Control Components
Control Components
Electronic Automatic Temperature Control (EATC)
The HVAC 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 (VSS)
- Engine Coolant Temperature (ECT) sensor
Using these inputs, the HVAC module determines the correct conditions for the following outputs:
- A/C clutch engagement
- Blower speed
- Temperature blend door position
- Floor/panel mode door position
- Panel/defrost mode door position
- Air inlet mode door position
Control System Inputs - Electronic Automatic Temperature Control (EATC)
HVAC Module
The HVAC 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 DTCs
- An On-Board Diagnostic (OBD) feature to supply the technician with 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. For removal and installation of the steering wheel audio/climate control switch, refer to Information and Entertainment Systems Service and Repair.
Ambient Air Temperature Sensor
The 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 HVAC module.
In-Vehicle 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
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.
Control System Outputs - Electronic Automatic Temperature Control (EATC)
Blower Motor Speed Control
The blower motor speed control is located on the A/C evaporator housing, in the engine compartment.
- The function of the blower motor speed control is to convert low power signals from the HVAC module to a high current, variable ground feed for the blower motor.
- Blower motor speed is infinitely variable and is controlled by the HVAC module software.
- A delay function provides a gradual increase or decrease in blower motor speed under all conditions.
Temperature Blend Door Actuator
The temperature 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 temperature 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 HVAC module and is matched with the expectation valve. The control module then drives the actuator motor in the direction necessary to make the actuator wiper voltage agree with the expected voltage.
Vacuum Control Motors
Vacuum control motors are used to move the floor/panel, panel/defrost and air inlet mode doors.
Manual Climate Control
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 the following:
- 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 - Manual Climate Control
Climate Control Assembly
The climate control assembly has 3 system controls:
- The function selector switch combines a vacuum selector valve with 2 electrical switches to supply battery voltage (B+) to the A/C clutch circuit and the blower motor control circuit
- Temperature selection is accomplished with a potentiometer connected to the temperature blend door actuator. Movement of the control knob from COOL (blue) to WARM (red) causes a corresponding movement on the 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 blower motor resistor
Control System Outputs - Manual Climate Control
Blower Motor Resistor
The blower motor 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 4 blower motor speeds
- Depending on the blower motor switch position, series resistance is added or bypassed in the blower motor circuit to decrease or increase blower motor speed
- A device to prevent overheating (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
Temperature Blend Door Actuator
The temperature blend door actuator has the following features:
- It is a self-controlled electric motor which monitors the position of the temperature control potentiometer in the climate control assembly
- It interprets the temperature control potentiometer signal as the desired temperature blend door position and automatically drives the temperature blend door to that position
- It is located on the top of the plenum chamber
Vacuum Control Motors
Vacuum control motors are used to move the floor/panel, panel/defrost and air inlet mode doors.