1. Field of the Invention
The invention relates to a control system for an automatic transmission.
2. Description of the Related Art
In a conventional automatic transmission, rotation from the engine is transmitted to a speed change unit through a hydraulic power transmission, and speed changes are effected in the speed change unit. A first clutch (an input clutch) is arranged between the hydraulic transmission and the speed change unit for change between a neutral range and a forward driving range by engaging/disengaging the first clutch.
In the automatic transmission, a neutral control is performed by disengaging the first clutch based on a hydraulic pressure modulated by a linear solenoid valve when the forward driving range is selected, an accelerator pedal is released, a brake pedal is depressed and the vehicle is in a "vehicle stopped state" defined as a vehicle speed of almost 0. Fuel consumption is improved by such neutral control because the load on the engine is reduced and vibration of the engine is prevented.
Within the neutral control, a hill-hold function is provided so that the vehicle will not move backward when the brake pedal is released for starting, after the vehicle has been stopped on an incline and facing up-hill. To provide the hill-hold function, when it is detected that the vehicle is in the "vehicle stopped state", a hill-hold output is output to establish a hill-hold state in the speed change unit by, for example, engaging a first brake and locking a designated one-way clutch.
However, in the conventional automatic transmission, when the first brake is engaged very shortly after the hill-hold output, the hill-hold state is established while engaging the first clutch. In this case, the automatic transmission is rotated to a small degree, in one direction around an input shaft, by a reaction force received before the hill-hold state is established and held. When the first brake is engaged before disengaging the first clutch in this state, the automatic transmission is held in a state wherein the automatic transmission receives the reaction force. The engine, connected to the automatic transmission, is thereby held in the same state. As a result, the neutral control is performed in a state wherein the engine is under a load. Therefore, the load on the engine increases accompanying the change in the conditions of the neutral control. As a result, the fuel consumption is not improved and vibration of the engine is not reduced to the extent that it cannot be felt in the driver's seat.