Speedometer Head: Description and Operation

SPEEDOMETER

The instrument panel cluster (IPC) displays the vehicle speed based on class 2 serial data message from the powertrain control module (PCM) or the engine control module (ECM). The speedometer will default to 0 km/h (0 mph) if:
- The PCM/ECM detects a malfunction in the vehicle speed sensor circuit.
- The IPC detects a loss of class 2 communications with the PCM/ECM.
- The transmission control module (TCM) detects a loss of GMLAN communications with the TCM.