Control Components

Part 1:

Part 2:

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 sensor
- ambient temperature sensor
- sunload sensor
- vehicle speed sensor
- engine coolant 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

MANUAL A/C
The manual climate control system heats or cools the vehicle interior depending on the function selector switch position and the temperature selected. The function selector position determines heating or cooling and air distribution. The temperature blend control setting determines air temperature.

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

CONTROL SYSTEM INPUTS - EATC

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.

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 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 INPUTS - 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 voltage (B+) to the A/C clutch circuit and the blower motor speed control circuit.
- Temperature selection is accomplished with a potentiometer connected to the electric blend door. Movement of the control knob from COOL (Blue) to WARM (Red) causes a corresponding movement on the air temperature blend 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 OUTPUTS - EATC

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.

System Response, Part1:

System Response, Part 2:

System Response

CONTROL SYSTEM OUTPUTS - MANUAL A/C

Blower Motor Switch Resistor
The heater blower motor switch resistor has the following features:
- The assembly is located in the engine compartment on the evaporator housing toward the center of the vehicle.
- Three resistor 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 A/C blower motor circuit to decrease or increase A/C blower motor speed.
- An overheating protective device (thermal limiter) will open the resistor coil 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.

A/C Electronic Blend Door Actuator
The Electronic Blend Door Actuator for the manual A/C system function is similar to the EATC system, except that the temperature potentiometer is controlled by the knob located in the center of the A/C manual control.

Vacuum Control Motors
Vacuum control motors for the Manual A/C system operate the same as the EATC system.