Heating and Air Conditioning: Description and Operation

Fig. 7 Electronic automatic temperature control system block diagram.:

The push button electronic automatic temperature control system uses a micro- computer to analyze four major inputs, then uses this information to determine the correct conditions of six outputs, Fig. 7.
The micro-computer analyzes input from the manually selected temperature and mode, in-vehicle temperature, ambient temperature and engine temperature. This information is used to determine the correct conditions of A/C compressor clutch engagement, blower speed, blend door position, mode door position, panel/ defrost door position and O/A recirc door position.
Small DC electric motors or actuators are used to operate each of four air distribution doors. A feedback circuit is used in each motor so that the computer can identify the door's position. The A/C compressor clutch and blower motor are controlled by the control assembly through the blower speed controller (BSC).

Fig. 8 Electronic automatic temperature control system operating chart.:

The BSC is required to convert the low power signal from the control head to the high power required to drive the clutch and blower. The control assembly response to the seven customer mode selections is shown in Fig. 8.
A self test feature is incorporated into the control assembly which provides error codes to determine the damaged component(s).