is outside this range. Therefore, if the current transmission temperature is within the predefined normal operating range, the algorithm  130  proceeds to step  136  at which the algorithm  130  waits for the next “regulated closed-throttle downshift” or the next “regulated garage shift”, and thereafter the algorithm  130  returns to step  132 . If the current transmission temperature is outside the predefined normal operating range, the algorithm  130  proceeds to step  138 . 
     At step  138 , the algorithm  130  determines whether the maximum throttle input to the engine  12  is less than a predetermined amount. This step is implemented because the method of the present invention learns the flow rate during a “regulated closed-throttle downshift” or a “regulated garage shift” in response to an overfill condition, which is described in detail hereinafter, and such overfill may be falsely detected unless the maximum engine throttle is less than a predetermined amount. If throttle is applied during a “regulated closed-throttle downshift”, the increase in turbine speed could be caused by either an overfilled condition or the off-going clutch releasing prematurely and letting the input speed be increased by the increase in engine torque. However, if throttle is near zero, torque is neutral or negative and an increase in input speed would only be caused by an overfilled condition. Therefore, if the maximum engine throttle is greater than or equal to the predetermined amount, the algorithm  130  proceeds to step  136  at which the algorithm  130  waits for the next “regulated closed-throttle downshift” or the next “regulated garage shift”, and thereafter the algorithm  130  returns to step  132 . If the maximum engine throttle is less than the predetermined amount, the algorithm  130  proceeds to step  140 . 
     At step  140 , the algorithm  130  determines whether the previous “regulated closed-throttle downshift” or “regulated garage shift” occurred at the normal shift point. This step is implemented because the method of the present invention preferably does not learn from a shift unless it occurs at the normal shift point. As an example, if the vehicle operator overrides the normally scheduled shift point by manually moving the shift selector (not shown), timing information pertaining to the manual shift is not implemented to learn the flow rate. Therefore, if the previous “regulated closed-throttle downshift” or “regulated garage shift” did not occur at the normal shift point, the algorithm  130  proceeds to step  136  at which the algorithm  130  waits for the next “regulated closed-throttle downshift” or the next “regulated garage shift”, and thereafter the algorithm  130  returns to step  132 . If the previous “regulated closed-throttle downshift” or “regulated garage shift” did occur at the normal shift point, the algorithm  130  proceeds to step  142 . 
     At step  142 , the algorithm  130  determines whether speed at which the transmission pump  46  (shown in  FIG. 1 ) is being driven is sufficient to meet the needs of the hydraulic system. This step is implemented to ensure the previous shift was regulated because, as previously indicated, the method  130  is preferably only applied to regulated shifts. The determination made at step  142  may be based on a conventional speed sensor attached to the engine  12  and/or the pump  46 . If the transmission pump speed is not sufficient to meet the needs of the hydraulic system, the algorithm  130  proceeds to step  136  at which the algorithm  130  waits for the next “regulated closed-throttle downshift” or the next “regulated garage shift”, and thereafter the algorithm  130  returns to step  132 . If the transmission pump speed is sufficient to meet the needs of the hydraulic system, the algorithm  130  proceeds to step  144 . 
     At step  144 , the algorithm  100  determines if an overfill condition has been identified. Overfill is a shift aberration wherein the on-coming clutch gains capacity too soon. Overfill during a “regulated closed-throttle downshift” is preferably identified when the turbine speed or the transmission input speed, which can be measured with a speed sensor, increases before it is scheduled to do so. Overfill during a “regulated garage shift” is preferably identified when the turbine speed or the transmission input speed, which can be measured with a speed sensor, decreases before it is scheduled to do so. If overfill has not been identified at step  144 , the algorithm  130  proceeds to step  146 . If overfill has been identified at step  144 , the algorithm  130  proceeds to step  148 . 
     At step  146 , the algorithm  130  iteratively decreases the estimated flow rate value in the table  98  (shown in  FIG. 2 ) corresponding to the current transmission temperature. According to the preferred embodiment, the iterative step is larger before a flow rate value is learned for the first time, and after a particular flow rate value has been learned the iterative step is reduced. If the current transmission temperature falls between two of the temperatures listed in table  98 , a flow rate estimation is obtainable by interpolating or scaling between the flow rate values in table  98  that correspond to the two closest temperatures. 
     At step  148 , the algorithm  130  increases the estimated flow rate value in the table  98  (shown in  FIG. 2 ) corresponding to the current transmission temperature by one iterative step. As overfill was identified at step  144 , the estimated flow rate value used to calculate clutch fill time during the previous ratio change is likely to be too low. Therefore, the estimated flow rate is increased at step  148  by one iterative step to provide a closer approximation of the actual flow rate. The iteratively increased flow rate is the “learned” flow rate value for the current transmission temperature and is saved into the table  98 . If the current transmission temperature falls between two of the temperatures listed in table  98 , a flow rate estimation is obtainable by interpolating or scaling between the flow rate values in table  98  that correspond to the two closest temperatures. Also at step  148 , after the flow rate value has been learned as described hereinabove, the iterative step for this value is preferably reduced to a minimal value (e.g., 2 cc/second) so that the process of learning can continue throughout the life of the vehicle and thereby account for changes to the system over time. The reduction of the iterative step is optional and is predicated on the assumption that the previously learned flow rate value is close to the actual and therefore any changes to the learned flow rate should be relatively small. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.