RPM Reference Signal
WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
The RPM is monitored by PCM through tach/pulse signals produced by either the Ignition Control Module (ICM) or Crankshaft Position (CKP) sensor. These signals are used by PCM for determining control of timing, fuel delivery, EGR function and idle speed.