Electronic Automatic Temperature Control (EATC)

With the use of a microcontroller, the Electronic Automatic Temperature Control (EATC) module analyzes input from the following major sources:
- temperature, air distribution and blower selection (made by the vehicle occupants)
- in-vehicle temperature
- ambient temperature
- sunload
- vehicle speed
- engine coolant temperature

Using these inputs, the microcontroller determines the correct conditions for the following outputs:
- A/C compressor clutch operation
- blower speed
- air temperature blend door position
- plenum/floor door position
- windshield defroster door position
- air inlet door position
- floor console door position.

Control System Inputs

Climate Control Assembly - Electronic Automatic Temperature Control (EATC)
The EATC module, located in the instrument panel, has the following features:
- a blower speed override control for manual input
- a vacuum fluorescent display for displaying set temperature, ambient temperature, function and Diagnostic Trouble Codes (DTCs)
- an On-Board Diagnostic (OBD) feature to supply the technician with diagnostic trouble codes (DTCs). These DTCs direct the technician to the inoperative component.

Evaporator Discharge Air Temperature Sensor
The PCM determines A/C compressor clutch cycling based on input from the evaporator air discharge temperature sensor. The evaporator air discharge temperature sensor:
- maintains evaporator core temperature, and prevents icing of the evaporator core by disengaging the A/C compressor when the evaporator discharge air temperature falls below a preset level, and engaging the A/C compressor when evaporator discharge air temperature rises above a preset level.For information regarding operating temperature specifications, refer to Specifications.
- contains a thermistor which receives a reference voltage from the PCM. The thermistor then varies the resistance to the reference voltage based on the evaporator discharge air temperature, and returns the resulting voltage to the PCM. The PCM determines evaporator air discharge temperature based on the return voltage value.
- is located in the heater core and evaporator core housing just downstream of the evaporator airflow.

A/C Ambient Air Temperature Sensor
The A/C ambient air temperature sensor:
- is located in front of the A/C condenser core near the center of the vehicle.
- contains a thermistor which measures the temperature of outside air as a resistance and sends that reading to the electronic automatic temperature control assembly.

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

Solar Amplifier (Sunload Sensor)
For additional information on the location and service of the solar amplifier, refer to Lighting and Horns/Ambient Light Sensor.

Control System Outputs

A/C Blower Motor Speed Control
The A/C blower motor speed control is located under the A/C evaporator housing, near the blower motor.
- The function of the A/C blower motor speed control is to convert low power signals from the electronic automatic temperature control module to a high current, variable ground feed for the blower motor.
- A/C blower motor speed is infinitely variable and is controlled by the electronic automatic temperature control module software.
- A delay function provides a gradual increase or decrease in blower motor speed under all conditions.

A/C Electronic Blend Door Actuator
The A/C electronic blend door actuator is located on the rear of the A/C evaporator housing.
- Its function is to move the air temperature blend doors on command from the control assembly.
- The A/C electronic blend door actuator 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 air temperature blend door.
- A 5 volt signal is applied to the ends of the potentiometer. The voltage available at the wiper indicates the position of the potentiometer. The expressed value of the actuator wiper voltage is sent to the electronic automatic temperature control module and is matched with the wiper voltage of the module potentiometer. The control module then drives the actuator motor in whichever direction is necessary to make the actuator wiper voltage agree with the control module wiper voltage.

Vacuum Control Motors
The vacuum control motors:
- are located on the A/C evaporator housing, A/C recirculating air duct and heater air plenum chamber.
- direct system airflow to the vehicle interior as determined by the electronic automatic temperature control module or manual override.

System Response, Part 1:

System Response, Part 2:

SYSTEM RESPONSE