EGR Control Solenoid: Description and Operation

Fig. 69 Signal Converter:

The signal converter, Fig. 69, consists of a housing with three vacuum ports and a movable piston suspended between two diaphragms and a spring. The piston acts upon a valve.
The vacuum signal runs from port 1 to port 2. As the vacuum at port 3 increases, the piston will move upwards, driven by spring force, sealing the opening to port 1. At the same time the EGR valve is vented through port 2. The EGR flow is thereby controlled in relation to engine load.
As long as the turbocharger remains idle, the pressure upstream of the butterfly valve will not be sufficient to operate the EGR valve.