Self-diagnosis system in an automatic transmission

An automatic transmission comprising controlling unit 5 for driving and controlling an actuator 3 until a rotation position, which is detected by a potentiometer 13 in response to the receipt of an instruction signal from a operation switch portion 17 , accords with the instructed shifting position, characterized by comprising self-diagnosis unit 9 for diagnosing a trouble after driving the actuator 3 to the position over the instructed shifting position for a predetermined amount on the basis of the first instruction signal by the operation switch portion 17 at a starting time of driving an automobile and after driving the actuator 3 in reverse to the instructed shifting position, as well as driving the actuator 3 by varying high or low the duty ratio in driving and controlling the actuator 3 , in accordance with the judgment whether or not an output signal of the potentiometer 13 corresponds to driving mentioned above.