Instrument Panel Cluster (IPC) Description and Operation: Speedometer
WARNING: This page is about a different car, the 2003 GMC Safari and 2003 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
The IPC displays the vehicle speed based on the information from the PCM. The PCM converts the data from the vehicle speed sensor to a 4000 pulses/mile signal. The IPC uses the vehicle speed signal circuit (4000 pulses/mile) from the PCM in order to calculate the vehicle speed. If the IPC detects a malfunction in the vehicle speed signal circuit, the IPC uses the class 2 message from the PCM indicating the vehicle speed.