Control Components

Component Locations:

The manual climate control system heats or Cools the vehicle interior depending on the function selector position and the temperature selected. Function selector position determines heating or cooling and air distribution. The temperature blend control setting determines air temperature.

The manual climate control components are used to:
- select air inlet source (outside or recirculated).
- select blower motor speed.
- select discharge air temperature (temperature blend).
- select discharge air location (defrost, panel, floor).
- select A/C compressor clutch operation.

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
- panel/floor door position
- panel/defrost door position
- air inlet door position

Control System Input-manual A/C
Climate Control Assembly
The climate control assembly has three system controls:
- The A/C heater function selector switch combines a vacuum selector valve with two electrical switches to supply battery positive voltage (B+) to the A/C clutch circuit and the blower motor control circuit.
- The temperature selection is accomplished with a potentiometer connected to the electric blend door actuator that controls positioning of the temperature blend door. Movement of the control knob from COOL (blue) to WARM (red) causes a corresponding movement on the air temperature control door and determines the temperature that the system will maintain.
- The blower motor switch controls blower motor speed by adding or bypassing resistors in the heater blower motor switch resistor.

Control System Input-EATC
Climate Control Assembly
The electronic automatic temperature control (EATC) module, located in the instrument panel, has the following features:
- 11 push buttons
- a blower speed override thumbwheel for manual input
- a vacuum fluorescent display for displaying set 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.

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 plenum assembly and the in-car temperature sensor.
- The automatic temperature control sensor hose and elbow takes air from the plenum assembly 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 sunload sensor :
- is located on the top 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 carried out is for an internal short circuit.

Control System Output-Manual A/C
Blower Motor Switch Resistor
The heater blower motor switch resistor has the following features:
- The assembly is located on the evaporator housing in the engine compartment.
- Three resistance elements are mounted on the resistor board to provide four blower motor speeds.
- Depending on the heater blower motor switch position, series resistance is added or bypassed in the blower motor circuit to decrease or increase blower motor speed.
- An overheating protective device (thermal limiter) will open the resistor coil circuit when the temperature reaches 121°C (250°F) interrupting the blower motor operation in all speeds except HI.
- The thermal limiter cannot be reset and is not serviceable.Control System Output-EATC Blower Motor Speed Control The blower speed control is located on the evaporator housing in the engine compartment.
- The function of the 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.
- The 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.

Control System Outputs-EATC
Blower Motor Speed Control
The blower speed control is located on the evaporator housing in the engine compartment.
- The function of the 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.
- The 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 Electric Blend Door Actuator
The A/C electric blend door actuator is located on the plenum assembly.
- its function is to move the air temperature blend door on command from the control assembly.
- The A/C electric 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.

EATC System Response

EATC System Response (Part 1 Of 2):

EATC System Response (Part 2 Of 2):

[a] 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 panel/floor door will shift from the defrost position to the floor.