As I mentioned here : http://www.xoutpost.com/801192-post39.html the 5HP24 is very unusual in not having a torsional vibration damper for the LUC (to save weight and space) and so a continuous slip mode is employed instead at lower engine & vehicle speeds. In the Range Rover (same transmission and very similar engine) the LUC is never fully locked below 56mph so the torque converter can spend a lot of its life in this continuous slip mode. I expect it's very similar for E53.





I hope you don’t mind me correcting your statement that the LUC’s default status is in the lockup position. The default is to have pressure/flow directed to the front of the LUC piston by the converter pressure valve which holds the converter open. It has to be designed this way so that if the transmission has to go into limp-home - say through failure of the electronics - the LUC stays open to allow the vehicle to pull away from rest (& then stay) in 4th gear. To activate the LUC, pressure from solenoid 4 moves the converter pressure valve so that it vents the front of the clutch piston to tank and at the same time moves the LUC shift valve causing the rear of the converter to be pressurised, holding the LUC locked. This can be seen clearly in the hydraulic circuit.





So basically the two valves work together to reverse the direction of flow through the converter. It either flows in through the turbine shaft and out through the stator shaft (LUC off - default) or in through the stator shaft (no flow, just pressure) with the turbine shaft earthed to tank (LUC on).

Phil