1977-79 Models
The EGR control valve is constructed as a three-diaphragm valve to provide better proportioning of exhaust gas recirculation. Depending on the position of the throttle valve, the center diaphragm chamber of EGR valve will receive more or less vacuum via the thermo/vacuum valve.
In the upper diaphragm there is always atmospheric pressure present via a vent bore. Depending on vacuum, the shut-off diaphragm with connecting pin moves downward against spring tension and the EGR valve will open. See Fig 1.
The opening and closing of the EGR valve is controlled by the exhaust pressure transducer which, depending on exhaust gas backpressure in the exhaust manifold, either vents or evacuates the diaphragm chamber above the working diaphragm.