The blower motor is a variable speed motor. The higher the voltage applied to the motor, the faster the speed.

Battery voltage to the blower motor is supplied by the HVAC controller by way of the blower motor relay. At low and medium speeds, the voltage is stepped down by the blower motor resistor. At high speed, the blower motor relay is energized, removing the blower motor resistor from the circuit. Battery voltage is then applied directly to the blower motor through the relay. The motor will then run at maximum speed.