In a vehicle having an automatic transmission, a clutch assembly smoothly engages a rotating engine crankshaft with a stationary driveshaft for transmission of power to the drive wheels, and also disengages the respective shafts to interrupt power transfer therebetween to permit, for instance, smooth shifting between various gears of a planetary gear set. Clutch assemblies or clutches are torque-transmitting devices typically having a series of friction elements, i.e. a clutch pack, located within a clutch housing and actuated by a clutch piston, the piston being powered or energized by a supply of hydraulic fluid. The hydraulic fluid supply is typically pressurized by a controllable pump. When hydraulic clutch pressure is reduced, the clutch is released or disengaged, and likewise, when clutch pressure is increased, the clutch is actuated or engaged. The hydraulic system may be further controllable to actuate other transmission components, such as a specific clutch or a series of clutches within an automatic transmission.
One such hydraulically-actuated clutch is a latched-pump applied clutch (LPAC). In an automatic transmission containing an LPAC, a controllable pump pressure engages the LPAC while a latching valve closed to thereby trap and substantially seal a supply of pressurized hydraulic fluid within the LPAC circuit. Once adequate clutch-apply pressure has been trapped or sealed within the LPAC circuit, controllable pressure in the main pump circuit may then be reduced as required in order to minimize spin losses elsewhere in the transmission without thereby diminishing available clutch-apply pressure within the LPAC circuit.
A leak or series of leaks within a sealed LPAC circuit may cause a decrease or reduction in available clutch-apply pressure, the effect varying with the severity and/or number of leaks in the circuit. Leakage or bypass might occur at various points within the circuit, including around piston seals, valve body gaskets, various component connections, within latching valves, or through the inherent porosity of cast transmission components. Such leaks, particularly in a relatively stiff of low-compliance system, can deplete available clutch-apply pressure within the LPAC circuit. When applied to a rotating-type clutch in particular, that is a stationary piston is used to clamp or apply a rotating clutch pack, a plurality of thrust bearings are commonly used to allow relative motion without an undue increase in drag. Under these circumstances, however, thrust bearing spin loss may increase along with the increase in clutch-apply pressure.