EGR Solenoid

EGR valve operating vacuum is controlled by a Pulse Width Modulated (PWM) solenoid. Battery voltage is applied to the solenoid when the ignition switch is in the on position, and the solenoid ground path is completed through the ECM. When the ECM completes the solenoid ground path, the solenoid is ``on'' and vacuum is applied to the EGR valve. The ECM cycles the solenoid ``on'' and ``off'' at a constant rate, while vacuum is regulated by varying the ``on'' time (dwell period) of each cycle from 0% to 100%. The ECM monitors EGR valve operating vacuum through the MAP sensor and corrects variations between calculated and actual EGR by increasing or decreasing the solenoid dwell period. The ECM also detects vacuum system malfunctions (leaks, blockage, etc.) through the MAP sensor, and if a malfunction is detected, the ``Check Engine'' lamp is illuminated and the EGR solenoid is de-energized until the ignition switch is turned off.