The automatic transmission speed sensor is measuring the rotational speed of the transmission.

The measured data is used by the engine management to determine the right gears, shifting times and shifting pressures.

The sensory unit is either based on the Hall-effect or has a magneto resistive sensor system.

If an automatic transmission speed sensor fails the engine management will issue a diagnostic trouble code and illuminate the check engine light.

Also the driver might experience shifting problems as well as problems with the cruise control system.

The main reasons for a defective automatic transmission speed sensor are broken cables or corrosion.