How Does a Blower Work in a Car?by Don Bowman
The blower is used for moving air from either an outside source--referred to as vented air--or from cabin air past the heater core or air conditioning evaporator. In doing so, it causes a heat transfer either to the inside of the car or from the inside to the outside. The majority of the vehicles have a blower motor under the dash, close to the kick panel on the right side of the car. There are, however, different locations for the heater core and evaporator. The blower will always be close to both. On a Cadillac, for example, the blower motor is under the hood in the center of the firewall right above the rear valve cover.
The blower motor's speeds are controlled by the blower motor resistor, which is located in close proximity to the blower and in the path of the air flow. There are two types of blower motor resistors. One uses coils of wire in different thicknesses to produce the resistance. It has one coil for each speed. On the low setting, 12 volts will go into the resistor and only 4 volts will come out to the blower. As the settings increase, different coils are used with less resistance, which will produce higher voltages for increased speed. Many of the newer vehicles use a transistorized type of resistor. It still operates the same and is located in the same location. The resistor is located in the air stream because a resistor that drops voltage creates heat and the air stream keeps its cool.
The control head can be either manual or automatic climate control. In the manual mode, the fan reacts strictly to the manual settings. In the automatic mode, a series of sensors comes into play. There are sensors for the inside temperature versus the outside. The automatic control head will attempt to keep the temperature constant. The automatic control is also tied in to the door and window mechanism. When it senses that the door or window is opened, the climate control will increase fan speed to maintain the inside temperature.
The majority of the time, a failure will be found in the blower motor resistor or the motor. The easiest way to check for a failure is to start the vehicle and turn the climate control on. Check the blower motor at its two-wire connector for power. If there is power, the motor is bad. If there is no power, check the resistor for power. If there is power, the resistor is bad. If no power is observed, then the control head is bad.