Back-up device for an electronic control system of an automatic transmission

In an electronic control system of an automatic transmission in an automotive vehicle, said automatic transmission having at least two speed ratios including a first speed ratio for lower speed running of said vehicle and a second speed ratio for higher speed running of said vehicle, and forming part of a power transmission system connected between an engine of said vehicle and driving wheels of said vehicle, said electronic control system including a first frictionally engaging element hydraulically actuatable for establishing said first speed ratio, a second frictionally engaging element hydraulically actuatable for establishing said second speed ratio, an operating fluid source, selector valve means for selectively connecting said operating fluid source to said first and second frictionally engaging elements, said selector valve means being adapted to selectively assume a first position for establishing said first speed ratio and a second position for establishing said second speed ratio, electromagnetic valve means for driving said selector valve means to select said first position or said second position, and electronic control means for selectively energizing and deenergizing said electromagnetic valve means in accordance with a predetermined gear shifting manner responsive to an electrical signal indicative of the speed of said vehicle and an electrical signal indicative of output from said engine, the improvement comprising a back-up device comprising vehicle speed sensing means for generating an output signal having a value thereof increasing as the speed of said vehicle increases, and control means for causing said selector valve means to assume said second position irrespective of the operation of said electromagnetic valve means, when the value of said output signal from said vehicle speed sensing means exceeds a predetermined value.