Lock-Up Operation

When the TCM recognizes lock-up clutch engagement to be necessary, it issues signals of a large duty ratio to the lock-up duty solenoid. These signals energizes the solenoid for long periods, causing large quantity of ATF to drain. This reduces the lock-up duty pressure, allowing the spool of the lock-up control valve to move to the position at which the ATF from the pressure regulator valve can flow through the lock-up engagement circuit into the torque converter. As a result, a pressure difference occurs between both sides of the lock-up piston with the higher of the pressures pressing the lock-up clutch against the impeller cover. Since the clutch is driven by the engine, the impeller cover that is now mechanically engaged with the clutch can transmit engine power directly to the transmission input shaft.