embodiments of the present disclosure are described herein . it is to be understood , however , that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms . the figures are not necessarily to scale ; some features could be exaggerated or minimized to show details of particular components . therefore , specific structural and functional details disclosed herein are not to be interpreted as limiting , but merely as a representative basis for teaching one skilled in the art to variously employ the present invention . as those of ordinary skill in the art will understand , various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described . the combinations of features illustrated provide representative embodiments for typical applications . various combinations and modifications of the features consistent with the teachings of this disclosure , however , could be desired for particular applications or implementations . fig1 schematically illustrates an all - wheel drive vehicle powertrain . the heavy lines indicate the flow of mechanical power , whereas the dashed lines indicate the flow of information . an internal combustion engine 10 generates mechanical power by converting stored chemical energy in a fuel source . transaxle 12 adapts the speed and torque of the mechanical power produced by the engine to suit the current needs of the vehicle . transaxle 12 includes a multiple - ratio gearbox and also a front differential that provides roughly equal torque to left and right front wheels 14 and 16 while accommodating slight differences in speed such as when the vehicle turns a corner . a power take - off unit ( ptu ) 18 selectively driveably connects the output of the multiple - ratio gearbox of transaxle 12 to driveshaft 20 . rear drive unit ( rdu ) 22 selectively transmits power from driveshaft 20 to rear differential 24 . rear differential 24 transmits roughly equal torque to left and right rear wheels 26 and 28 while accommodating slight differences in speed . wheel - end disconnect 30 includes a controlled clutch that selectively couples or decouples right rear wheel 28 from the corresponding side of rear differential 24 . wheel - end disconnect 30 and the ptu disconnect may be synchronizers that include friction clutches with sufficient torque capacity to bring the components to the same speed before engaging a positive engagement dog clutch . alternatively , one of them may be only a dog clutch and the rdu clutch may be used to synchronize the speeds before engagement . alternatively , engagement may be limited to times when the vehicle is stationary . controller 32 adjusts the state if several components including ptu 18 , rdu 22 , and wheel - end disconnect 30 . specifically , when the vehicle is in a front wheel drive ( fwd ) operating mode , controller 32 signals ptu 18 to disconnect driveshaft 20 from transaxle 12 and signals wheel - end disconnect 30 to disconnect the right rear wheel from rear differential 24 . by so doing , the driveshaft can remain stationary as the vehicle moves , reducing the parasitic drag associated with driveshaft rotation and improving fuel economy . when the vehicle is in an all wheel drive ( awd ) operating mode , the controller signals ptu 18 and wheel - end disconnect 30 to drivably connect the corresponding components . then , controller 32 monitors vehicle traction and signals rdu 22 to transmit power from driveshaft 20 to rear differential 24 when it detects a low of traction at the front wheels . controller 32 may also signal rdu 22 to complete this power flow path in advance of maneuvers that are likely to cause loss of front wheel traction . controller 32 may receive input signals from other components such as ptu 18 , rdu 22 , and rear differential 24 . controller 32 may be a stand - along driveline controller and may be integrates into another controller such as a transaxle controller or powertrain controller . fig2 illustrates the structure of rear differential 24 . the differential is encased in a housing 40 . in vehicles with independent rear suspension , housing 40 is fixed to vehicle structure . in vehicles with a solid rear axle , housing 40 is suspended and moves with the axles . the housing does not rotate in either type of vehicle . housing 40 is partially filled with fluid the provides lubrication to moving components and convects heat generated by friction between components to the housing , from which the heat is dissipated . temperature sensor 42 sends an electronic signal to the controller indicating the temperature of the fluid . power enters the rear differential from the rdu via stub shaft 44 which is fixed to drive pinion 46 . drive pinion 46 is in continuous meshing contact with ring gear 48 . the axes of rotation of pinion 46 and ring gear 48 are offset by 90 degrees . they may also be vertically offset , in which case hypoid gear geometry is common . ring gear 48 is fixed to differential carrier 50 . a number of planet shafts 52 are fixed to an interior of differential carrier 50 . a number of beveled planet gears are supported for rotation with respect to each of the planet shafts . alternatively , the planet gears could be fixed to the planet shafts and the planet shafts could rotate with respect to the carrier . each of the beveled planet gears 54 mesh with a left side gear 56 and a right side gear 58 . the left side gear is fixed to a left axle shaft 60 while the right side gear is fixed to a right axle shaft 62 . when the vehicle is traveling in a straight line with wheel - end disconnect 30 engaged , differential carrier 50 , left axles shaft 60 , and right axle shaft 62 rotate at the same speed and in the same direction . in this condition , there is no motion among the planet gears 54 , the side gears 56 and 58 , and the planet shafts 52 . stub shaft 44 rotates at a speed proportional to the speed of differential carrier 50 with the ratio determined by the tooth counts of pinion gear 46 and ring gear 48 . when the vehicle turns to the left , left axle shaft 60 rotates slightly slower than differential carrier 50 and right axle shaft 62 rotates slightly faster than differential carrier 50 . planet gears 54 rotate about planet shafts 52 at a speed proportional to the difference in speed between left axle 60 and right axle 62 . speed sensors 64 and 66 send electronic signals to the controller indicating the rotational speeds of left axle shaft 60 and right axle shaft 62 respectively . due to the speed relationships among components , two sensors are sufficient to calculate the speeds of all rotating components within the differential . alternatively , one or both speed sensors could sense the speed of a different component and the controller could calculate the speed of each axle shaft based on speed relationships . when wheel - end disconnect 30 is disengaged , the speed of left axle shaft 60 is proportional to vehicle speed , but the speeds of other components are determined by parasitic drag . generally , because the drag on components linked to differential carrier 50 is greater than the drag on components linked to right axle 62 , carrier 50 will be stationary . with differential carrier 50 stationary , right axle shaft will rotate at the same speed as left axle shaft but in the opposite direction . planet gears 54 rotate with respect to planet shafts 52 at relative speeds much higher than during connected speed differentiation . furthermore , this condition may be maintained for much longer periods of time than the duration of turning maneuvers . during this high planet gear relative speed operation , some heat is generated by friction between the planet gears and the planet shafts . if fluid is flowing past this interface , the quantity of heat generated in substantially reduced due to the lubrication effect and that heat effectively removed from the area by movement of the fluid . however , if fluid is not flowing past this interface for some reason , a large amount of heat is generated making the components in the vicinity of the interface extremely hot . when the relative motion ceases , for example because the vehicle momentarily stops , the planet pinions may become effectively welded to the planet shafts . fluid flow may be interrupted for a couple of reasons . if there is a leak in housing 40 , the quantity of fluid decreases . the differential carrier 50 may stop in a position in which one of the planet shafts and corresponding planet pinion is above the fluid level . if this welding phenomenon occurs , then differential carrier 50 , left axle shaft 60 , and right axle shaft 62 are constrained to rotate at the same speed . when wheel - end disconnect 30 is disengaged , this results in a slight increase in parasitic drag which is not a serious detriment to vehicle operation . however , if wheel - end disconnect 30 is engaged , then left rear wheel 26 is constrained to rotate at the same speed as right rear wheel 28 . in this condition , one or both wheels are likely to lose traction when the vehicle turns , which could result in under - steer or over - steer . fig3 and 4 illustrate a method of avoiding the vehicle handling effects if the differential fails to differentiate . when the driver selects front wheel drive mode , the vehicle operates in fwd mode at 70 . the driveshaft is disconnected at both the ptu and the wheel - end to reduce parasitic drag . the rdu clutch is disengaged . while operating in this mode , a timer is set to expire at regular intervals , such as every 90 seconds . when the timer expired , or when the driver requests activation of the all wheel drive feature , the controller performs a safety check at 72 . the details of performing this safety check are discussed below . three outcomes of the safety check are possible . if the differential appears to be differentiating properly but the fluid temperature indicates a risk of failure , the safety check return warning and the controller remains in the fwd mode . if the safety check indicates that the differential is unsafe , a message is displayed to the driver at 74 and the controller enters a disabled mode at 76 . in the disabled mode , the driveshaft disconnects and rdu continue to operate to provide front wheel drive only . the controller does not exit disabled mode until service has been performed and the controller is manually reset . if the safety check indicates that the differential is operating normally , the controller checks at 78 whether awd is selected . if awd is not selected , implying the safety check was triggered by the timer , then the controller remains in fwd mode . the safety check may not properly detect an unsafe condition when the vehicle is stationary . running the safety check periodically while the vehicle is moving ensures that any unsafe condition is detected even if the driver requests awd while the vehicle is stationary . also , running the safety check periodically provides an opportunity to repeat the safety check if a previous safety check returned warning . if awd is selected , then the controller engages the disconnects at 80 and transitions to awd mode at 82 . in awd mode , the rdu is engaged when front wheel slip is detected to transfer some of the engine power to the rear wheels . the rdu may also be engaged pre - emptively in conditions that are likely to cause front wheel slip , such as a rapid increase in driver demanded power . fig4 indicates how the safety check is performed . at 90 , the controller computes the speed of the differential carrier and wheel 26 . at 92 , the controller computes the ratio of wheel speed divided by carrier speed . in normal operation , the carrier speed would be zero , so this ratio should be zero . if the differential is failing to differentiate at all , then the ratio will be equal to one . at 94 , the controller compares the ratio to a calibratable threshold . if the ratio is less than the calibratable threshold , the safety check returns unsafe . if the ratio is greater than the threshold , the controller proceeds to check the fluid temperature at 96 . if the temperature is below a second calibratable threshold , the controller resets a counter to zero at 98 and returns safe . if the temperature is above the second threshold , the controller increments the counter at 100 . then , if the counter is less than a third calibratable threshold at 102 , the controller return warning . if the counter is equal to the third threshold , indicating that the fluid temperature has remained above the second threshold for an extended time , the controller returns unsafe . while exemplary embodiments are described above , it is not intended that these embodiments describe all possible forms encompassed by the claims . the words used in the specification are words of description rather than limitation , and it is understood that various changes can be made without departing from the spirit and scope of the disclosure . as previously described , the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated . while various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics , those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes , which depend on the specific application and implementation . as such , embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications .