Exhaust Gas Recirculation: Description and Operation

The EGR system is only used on California models.
The EGR vacuum control valve assembly is installed onto the injection pump. With the vacuum supply connected, the valve modulates under increasing load and pressure drop.
The pressure modulated by the pressure control valve is converted for EGR system with the pressure converter. The EGR valve is activated through a switch-over valve. The modulated pressure can be continuously changed by using the adjusting screw, located under a red protective cap.
A pneumatically operated pressure control flap is located in the air guide housing to increase the vacuum within the intake manifold. When the engine is operating in the EGR mode, the pressure control flap closes the fresh air duct. In the closed position, a minimum opening remains between the pressure control flap and the air guide housing.
After turning the ignition switch to the ON position, the control unit is connected to battery voltage. The control unit operates at a minimum voltage of 11 volts. The control unit is located behind the battery.
An electric switch-over valve releases vacuum for the EGR valve under certain engine operating conditions. The control unit activates the valve depending upon coolant temperature and engine speed.
A micro switch turns off EGR flow and the A/C compressor prior to full load (A/C compressor only if air conditioning-comfort circuit is operating).
Two RPM sensors are used on this EGR system, one installed on transmission flange and the other on the tachometer assembly (on manual transmission models). The RPM sensor located on the transmission flange, comprises of a magnetic core and a coil. The sensor senses engine speed and transmits a corresponding signal (AC voltage signal) to the control unit which will control A/C compressor activation (ON/OFF). The RPM sensor located on the tachometer (on manual transmission vehicles), senses vehicle speed. At approximately 48 mph or above, the switching unit will automatically switch off EGR flow.