The 78-80 300SD were the same. They all should have electronic kickdown. Normally adjusted, the control pressure linkage alone will not get the 1st gear downshift.

The problem most often was the relay in the right kickpanel. Every one I ever had bad was fixable by resoldering the PC board inside the relay. Sometimes the problem was caused by shorted magnets on the solenoid. Often the problem was simply a open magnet.