How Does Fan Control Unit Work?by Don Bowman
Radiator cooling fans are controlled by a computer. Information collected from the coolant temperature sensor and from the air conditioning control switch provide the fan control with information. Because most vehicles today are front-wheel drive--which require the engine to be situated sideways versus straightforward--there is no way to use a conventional fan in them. Rather, electric fans are used, which have the benefit of saving fuel since they do not draw any horsepower from the engine. Since they are electric and not engine-driven, they do not have to operate when the car is in motion.
A car will not overheat because of the relative wind moving through the radiator. However, when the car comes to a stop, the engine temperature begins to rise. When the temperature rises to about 210 degrees, a sensor sends a signal to the computer, and the computer turns the fan on. If the temperature continues to rise, the computer will activate a high-speed relay, causing the fans to pick up speed. As soon as the car is once again in motion and the temperature begins to drop, the computer shuts the fans off. When the air conditioning is turned on, the computer will automatically turn the condenser fan on and keep it on as long as the air conditioning is on so as to keep the head pressure down.
The fan control for the heater and air conditioning fan inside the car is controlled by a combination of the control head on the dash and the blower motor resistor, which controls the fan speeds. The resistor, located very close to the blower motor, is a small unit that supplies varying amounts of resistance to the blower motor at all speeds. In most cases, if the fan only works on one or two speeds, the resistor is bad. If the blower does not work at all, it could be either the resister or the blower. If no power at all is going to the blower, the resistor is bad. If there is power, this means the blower itself is defective.