3X Reference PCM Input

From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position at engine speeds above 1200 RPM. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, DTC P1374 will set and the PCM will use the 24X reference signal circuit for fuel and ignition control. The engine will continue to start and run using the 24X reference signal only.