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.