a device for sensing the input shaft speed of an automotive automatic transmission that is driven by the engine through the fluid coupling of a torque converter . the speed sensor device includes circumferentially spaced markings about the transmission input shaft and a speed sensor that is placed at close proximity to the circumferentially spaced markings through a hole in the torque converter stator shaft . an electronic control unit analyzes the sensor output signal and in the case of an active speed sensor it also functions as its power source . during vehicle operation , the transmission input shaft rotates the target wheel in front of the speed sensor causing modulation of its output signal . the electronic control unit analyzes the signal modulation and calculates the input shaft rotational speed . a variety of sensor / target - wheel options and sensor mounting techniques could be used depending on the application constraints .