Section 12 (Instrument Panel): Operation: Speedometer/Odometer
WARNING: This page is about a different car, the 1994 Chevrolet Lumina APV. However, it is still accessible from the selected car via links, so may be relevant.
The Vehicle Speed Sensor (VSS) generates an AC signal with frequency proportional to vehicle speed. This signal goes to a VSS buffer, which converts the signal to a square wave. This signal is interpreted by the speedometer and cruise control system as vehicle speed. On 3.1L, the VSS buffer is a separate component; on 3.8L it is incorporated into the Powertrain Control Module (PCM).