The blower motor supplies the air flow to move air around the inside of the passenger compartment.
It is located in the ventilation system before the air-conditioning evaporator.
In a manual air conditioner the switch connects resistors in series with the motor to control its speed. The higher the resistance connected in series with the motor the slower that the motor will operate. Reducing the series resistance increases the motor operating speed.
Most air-conditioning systems use two resistors to give 3 driver-selectable speeds of the fan motor.
In climate control systems the speed of the fan motor can be controlled electronically by the control unit modulating a pulse width signal: a longer pulse width will produce a higher fan speed.
The electronic control unit changes the fan motor speed automatically depending on the difference between the interior cabin temperature and the set temperature. When the difference in temperature is large the fan speed is increased to increase air circulation through the cabin. When the difference in temperature decreases the fan motor speed decreases.
This gives an infinitely variable fan speed between zero and maximum speed. The change is progressive.
Source: CDX Global
