Instrument Cluster Description and Operation: Speedometer
WARNING: This page is about a different car, the 2022 Cadillac CT4. However, it is still accessible from the selected car via links, so may be relevant.
The Instrument Panel Cluster Control Module displays the vehicle speed based on the information from the ECM. The ECM sends the vehicle speed information via serial data to the BCM. The BCM then sends the vehicle speed information via serial data to the Instrument Panel Cluster Control Module to display the vehicle speed.