Variables

Hello to all: I am building a real panel hardware of an autopilot of a 747-200. This panel has a selection of "IAS HOLD" that maintains the airspeed "IAS" constant, varying the RATE of CLIMB to maintain the IAS constant. This is made to constant thrust power of the engines. I was proving with the variable of the FSUIPC $3460 but I don't achieve him to work. Can somebody help me???. Thanks.Jose