Engine Speed Sensor: Description and Operation

The ECM monitors engine RPM through a ``tach'' lead from the Delcotron ``R'' terminal. Engine RPM is used to determine EGR solenoid dwell period and to control TCC engagement. The ``Check Engine'' lamp will remain illuminated whenever the ignition key is on and the engine speed signal is less than 400 RPM. However, if the ``tach'' signal is lost, the light will remain illuminated even after engine speed exceeds 400 RPM.