Control Components
Control Components
Electronic Manual Temperature Control (EMTC)
The Electronic Manual Temperature Control (EMTC) 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 request
- Auxiliary heater operation (optional on diesel vehicles only)
Control System Inputs
The EMTC system has 2 system control inputs.
HVAC Module
The HVAC module integrates the temperature control, airflow mode selection, A/C request button, recirculated air request button and rear defog switch into a single unit.
The temperature control selector setting determines air temperature. Movement of the temperature display from COOL (blue) to WARM (red) causes a corresponding movement of the temperature blend door and determines the discharge air temperature that the air distribution system will maintain. The temperature control selector is an integral part of the HVAC module and cannot be installed separately.
The airflow mode setting determines air discharge location. Movement of the airflow mode selector causes a corresponding movement of the airflow mode doors and determines the air discharge location. The airflow mode selector is an integral part of the HVAC module and cannot be installed separately.
The A/C request button determines A/C compressor operation, except when the airflow mode selector is in the OFF, MAX A/C or DEFROST mode. The A/C request button is an integral part of the HVAC module and cannot be installed separately.
The recirculated air request button can select recirculated air in any mode except DEFROST, and fresh air in any mode except MAX A/C or OFF. The recirculated air request button is an integral part of the HVAC module and cannot be installed separately.
The rear defog button signals activation of the heated backlight. The rear defog button is an integral part of the HVAC module and cannot be installed separately.
The HVAC module is equipped with self-test capabilities.
Blower Motor Switch
The blower motor switch is mounted in the HVAC module and controls blower motor speed by adding or bypassing resistors in the blower motor resistor in all modes except OFF.
Control System Outputs
The EMTC system has 4 system outputs.
Blower Motor Resistor
The blower motor switch varies the blower motor ground circuit resistance by adding or bypassing series resistance in the blower motor resistor. Increased resistance will lower the blower motor speed and lowered resistance will increase blower motor speed. When the blower motor is in the HI position, the blower motor resistor is bypassed.
The blower motor resistor is located on the heater core and evaporator core housing near the blower motor.
Temperature Blend Door Actuator
The temperature blend door actuator moves the temperature blend door on command from the HVAC module.
The temperature blend door actuator contains a reversible electric motor and a potentiometer. The potentiometer circuit consists of a 5-volt reference signal connected to one end of a variable resistor and a signal ground connected to the other. A signal return circuit is connected to a wiper, which is driven along the variable resistor by the actuator shaft. The signal returned to the HVAC module from the wiper indicates the position of the temperature blend door. The HVAC module drives the actuator motor in whichever direction is necessary to make the actuator wiper voltage agree with the expected HVAC module wiper voltage value.
The temperature blend door actuator is located on the top of the heater core and evaporator core housing.
Airflow Mode Door Actuator
The airflow mode door actuator uses a cam and lever assembly to position the airflow mode doors on command from the HVAC module. The mode door actuator contains a reversible electric motor and potentiometer. The potentiometer allows the HVAC module to monitor the position of the airflow mode doors. The potentiometer circuit consists of a 5-volt reference signal connected to one end of a variable resistor and a signal ground connected to the other. A signal return circuit is connected to a wiper, which is driven along the variable resistor by the actuator shaft. The signal returned to the HVAC module from the wiper indicates the position of the airflow mode doors.
The airflow mode door actuator is located on the plenum chamber.
Air Inlet Mode Door Actuator
The air inlet mode door actuator moves the air inlet door between the FRESH and RECIRC positions on command from the HVAC module. The air inlet mode door actuator is driven to, and will automatically stop at, the full RECIRC or full FRESH air inlet position and does not require a potentiometer circuit to monitor its position.
The air inlet mode door actuator is located on the air inlet duct.
Electronic Automatic Temperature Control (EATC)
The HVAC module analyzes input from the following major sources:
- Temperature, airflow mode, blower, A/C and RECIRC selection (made by the vehicle occupants)
- In-vehicle temperature sensor
- Ambient temperature sensor
- Solar radiation sensor
- Vehicle speed
- Engine coolant temperature
Using these inputs, the HVAC module determines the correct conditions for the following outputs:
- A/C compressor operation
- Blower speed
- Temperature blend door position
- DEFROST/PANEL/FLOOR door position
- Air inlet door position
- Auxiliary heater operation (optional on diesel vehicles only)
Control System Inputs
The dual-zone Electronic Automatic Temperature Control (EATC) system has 4 control system inputs.
HVAC Module
The HVAC module provides an interface for the vehicle occupants to control the climate control system, or automatically controls the climate control system in the AUTO mode. The HVAC module has a vacuum fluorescent display for displaying set temperature, airflow direction, blower speed and DTCs. An On-Board Diagnostic (OBD) feature is incorporated to supply the technician with DTCs. These DTCs help direct the technician to the inoperative component.
In-Vehicle Temperature Sensor
The in-vehicle temperature sensor contains a thermistor, which measures the in-vehicle air temperature and sends that reading to the HVAC module. An aspirator hose and elbow is connected between the heater core and evaporator core housing and the in-vehicle temperature sensor. The aspirator hose uses airflow through the heater core and evaporator core housing to create a venturi-type suction to draw in-vehicle air through the in-vehicle temperature sensor and across the thermistor.
The in-vehicle temperature sensor is located at the RH lower of the instrument cluster finish panel.
Solar Radiation Sensor (Sunload Sensor)
The solar radiation sensor supplies information to the HVAC module indicating sunload.
The solar radiation sensor is located at the top center of the instrument panel.
Ambient Temperature Sensor
The ambient temperature sensor signal is received by the HVAC module and indicates the outside air temperature. The ambient temperature sensor is located in front of the radiator on the radiator support.
The ambient temperature sensor signal is not the sole factor considered by the HVAC module for the outside air temperature display. Therefore, the ambient temperature PID monitored with a scan tool may not exactly match the HVAC module display.
The ambient temperature sensor is located above the RH headlamp.
Control System Outputs
The EATC system has 4 control system outputs.
Blower Motor Speed Control
The blower motor speed control controls the blower motor speed by converting low power signals from the HVAC module to a high current, variable ground feed for the blower motor. A delay function is used to provide a gradual increase or decrease in blower motor speed under all conditions.
Temperature Blend Door Actuators
The dual-zone EATC system uses 2 temperature blend door actuators to control 2 separate temperature blend doors. The temperature blend doors independently vary the LH side and RH side temperature settings, as desired. The temperature blend door actuators each contain a reversible electric motor and a potentiometer. The potentiometer circuit consists of a 5-volt reference signal connected to one end of a variable resistor and a signal ground connected to the other. A signal return circuit is connected to a wiper, which is driven along the variable resistor by the actuator shaft. The signal returned to the HVAC module from the wiper indicates the position of the temperature blend door. The HVAC module drives the actuator motor in whichever direction is necessary to make the actuator wiper voltage agree with the expected HVAC module wiper voltage value.
The LH temperature blend door actuator is located on the bottom of the heater core and evaporator core housing. The RH temperature blend door actuator is located on the top of the heater core and evaporator core housing.
Airflow Mode Door Actuator
The airflow mode door actuator uses a cam and lever assembly to position the airflow mode doors on command from the HVAC module. The mode door actuator contains a reversible electric motor and potentiometer. The potentiometer allows the HVAC module to monitor the position of the airflow mode doors. The potentiometer circuit consists of a 5-volt reference signal connected to one end of a variable resistor and a signal ground connected to the other. A signal return circuit is connected to a wiper, which is driven along the variable resistor by the actuator shaft. The signal returned to the HVAC module from the wiper indicates the position of the airflow mode doors.
The airflow mode door actuator is located on the plenum chamber.
Air Inlet Mode Door Actuator
The air inlet mode door actuator moves the air inlet door between the FRESH and RECIRC positions on command from the HVAC module. The air inlet mode door actuator is driven to, and will automatically stop at, the full RECIRC or full FRESH air inlet position and does not require a potentiometer circuit to monitor its position.
The air inlet mode door actuator is located on the air inlet duct.