Vehicle Speed Sensor (VSS)
WARNING: This page is about a different car, the 1995 Ford Windstar. However, it is still accessible from the selected car via links, so may be relevant.
VSS is a variable reluctance sensor that generates a waveform with a frequency that is proportional to vehicle speed. When vehicle is moving slowly, sensor produces a low frequency signal. As vehicle speed increases, sensor produces a higher frequency signal. The PCM uses this signal to control fuel injection, ignition timing and transmission shift points.