Freezer much too cold (-20)

Originally food compartment was too cold as well but after unloading both sides and letting refrigerator thaw out chunk of ice fell from the passage between freezer and food compartments. Also cleaned coils, front and back. No ice seen on either. Evaporator pan was dry. No obvious loose connections, no evidence of water or ice dripping. Turned it back on and 24 hours later food compartment temperature is correct, but freezer still running at -20. In retrospect, this problem has probably been developing gradually over time. Obviously I am not a refrigerator tech, but am pretty handy and I fix all kinds of things around the house. Have found references on the net to diagnostics that can be run from control panel also need wiring diagrams so I can check what I can with my meter before randomly throwing parts at it.