Vehicle Speed Sensor (VSS)
WARNING: This page is about a different car, the 1995 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.
The transmission-mounted VSS sends a pulsing signal to PCM when vehicle is moving. The VSS generates pulses with axle shaft revolution. The PCM interprets speed sensor input along with TP sensor closed throttle input.
This input enables PCM to differentiate between closed throttle deceleration and closed throttle idle (vehicle stopped) conditions. During deceleration, PCM controls Idle Air Control (IAC) valve to maintain a desired manifold pressure. During idle, PCM controls IAC motor to maintain a desired idle speed.