The present invention relates to an automatic transmission for an automotive vehicle for driving front and rear wheels, and more particularly to an automatic transmission of the trans-axle type which is adapted to drive either two-wheels or four-wheels.
In recent years, the front wheel drive automobile in which the engine is mounted in a front position of the vehicle has been widely used, because of light weight and a safe steering operation. Further, there has been provided a front wheel drive automobile with an automatic transmission, in which the automatic transmission is integrally assembled with the final reduction gear for the front wheel drive.
It occasionally is difficult to drive the two-wheel drive vehicle at a low speed under a heavy load in such driving conditions as a rapid starting, rough road driving or steep slope climbing. To eliminate such a defect, there has been provided an automobile having a transmission and a clutch adapted to drive either the front two wheels or both the front and rear four-wheels in which the rear wheels are driven through a clutch means. Since the clutch means is a dog clutch, operation for coupling the clutch must be performed in a stopped state of the vehicle or in a straight forward driving state where no rotational difference between the front and rear wheels exists. In an automatic transmission for the rear-wheel drive a shift lever for the clutch means must be operated after the select lever of the transmission has been shifted to the neutral position (N) or the parking position (P) for cutting off the power. Accordingly, manipulation of the levers is complicated, which means a decrease in the utility of the automatic transmission. Therefore, it is desirable that the front wheel drive is automatically changed to the four-wheel drive in accordance with the driving condition. Further, the four-wheel drive is desirable during rearward driving, on a slope and a rough road.