Patent Publication Number: US-RE39598-E

Title: Variable resistance shift rail detent assembly and shift control method employing same

Description:
RELATED APPLICATIONS 
     This application is a divisional application of reissue application Ser. No.  10 / 124 , 934 , filed on Apr.  18 ,  2002 .  
     This application is a continuation of U.S. Ser. No. 08/928,234 now abandoned filed Sep. 12, 1997, and assigned to EATON CORPORATION, the assignee of this application. 
     This application is related to U.S. Ser. No. 08/646,225 filed May 6, 1996, now U.S. Pat. No. 5,785,543, entitled SHIFT LEVER ASSEMBLY FOR MINIMIZING JUMPOUT and Ser. No. 08/902,603 filed Aug. 7, 1997, now U.S. Pat. No. 5,904,635 entitled PARTIALLY AUTOMATED LEVER-SHIFTED MECHANICAL TRANSMISSION SYSTEM, both assigned to EATON CORPORATION, the assignee of this application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to shift rail detent mechanisms for providing a selectively variable resistance to axial or rotational movement of a shift rail for minimizing the occurrence of jumpout. In a particular preferred embodiment, the present invention relates to such a detent mechanism for a lever-shifted transmission system having means to determine a driver intent to initiate or continue a lever shift and, upon sensing such an intent, to cause the detent mechanism to provide a decreased resistance to shift rail movement. 
     2. Description of the Prior Art 
     Manually shifted vehicular transmissions of the simple and/or compound types and of the synchronized, blocked and/or non-synchronized types are well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 5,000,060 and 5,390,561, the disclosures of which are incorporated herein by reference. 
     The prior art manually shifted transmissions, especially as utilized for heavy-duty vehicles such as straight trucks and conventional (i.e., not cab-over-engine) tractor/semi-trailers, utilized a manually manipulated shift lever extending upwardly from a shift tower subassembly mounted directly on the transmission housing and interacted with a multiple-rail or single shift shaft shifting mechanism of the types shown in U.S. Pat. Nos. 4,455,883; 4,550,627; 4,920,815 and 5,272,931, the disclosures of which are incorporated herein by reference. 
     While such transmissions are widely used and commercially successful, they are not totally satisfactory, as under certain severe road conditions, the transmissions may experience shift lever-induced jumpout (i.e., unintended disengagement of a gear ratio). This situation usually is associated with transmissions utilized in relatively heavy-duty vehicles (i e., such as MVMA Class 5 and larger vehicles), which tend to have relatively long shift levers having relatively large shift knobs, often including master valving for controlling range and/or splitter shifts, at the ends thereof. 
     As is known in the prior art, shift rail detent mechanisms are used to maintain the shift rails in a fixed position to resist jumpout, such as shift lever-induced jumpout. Examples of such detect mechanisms may be seen by reference to U.S. Pat. Nos. 4,550,627; 4,614,126; 4,920,815; 5,000,060 and 5,350,561, the disclosures of which are incorporated herein by reference. Shift lever detents are also useful to maintain a transmission in neutral when the engine is left running to keep the heater operational. Such mechanisms are not totally satisfactory, as the magnitude of resistance to shift rail movement needed to provide a significant resistance to jumpout or to resist accidental shifting from neutral, often objectionably increased the operator effort associated with a lever shift. 
     Partially automated mechanical transmission systems providing automatic assistance, such as automatic engine fuel control, for manual lever-shifted transmissions are known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,593,580; 5,569,115; 5,571,059; 5,573,477 and 5,582,558, the disclosures of which are incorporated herein by reference, and to co-pending U.S. Ser. Nos. 08/649,829 now U.S. Pat. No. 5,682,790, 08/649,830 now U.S. Pat. No. 5,735,771, 08/649,831, now abandoned, and 08/666,164, all assigned to EATON CORPORATION, the assignee of this application. These systems utilize automatic engine fueling controls and/or range and/or splitter shift actuators, actuated by a driver indication of an intent to shift, allowing an old gear to be disengaged and a new or target gear to be engaged without requiring the driver to manipulate the clutch pedal (required only for vehicle launch and stop) or the throttle pedal. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, the drawbacks of the prior art are minimized or overcome by the provision of a selectively variable detent mechanism for a transmission system having a means for sensing a driver intent to initiate a lever shift, which provides a significant resistance to shift lever-induced jumpout without objectionably increasing the operator effort required to make an intended lever shift. 
     The foregoing is accomplished by providing a detent mechanism which may be controlled to a first condition for providing a greater resistance to shift rail movement or to a second condition for providing a lesser resistance to shift rail movement. Upon determining a driver intent to initiate a lever shift, and preferably until configuring engagement of a target gear ratio, the detent mechanism is caused to assume the second condition wherein detent resistance to shift rail movement (and, thus, to lever shifts) is minimized. When not at the initiation of or during a lever shift operation, the detent mechanism is caused to assume the first condition wherein a significant detent resistance to shift rail movement (and, thus, to shift lever-induced jumpout) is applied. 
     Alternatively, operation of the vehicle heater when the transmission is allowed to remain in neutral may cause the detent mechanism to assume the first condition. 
     Accordingly, it is an object of the present invention to provide a new and improved shift rail detent mechanism for mechanical transmission systems. 
     This and other objects and advantages of the present invention will become apparent from a reading of the following description of the preferred embodiment taken in connection with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a symbolic representation of a vehicular drive line utilizing the improved shift rail detent assembly of the present invention. 
         FIG. 2  is a symbolic illustration of the parameters affecting shift lever-induced jumpout torque. 
         FIG. 3  is a symbolic illustration of the parameters affecting detent torque. 
         FIGS. 4A-4C  are symbolic representations of a heavy-duty, automatically assisted, manually shifted transmission system of the type advantageously utilizing the present invention. 
         FIGS. 5 and 6  are schematic illustrations of alternate variable resistance shift rail detent mechanisms. 
         FIG. 6A  is a schematic illustration of the detent mechanism of  FIG. 6  in a retracted position. 
         FIG. 7  illustrates a further alternate embodiment of the present invention. 
         FIG. 8  is a representation, in flow chart format, of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Certain terminology will be used in the following description of the preferred embodiment for convenience only and will not be limiting. The terms “upward,” “downward,” “rightward” and “leftward” will designate directions in the drawings to which reference is made. The terms “forward” and “rearward” will refer, respectively, to the front and rear ends of the drive train components as conventionally mounted in the vehicle, being, respectively, to the left and right sides of the various drive train components, as illustrated in FIG.  1 . The terms “clockwise” and “counterclockwise” will refer to rotational directions as viewed from the left side of the vehicle, as shown in FIG.  1 . Said terminology includes the words above specifically mentioned, derivatives thereof and words of similar import. 
     The preferred embodiment of the present invention is illustrated as utilized in a partially automated, lever-shifted mechanical transmission system of the type illustrated in aforementioned U.S. Pat. Nos. 4,593,580; 5,569,115 and 5,582,558, and in aforementioned co-pending U.S. Ser. No. 08/902,603, now U.S. Pat. No. 5,904,635, entitled PARTIALLY AUTOMATED LEVER-SHIFTED MECHANICAL TRANSMISSION SYSTEM. Although the present invention is particularly advantageously utilized in such systems, its application is not so limited. 
     A typical vehicular powertrain system  10  advantageously utilizing the present invention may be seen by reference to FIG.  1 . Powertrain  10  is of the type commonly utilized in heavy-duty vehicles, such as the conventional tractors of tractor/semi-trailer vehicles, and includes an engine, typically a diesel engine  12 , a master friction clutch  14  contained within a clutch housing, a multiple-speed compound transmission  16 , and a drive axle assembly  18 . The transmission  16  includes an output shaft  20  drivingly coupled to a vehicle drive shaft  22  by a universal joint  24  for driving the drive axle assembly, as is well known in the prior art. The transmission  16  is housed within a transmission housing  26  to which is directly mounted the shift tower  28  of the shift lever assembly  30 . 
       FIG. 4A  illustrates a shift pattern for assisted manual shifting of a combined range-and-splitter-type compound transmission manually shifted by a manually operated shift lever. Briefly, the shift lever  31  is movable in the side-to-side or X—X direction to select a particular ratio or ratios to be engaged and is movable in the fore and aft or Y—Y direction to selectively engage and disengage the various ratios. The shift pattern may include an automatic range shifting feature and automatically selected and/or implemented splitter shifting, as is known in the prior art. Manual transmissions utilizing shift mechanisms and shift patterns of this type are well known in the prior art and may be appreciated in greater detail by reference to aforementioned U.S. Pat. Nos. 5,000,060 and 5,390,561. 
     Typically, the shift lever assembly  30  will include a shift finger or the like (not shown) extending downwardly into a shifting mechanism  32 , such as multiple-rail shift bar housing assembly or a single shift shaft assembly, as is well known in the prior art and as is illustrated in aforementioned U.S. Pat. No. 4,455,883; 4,550,627; 4,920,815 and 5,272,931. 
     In the prior art transmissions of the general type illustrated in  FIG. 1  but not incorporating the improved shift rail detent assembly of the present invention, it is known that annoying shift lever jumpout may occur if road conditions are severe. Briefly, shift lever jumpout is the unintended disengagement of the jaw clutches of a manually shifted transmission caused by shift lever oscillations in the Y—Y direction about the Y—Y pivot axis  34  of the shift lever assembly. It is the purpose of the shift rail detent assembly of the present invention to minimize the occurrences of such shift lever-induced jumpout while not objectionably increasing shift effort. 
     In a typical heavy-duty vehicle powertrain, the engine-clutch-transmission assemblage will tend to move, during severe road conditions, in a vertical manner (as illustrated by arrow  36 ) and in a pivoting manner about a pivot point or axis  38  (usually located in the area of the vehicle clutch). As is indicated by arrow  40 , an upward movement of the assemblage almost always is associated with a counterclockwise rotation of the assemblage around pivot axis  38 , while, as indicated by arrow  42 , a downward movement of the assemblage almost always is accompanied by a clockwise rotation of the assemblage about the pivot axis  38 . 
     As understood, shift lever-induced jumpout is forced by the inertial effects of excessive road-induced vibration in the vehicle drive train. This road-induced shock causes the engine-clutch-transmission assemblage to pitch on its mounts, as shown in FIG.  1 . This pitching occurs at the natural frequency of the engine-clutch-transmission-mount system, usually between about 7 and 10 Hz. This pitching induces relatively high vertical, fore-aft and rotational accelerations on the transmission and, in particular, the shift lever assembly. The shift lever assembly then develops an inertial jumpout torque T j  about its pivot  34  as determined by the sum of the inertial torques thereon, as will be described in greater detail below and as schematically illustrated in FIG.  2 . It is noted that the typical rearward offset in transmission lever tends to increase the jumpout torque. 
     As will be described in greater detail below and as is schematically illustrated in  FIG. 3 , jumpout torque T j  is resisted by the shift rail or shift shaft detent force multiplied by its moment arm determined by the distance between the pivot  34  and the shift rail or shaft (i.e., detent torque T o ). Detent force may include the forces required to overcome a detent mechanism, torque lock in the engaged jaw clutches, and frictional forces in the shift mechanism. When the jumpout torque overcomes the detent torque, shift lever jumpout occurs. This tends to occur when the drive train has a very low torque, such as vehicle coast conditions, since the friction from so-called torque lock in the drive train during driving conditions tends to lock the engaged sliding clutch members in engagement and greatly overcomes any jumpout forces imposed thereon. 
     As the shift lever assembly  30  itself is a dynamic system, it has its own natural frequency. Unfortunately, this also usually occurs between 7 and 10 HZ. This frequency is determined by lever height, lever offset, tower height, and isolator stiffness. If the natural frequency of the engine-clutch-transmission assemblage matches that of the shift lever assembly, propensity for jumpout is greater because the engine-amplified inertial forces are amplified further by the lever resonance. 
     In  FIG. 2 ,
         T j =a x My−a y Mx+I where:
           T j =Jumpout torque   M=Mass of lever   I=Moment of inertia of lever   a x =Fore/aft acceleration   a y =Vertical acceleration   =Angular acceleration of lever   x=Distance between cg of lever and pivot   y=Vertical distance between cg of lever and pivot   cg=Center of gravity
 
while in  FIG. 3 ,
   
           T o =F x d where:
           T o =Detent torque   F x =Detent force   D=Distance between pivot and rail   
               

       FIG. 2  illustrates a mathematical model for calculating the jumpout torque T j  induced in a shift rail by shift lever whip. It is noted that jumpout torque will be applied in both the counterclockwise and clockwise directions about the shift lever pivot axis  34  but will tend to cause jumpout only in one of these two directions, depending upon the currently engaged gear ratio. 
     One method of minimizing shift lever-induced jumpout is to increase the detent force F x  such that detent torque will almost always exceed jumpout torque. Unfortunately, such an increased detent force, if not relieved at the time of shifting, will result in objectionably high shift effort. 
     In a preferred embodiment of the present invention, the forward shifting of transmission  16 , comprising main section  16 A coupled to auxiliary section  16 B, is semi-automatically implemented/assisted by the vehicular semi-automatic transmission system  100 , illustrated in  FIGS. 4A-4C . Main section  16 A includes an input shaft  50 , which is operatively coupled to the drive or crank shaft  110  of the vehicle engine  12  by master clutch  14 , and output shaft  20  of auxiliary section  16 B is operatively coupled, commonly by means of a drive shaft to the drive wheels of the vehicle. The auxiliary section  16 B is a splitter type, preferably a combined range-and-splitter type, as illustrated in U.S. Pat. No. 5,390,561. 
     The change-gear ratios available from main transmission section  16  are manually selectable by manually positioning the shift lever  31  according to the shift pattern prescribed to engage the particular change gear ratio of main section  16 A desired. As will be described, manipulation of the master clutch (other than when bringing the vehicle to or when launching the vehicle from an at-rest condition) and manual synchronizing are not requiring. The system includes means to signal an intent to shift into a target ratio and will automatically take actions to minimize or relieve torque-lock conditions, allowing, if required, an easier shift into main section neutral from the engaged main section ratio and further allowing required splitter shifts to be automatically and rapidly completed upon a shift into neutral. Upon sensing a neutral condition, the system will cause engine to rotate at a substantially synchronous speed for engaging a target gear ratio. 
     The system  100  includes sensors  106  for sensing engine rotational speed (ES),  108  for sensing input shaft rotational speed (IS), and  110  for sensing output shaft rotational speed (OS) and providing signals indicative thereof. As is known, with the clutch  14  engaged and the transmission engaged in a known gear ratio, ES=IS=OS*GR (see U.S. Pat. No. 4,361,060). 
     Engine  12  is electronically controlled, including an electronic controller  112  communicating over an electronic data link (DL) operating under an industry standard protocol such as SAE J-1922, SAE J-1939, ISO 11898 or the like. Throttle position (operator demand) is a desirable parameter for selecting shifting points and in other control logic. A separate throttle posit on sensor  113  may be provided or throttle position (THL) may be sensed from the data link. Gross engine torque (T EG ) and base engine friction torque (T BEF ) also are available on the data link. 
     A manual clutch pedal  115  controls the master clutch, and a sensor  114  provides a signal (CL) indicative of clutch-engaged or disengaged condition. The condition of the clutch also may be determined by comparing engine speed to input shaft speed. A splitter actuator  116  is provided for operating the splitter section clutch (not shown) in accordance with compound output signals. The shift lever  31  has a knob  118  which contains selector switch  120  by which a driver&#39;s intent to shift may be sensed. A preferred embodiment of selector switch  120  may be seen by reference to  FIGS. 4A-4C . Switch  120  includes a body  120 A in which is pivotably mounted a rocker member  120 B. The rocker is spring-based to the centered, non-displaced position illustrated. The operator may press surface  120 C or  120 D of the rocker member to cause the rocker switch to be pivoted in the direction of arrows  120 E or  120 F, respectively, to select an up- or downshift, respectively. The rocker may be moved in the direction of the arrows and then released to provide an “up” or “down” pulse or may be moved to and retained at the “up” or “down” positions to achieve different control results, as will be described in detail below. The rocker may be used to provide multiple pulses to request a skip shift (see U.S. Pat. No. 4,648,290). Alternatively, rocker  120 B may be replaced by a toggle, pressure-sensitive surfaces, separate “up” and “down” buttons, or the like. 
     A driver&#39;s control display unit  124  includes a graphic representation of the six-position shift pattern with individually lightable display elements  126 ,  128 ,  130 ,  132 ,  134  and  136  representing each of the selectable engagement positions. Preferably, each half of the shift pattern display elements (i.e.,  128 A and  128 B) will be individually lightable, allowing the display to inform the driver of the lever and splitter position for the engaged and/or target ratio. In a preferred embodiment, the engaged ratio is steady lit, while the target ratio is indicated by a flashing light. 
     The system includes a control unit  146 , preferably a microprocessor-based control unit of the type illustrated in U.S. Pat. Nos. 4,595,986; 4,361,065 and 5,335,566, the disclosures of which are incorporated herein by reference, for receiving input signals  148  and processing same according to predetermined logic rules to issue command output signals  150  to system actuators, such as the splitter section actuator  116 , the engine controller  112  and the display unit  124 . A separate system controller  146  may be utilized, or the engine ECU  112  communicating over an electronic data link may be utilized. 
     As shown in co-pending patent application U.S. Ser. No. 08/597,304 now U.S. Pat. No. 5,661,998, the splitter actuator  116  is, preferably, a three-position device, allowing a selectable and maintainable splitter section neutral. Alternatively, a “pseudo” splitter-neutral may be provided by deenergizing the splitter actuator when the splitter clutch  80  is in an intermediate, nonengaged position. 
     Forward dynamic splitter-only shifts, other than for the more fully automatic 9-10 and 10-9 splitter shifts, such as third-to-fourth and fourth-to-third shifts, are automatically implemented upon driver request by use of the selector switch  120 . By way of example, assuming a three-position splitter actuator, upon sensing that a splitter shift is required, by receiving a single “up” signal when engaged in first, third, fifth or seventh, or receiving a single “down” signal when engaged in second, fourth, sixth or eighth, the ECU  146  will issue commands to the actuator  116  to bias the actuator toward neutral, and to engine controller  112  to minimize or break torque. This may be accomplished by causing the engine to dither about a zero flywheel torque value (see aforementioned U.S. Pat. No. 4,850,236). As soon as splitter neutral is sensed, the engine will be commanded to a substantially synchronous engine speed for the target gear ratio at current output shaft speed (ES=IS=OS*GR T ±E RROR ). The engagement is timed, in view of reaction times and shaft speeds and accelerations, to occur just off synchronous to prevent clutch butting. Automatic splitter shifting of this general type is illustrated in aforementioned U.S. Pat. Nos. 4,722,248 and 5,435,212. 
     The more fully automated 9-10 and 10-9 splitter shifts are implemented in the same manner but are initiated by the ECU, not the selection switch  120 , in accordance with predetermined shift schedules. 
     The engaged and neutral (not engaged) conditions of transmission  10  may be sensed by comparing the input shaft/output shaft rotational speeds to known gear ratios (ISO/OS=GR i=1 tp 10 ±Y?) for a period of time. Position sensors may be utilized in lieu of or in addition to input shaft and output shaft speed logic. 
     When synchronizing to engage a target ratio, the engine is directed to achieve and remain at a speed about 30 to 100 RPM (preferably about 60 RPM) above or below (preferably below) true synchronous speed (ES SYNCHRO =(OS×GR T )−45 RPM) to achieve a good quality jaw clutch engagement without butting. To verify engagement of a target ratio, the system looks for input shaft speed equaling the product of output shaft speed and the numerical value of the target ratio, plus or minus about 10 to 30 RPM (IS=(OS*GR T )±20 RPM) for a period of time, about 100 to 400 milliseconds. 
     The foregoing logic allows transmission engaged and neutral conditions to be determined on the basis of input and output shaft speeds without false engagement sensing caused by engine synchronizing for engagement of a target ratio (see co-pending U.S. Ser. No. 08/790,210, now U.S. Pat. No. 5,974,354). 
     When in an even numbered ratio (i.e., when in the high splitter ratio) and a single upshift is required, a lever upshift (with splitter downshift) is appropriate and the system, if requested by the driver, will automatically assist in implementing same. Similarly, when in an odd numbered ratio (i.e., when in the low splitter ratio) and a single downshift is requested, a lever downshift (with splitter upshift) is appropriate and the system, if requested by the driver, will automatically assist in implementing same. It is noted that in system  100 , splitter-only shifts may be automatically implemented, while lever shifts, with accompanying splitter shifts, require driver initiation and main section jaw clutch manipulation. 
     When a combined lever-and-splitter shift is requested, a single pulse of the selector in the appropriate direction (as opposed to maintaining the rocker  120 B in the appropriate displaced position) is taken as simply a request for an appropriate splitter shift with no automatic assistance, and the splitter will be preselected to shift to the appropriate splitter position and will do so when the operator manually shifts to neutral or otherwise breaks torque. The driver is then required to engage the appropriate main section ratio without intervention by the controller  148 . This is substantially identical to the operation of a fully manual splitter-type transmission. 
     If the driver wishes automatic assistance for a combined lever-and-splitter shift, the rocker member  120 B of the selector is moved to and retained (for at least 50 milliseconds to 1 second) in the appropriate position to request an assisted up- or downshift. The controller  148 , upon receiving such a request, will automatically cause (for a period of about 2-5 seconds) the engine to be fueled to dither about a zero flywheel torque, thereby reducing or eliminating torque lock conditions and allowing the operator to easily manually shift to main section neutral (see U.S. Pat. Nos. 4,850,236 and 5,573,477). The display  124  will steadily light the old gear ratio and flash or otherwise indicate the selected ratio. The ECU  148  will sense for neutral conditions by comparing the ratio of input shaft speed (IS) to output shaft speed OS) to known gear ratios. Alternatively or in combination, position sensors may be utilized. The logic will determine the identity of the target gear ratio GR T  as a direct or indirect function of current gear ratio GR C  and the direction of the requested shift. 
     When main section neutral is sensed, the display element corresponding to the disengaged gear ratio will not be lighted, the splitter will automatically be caused to shift to the appropriate splitter ratio and the engine will automatically be caused (for a period of about 2-5 seconds) to rotate at a substantially synchronous speed (ES=OS*GR T ) for engaging the target gear ratio (GR T ), allowing the operator to easily manually utilize the shift lever  31  to engage the indicated main section ratio. Preferably, the engine will automatically be caused to rotate at an offset from or to dither about true synchronous speed (see U.S. Pat. Nos. 5,508,916 and 5,582,558). Upon sensing engagement of the target ratio, the display indicator elements corresponding to the newly engaged ratio will be steadily lit and engine fuel control will be returned to the operator. The assisted combined lever and splitter shift is accomplished without requiring the operator to manipulate the clutch pedal  115  or the throttle pedal  113 . 
     When in or after shifted to the “A” position  136  (i.e., 9/10), the ECU  146  will command the fuel controller  112  and splitter operator  116  to automatically select and implement appropriate 9-10 and 10-9 shifts. Automatic operation within an upper group of ratios is disclosed in aforementioned U.S. Pat. Nos. 4,722,248; 4,850,236 and 4,850,236 and 5,498,195. Systems incorporating this feature are sold by Eaton Corporation under the “Super 10 Top-2” trademark and by Dana Corporation under the “Automate-2” trademark. 
     To shift out of the “A” position, the operator may simply use the clutch pedal  115 , throttle pedal  113  and shift lever  57  to perform a fully manual shift to another ratio. If an assisted lever shifts from “A” to eighth (or a lower ratio) is required, the selector rocker  120 B may be retained in the “down” position, which will cause the ECU  146  to command the fuel controller  112  and/or splitter actuator  116  to assist the lever or combined lever-and-splitter shift from the engaged “A” ratio (ninth or tenth) to a selected target ratio. Pulses of the selector (and “up” continuing displacement), when in the “A” position, are ignored by the ECU. 
     In transmission systems such as system  100 , and in more automated systems, the system is provided with a signal indicating, or with a means for determining, that a shift in the main transmission section  16 A is to be initiated. 
     According to the present invention, a detent mechanism is provided which will provide a variable resistance to shift rail movement from an engaged position. When the system senses a desire to remain in an engaged ratio, the detent provides a detent force which will provide an exceedingly nigh resistance to shift rail movement which will resist shift lever-induced jumpout. As a shift is not occurring, this will have no adverse effect on shift quality. When an intent to do a lever shift or a shift in progress is sensed, there is no requirement to prevent jumpout and detent resistance or force is minimized to improve shift quality by reducing shift effort. In more automated systems, this will allow smaller shift actuators to be utilized. 
       FIG. 5  illustrates one embodiment of variable shift rail detent mechanism. Shift rail  150  (also called a “shift shaft”) has in-gear notches  152  and  154  which will align with a detect mechanism  156  when the transmission is engaged in 1/2, 5/6 or 9/10(A) or in R, 3/4 or 7/8, respectively. A land  158  exists between notches  152  and  154 . Alternatively, a small neutral detent (shown in dashed lines) may be utilized. 
     Shift rail  150  will typically carry one or more shift forks  151  for axially positioning clutch members  151 A in engaged or disengaged position, as is well known in the prior art. 
     The detect mechanism includes a plunger  160  having tapered tip  162  receivable in the notches and a piston end  164  receivable in a cylinder  166 . A light compressor spring  168  biases the plunger downwardly into contact with the notches. The piston and cylinder define a selectively pressurized and exhausted chamber  170  which is controlled by an actuator valve  172  under command from ECU  146 . 
     Upon sensing an intent to shift, chamber  170  is exhausted to minimize the resistance to axial movement of shaft rail  150 . Upon sensing a desire to remain engaged, the chamber  170  is pressurized to maximize the detent force and, thus, the resistance to axial movement of the shaft rail to resist shift 1(ever-induced jumpout. An onboard source S of pressurized fluid, such as hydraulic fluid or pressurized air, may be used to pressurize chamber  170 . 
     The detent mechanism of  FIGS. 6 and 6A  is similar to that illustrated in  FIG. 5  in that a shift rail is provided with in-gear notches  178  and  180  corresponding generally to notches  152  and  154 , respectively. Notches  17 E 8  and  180 , however, are not tapered. The notches  178  and  180  cooperate with a non-tapered tip  182  of a plunger member  184  of a detent mechanism  186 . Plunger member  184  includes a two-sided piston portion  188  slidably and sealingly received in a cylinder  190 . The piston portion  188  and cylinder  190  define two separate chambers  192  and  194 , which are alternately pressurized and exhausted by control valve  196  under command from ECU  146  to cause the plunger to assume an extended or retraced position. The retracted position of the plunger is illustrated in FIG.  6 A. 
     The mechanism in  FIG. 6  provides a positive resistance to axial movement of the shift rail  176 , as opposed to the resilient resistance to axial movement of shift rail  150  provided by the mechanism illustrated in FIG.  5 . Both types of mechanisms, and modifications thereof, are suitable for the present invention.  FIG. 8  illustrates, in a flow chart format, the method of the present invention. 
     The embodiment illustrated in  FIG. 7  is substantially identical to that of  FIG. 5 , except that neutral detent  158 A is intended to positively retain the shift shaft  150  in the neutral condition and a control unrelated to dynamic shifting, such as a heater control, provides a control input. Such a control is shown in the form of a switch  200 . As is known, in a heavy-duty truck, often it is desirable to leave the engine running in neutral for a long period of time to power the heater, the refrigeration unit or the like. Under such conditions, it is very desirable to positively lock the transmission in neutral. Plunger  160  will cooperate with detent  158 A to provide such a positive locking. 
     Although the present invention has been described with a certain degree of particularity, it is understood that the description of the preferred embodiment is by way of example only and that numerous changes to form and detail are possible without departing from the spirit and scope of the invention as hereinafter claimed.