Hi Kentrayl, In a blower motor, the AC relay supplies the power that allows the fans to circulate warmed air. Essentially an on/off switch, the blower motor relay's reactions start and end the heating process in HVAC systems by controlling the flow of current between the power source and the motor. While a blower motor resistor is responsible for controlling the blower motor's fan speed. ... A failed blower motor resistor can cause issues with the operation of the entire heating and air conditioning system. Still, for accurate and extensive troubleshooting procedure, it is best to consult your trusted local auto repair shop for this one. A vehicle specific service repair manual is also helpful if you can acquire or get one as well.