Electronic Automatic Temperature Control

ELECTRONIC AUTOMATIC TEMPERATURE CONTROL

The electronic automatic temperature control (EATC) module analyzes input from the following major sources:
- temperature, airflow direction, blower, A/C and RECIRC selection (made by the vehicle occupants)
- in-vehicle temperature sensor
- ambient temperature sensor
- solar radiation sensor
- vehicle speed
- engine coolant temperature

Using these inputs, the EATC module determines the correct conditions for the following outputs:
- A/C compressor operation
- blower speed
- temperature blend door position
- defrost/panel/floor door position
- air inlet door position

Control System Inputs
The EATC system has four control system inputs.

EATC Module
The EATC module:
- is located in the instrument panel.
- has a vacuum fluorescent display for displaying set temperature, airflow direction, blower speed and diagnostic trouble codes (DTCs).
- utilizes an on-board diagnostic (OBD) feature to supply the technician with diagnostic trouble codes (DTCs). These DTCs direct the technician to the inoperative component.
- display will switch between Celsius and Fahrenheit when the instrument cluster display is toggled.

In-Vehicle Temperature Sensor
The in-vehicle temperature sensor operates in the following manner:
- A thermistor in the in-vehicle temperature sensor measures air temperature inside the passenger compartment.
- An aspirator hose and elbow is connected between the heater core and evaporator core housing and the in-vehicle temperature sensor.
- The aspirator hose and elbow takes air from the heater core and evaporator core housing air stream to create a suction in the in-vehicle temperature sensor.
- The suction draws in-vehicle air into the in-vehicle temperature sensor and across the thermistor.

Solar Radiation Sensor
The solar radiation sensor supplies information to the EATC module indicating sunload.

Ambient Temperature Sensor
The ambient temperature sensor signal is received by the smart junction box (SJB) which communicates it to the EATC module via the vehicles SCP network.

Control System Outputs
The EATC system has three control system outputs.

Blower Motor Speed Control
The blower speed control:
- is located on the heater core and evaporator core housing near the blower motor.
- PWM signals from the EATC module to control a high-current, variable ground feed for the blower motor.
- varies the blower motor speed and is controlled by the EATC module software.
- has a delay function to provide a gradual increase or decrease in blower motor speed under all conditions.

Temperature Blend Door Actuator(s)
The temperature blend door actuator(s):
- are located on the heater core and evaporator core housing.
- moves the temperature blend door(s) on command from the EATC module.
- contains a reversible electric motor and a potentiometer. The potentiometer wiper is connected to the actuator output shaft and moves with the output shaft to indicate the position of the temperature blend door(s).
- applies a 5 volt signal to one end of the potentiometer and ground to the other. The voltage available at the wiper indicates the position of the potentiometer. The expressed value of the actuator wiper voltage is sent to the EATC module and is matched with an expected wiper voltage value. The EATC module then drives the actuator motor in whichever direction is necessary to make the actuator wiper voltage agree with the expected EATC module wiper voltage value.

Mode Door Actuators
The two mode door actuators:
- include the air inlet door and defrost/panel/floor mode door actuators.
- position the plenum doors to direct system airflow to the vehicle interior as determined by the EATC module settings.
- contains a reversible electric motor
- (defrost/panel/floor mode door actuator only) contains a potentiometer. The potentiometer wiper is connected to the actuator output shaft and moves with the output shaft to indicate the position of the mode door.
- (defrost/panel/floor mode door actuator only) apply a 5 volt signal to one end of the potentiometer and ground to the other. The voltage available at the wiper indicates the position of the potentiometer. The expressed value of the actuator wiper voltage is sent to the EATC module and is matched with an expected wiper voltage value. The EATC module then drives the actuator motor in whichever direction is necessary to make the actuator wiper voltage agree with the expected EATC module wiper voltage value.