Abstract: A transmission system of the present invention which has a main transmission and a sub transmission includes a first output shaft speed detecting means for detecting the rotational speed of the output shaft of the main transmission and a second output shaft speed detecting means for detecting the rotational speed of the sub transmission. The thus arranged transmission system of this invention makes it possible to accurately detect the state of the main transmission by providing the first output shaft speed detecting means for detecting the rotational speed of the output shaft of the main transmission as well as the second output shaft speed detecting means for detecting the rotational speed of the output shaft of the sub transmission.

Abstract: A transmission control device for controlling the operation of a transmission which is composed of a main transmission portion and a sub transmission portion which is capable of manual change-over is disclosed. The control device inhibits the shifting operation of one transmission while the other transmission is in operation.This structure of the control device avoids the occurrence of synchronization of the shifts made by means of the main transmission portion and the sub transmission portion, and thereby eliminates the amplification of transmission shock caused by the synchronization of the shifts of the main transmission portion and the sub transmission portion, whereby optimum shifting characteristics are obtained.

Abstract: A transmission control system for an automatic transmission for a vehicle having a main transmission unit which provides a plurality of speed stages and an auxiliary transmission unit which is connected in series with the main transmission unit and provides two very different speed reduction ratios such as 1 and 3 such as desirable for a transmission selectively operative in a two wheel drive operational mode or in a four wheel drive operational mode, the auxiliary transmission being manually switched over between the two different speed reduction ratios, wherein there is provided a means for controlling the auxiliary transmission unit to override the manual control of the auxiliary transmission in a manner only to prevent switching over of the auxiliary transmission unit to the larger speed reduction ratio according to at least one parameter with regard to operational conditions of the vehicle and also according to which of the plurality of speed stages the main transmission unit is set.