Instrument Panel Cluster (IPC) Description and Operation: Tachometer
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 Chevrolet Tahoe, 2003 Chevrolet Suburban, and 2003 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
The IPC displays the engine speed on the analog tachometer based on the engine speed signal from the PCM. The IPC uses the engine speed signal circuit - 2 pulses/engine revolution - from the PCM in order to calculate the engine speed.
The IPC also receives a class 2 message from the PCM indicating the engine speed. The IPC compares the class 2 value to the engine speed signal from the PCM to determine if the engine speed is valid. When the IPC detects a malfunction in the engine speed signal circuit, the IPC uses the class 2 message from the PCM to indicate the engine speed.