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Control Components

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Electronic Automatic Temperature Control
With the use of a microcontroller, the Electronic Automatic Temperature Control (EATC) module analyzes input from the following major sources:
- temperature, function 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 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, and installation of the steering wheel audio/climate control switch, refer to Radio,Stereo,and Compact Disc.

Rear Audio/Climate Control Switch
The rear audio/climate control switch is equipped on limousine (long wheel base) vehicles and is located in the rear armrest. The rear audio/climate control switch allows the rear passengers to adjust the passenger compartment temperature setting and manually override the blower motor speed setting. For removal of the rear audio/climate control switch, refer to Radio,Stereo,and Compact Disc.

A/C Ambient Air Temperature Sensor
The A/C ambient air temperature sensor and bracket:
- 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.

A/C Sunload Sensor
The A/C sun load sensor:
- is located on the top side of the instrument panel.
- 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 whichever direction is necessary to make the actuator wiper voltage agree with the control module wiper voltage.

A/C Electronic Recirc Door Actuator
The A/C electronic recirc door actuator is located on the plenum housing.
- It's function is to move the air inlet duct door on command from the control assembly.
- The operation of the A/C electronic recirc door actuator is the same as the A/C electronic blend door actuator.

Rear Audio/Climate Control Module
The rear audio/climate control module is equipped on limousine (long wheel base) vehicles and is mounted to the back of the rear seat backrest. The rear audio/climate control module converts an input from the rear audio/climate control switch, then signals the Audio Control Module (ACM) via the Audio Corporate Protocol (ACP) bus. The ACM then signals the EATC module over the J1850 communications network (Standard Corporate Protocol [SCP] bus).

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


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SYSTEM RESPONSE
[a]: The A/C clutch circuit is energized by the EATC module but the final control of clutch operation is determined by the powertrain control module. The A/C clutch can be shut off (or kept off) for several seconds at engine start-up, during high engine speeds, during acceleration, during low engine idle conditions (approximately 200 rpm below idle specs) and when the engine coolant temperature exceeds approximately 120°C (248°F).
[b]: If the engine coolant temperature fails to reach 50°C (122°F) after 3 1/2 minutes of operation, the blower motor speed will increase and the air flow will shift from the DEFROST position to the FLOOR position.
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