A fan motor speed control system for controlling the fan motor speed of an air conditioning system includes a power output circuit including a power triac which is turned on and off by an opto-isolator connected to a pulse generator circuit for varying an AC voltage waveform imposed on the fan motor. The pulse generator circuit is connected to heating and cooling ramp circuits and a minimum speed circuit to provide a variable voltage signal imposed on the pulse generator circuit corresponding to the temperature difference sensed by a return air sensor and a heating or cooling sensor or by separate heating and cooling sensors disposed adjacent respective heating and cooling heat exchangers of the air conditioning system. An adjustable minimum speed circuit and a cutoff circuit are provided to control motor minimum speed or motor shutoff when a predetermined minimum speed is reached to prevent motor bearing failure or overheating. Sensor protection circuits in the control system operate to drive the motor to full speed if any of the temperature sensors experience an open or short circuit condition. The control system circuit maximizes air conditioning system efficiency by capturing additional heating or cooling effect, reduces noise associated with motor startup and shutdown, and reduces rapid change in the sensed temperature in the air conditioned space during motor startup and shutdown.