US Patent 7815545 Hydraulic control system for automatic transmission

An automatic transmission hydraulic control apparatus includes an oil pump driven by an engine, a forward clutch arranged to be engaged by an engagement pressure from the oil pump at a start of the vehicle, a hydraulic pressure sensing section configured to sense a hydraulic pressure in the automatic transmission, a filling start judging section configured to judge a start of filling to the forward clutch in accordance with the sensed hydraulic pressure; and an engagement pressure control section configured to control the engagement pressure to go through a filling phase and an engagement phase. The engagement pressure control section shifts from the filling phase to the engagement phase when a predetermined time elapses from the judgment of the start of the filling to the forward clutch.