The rotation of your wheels noticed by the Toyota ABS wheel speed sensors during the braking procedure is how the locking up of the wheels is prevented. This is one of the many new safety devices that were designed to help the driver stay in control of their vehicle when they encounter adverse and slippery weather conditions. There is an ABS wheel speed sensor on all four wheels of new Toyota models, and only two on some older models. They are in place to modulate the braking forces at each wheel. This will help keep the wheels rotating at controlled speeds when their tendency is to either lock up or spin too fast when the roads surface is covered with rain or ice. By the front wheels rotating slowly, input from the steering wheel will still effect the direction of the vehicle while it is in motion. Since the Toyota ABS wheel speed sensors are under the vehicle where the presence of dirt and mud are, they can become covered and no longer have the ability to sense the movement of the wheel. A piece of road debris can also come in contact with the ABS wheel speed sensor causing a failure of this sensor. In either situation, the ABS light will become illuminated on the dash board informing the driver of the Toyota there is a problem with this system. Because the ABS wheel speed sensor is an electrical component, it may eventually fail over time. When any of these conditions occur an inspection must be made of the sensors in the ABS system for the problem to be rectified. The first step in this inspection of your Toyota should be to make sure the wires are connected to the ABS wheel speed sensor and to clean off any debris that might be on them. If that does not correct the problem then replacement of the sensor is warranted.