Circuit/System Description

The engine control module creates the engine speed signal by pulsing the engine speed signal circuit to ground at a rate proportional to engine revolution speed. The instrument cluster converts the 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 engine control module. The tachometer signal is pulled up to B+ in the instrument cluster. The engine control module monitors the voltage at the engine speed signal circuit to determine when the voltage is out of the normal operating range.

In some vehicle/option configurations, the discreet engine speed signal circuit is not used for tachometer operation. In these configurations, the Engine Control Module sends engine speed information to the Instrument Cluster via serial data. The Instrument Cluster then uses the serial data messages for tachometer operation.