Electronic Automatic Temperature Control

ELECTRONIC AUTOMATIC TEMPERATURE CONTROL

With the use of a microcontroller, the climate control assembly (EATC module) analyzes input from the following major sources:
- temperature, function and blower selection (made by the vehicle occupants)
- in-vehicle temperature sensor
- ambient temperature sensor
- sunload sensor
- vehicle speed sensor
- engine coolant temperature sensor
- evaporator discharge air temperature sensor

Using these inputs, the microcontroller determines the correct conditions for the following outputs:
- A/C clutch engagement
- blower speed
- air temperature blend door position
- floor/panel door position
- panel/defrost door position
- air inlet duct door position

Control System Inputs

Climate Control Assembly
The electronic automatic temperature control (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.

Steering Wheel Audio/Climate Control Switch
The steering wheel audio/climate control switch, located on the right side of the steering wheel, allows the driver to adjust the passenger compartment temperature setting and manually override the blower motor speed setting. For removal and installation of the steering wheel audio/climate control switch, refer to Radio, Stereo, and Compact Disc.

Evaporator Discharge Air Temperature Sensor
The PCM determines A/C compressor clutch cycling based on input from the evaporator discharge air 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 clutch when the evaporator discharge air temperature falls below a preset level, and engaging the A/C compressor clutch when the discharge air temperature rises above a preset level.
- 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. The resulting voltage is returned to the PCM where it is interpreted as an evaporator air discharge temperature reading.
- is located in the 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-vehicle temperature sensor operates in the following manner:
- A thermistor in the in-vehicle 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-vehicle 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-vehicle temperature sensor.
- The suction draws in-vehicle air into the in-vehicle temperature sensor and across the thermistor.

A/C Sunload Sensor
The A/C sunload sensor:
- is located on the top RH side of the instrument panel above the glove compartment.
- contains a photovoltaic diode that is sensitive to light.
- has some unspecified resistance across the terminals depending upon the amount of light reaching the photovoltaic diode; therefore the only test that can be performed is for an internal short circuit.

Control System Outputs

A/C Blower Motor Speed Control
The A/C blower motor speed control is located on the A/C evaporator housing, in the engine compartment.
- 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.
- 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 top of the plenum housing.
- Its function is to move the air temperature blend door 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 the direction necessary to make the actuator wiper voltage agree with the control module wiper voltage.

Vacuum Control Motors
Vacuum control motors are used to move the floor/panel and panel/defrost doors.