As with the previous advance system, the rotorโs force is transferred to the cam ring during injection. This force continually urges the piston toward the retard position. However, an opposing force is supplied by transfer pump pressure acting on one end of the servo advance piston. (Figure 4-62).
The position of the servo valve in the advance piston bore regulates this force, and determines the degree of advance achieved at any throttle setting or load. (Figure 4-63).
Additional advance at low throttle settings is provided by the face cam to rocker lever action which changes the reference point of the spring. (Figure 4-64).
This allows the servo-advance valve to open further and provide a greater degree of advance at low throttle settings. The end result of both of these advance mechanisms is a vast improvement in the driveability of diesel equipped vehicles.
