Circuit/System Description
WARNING: This page is about a different car, the 2019 Chevrolet Malibu. However, it is still accessible from the selected car via links, so may be relevant.
The engine control module (ECM) creates the engine speed signal by pulsing the engine speed signal circuit to ground at a rate of 2 pulses per engine revolution. The Instrument Cluster converts the 2 pulses per engine revolution signal to a tachometer position indicating the engine speed. The engine speed is sent to the Instrument Cluster over a discreet wire from the ECM. The tachometer signal is pulled up to B+ in the IPC. The ECM monitors the voltage at the engine speed signal circuit to determine when the voltage is out of the normal operating range.