Vehicle Speed Sensor (VSS)
WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2011 Mercury Milan, 2011 Mercury Mariner, 2011 Mercury Grand Marquis, 2011 Lincoln Town Car, and 2011 Lincoln Navigator L. However, it is still accessible from the selected car via links, so may be relevant.
The VSS is a variable reluctance or Hall effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low speed, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission or transaxle shift scheduling, and TCC scheduling.