Reference Low

This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground through the ICM only. Although this circuit is electrically connected to the PCM, the circuit is not connected to ground at the PCM. The PCM compares the voltage pulses on the 18X and the 3X reference input circuits to any on the reference low circuit, and ignores pulses that appear on both circuits. If the circuit is open, or connected to ground at the PCM, reduced engine performance and MIL illumination with no DTC set may result.