Control Components

CONTROL COMPONENTS

Two climate control systems are available:
- Electronic manual temperature control (EMTC) (Expedition only)
- Dual-zone electronic automatic temperature control (EATC)

Refer to the appropriate heading below for information concerning each system.

Manual Climate Control
The manual climate control 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 operation

Control System Inputs - Electronic Manual Temperature Control (EMTC)

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 setting determines air temperature. Movement of the temperature control from COOL (blue) to WARM (red) causes a corresponding movement of the temperature blend door and determines the air discharge temperature that the air distribution system will maintain. The temperature control switch is an integral part of the HVAC module and cannot be installed separately.

The airflow mode setting determines discharge air location. Movement of the mode selector causes a corresponding movement of the airflow mode door actuators. The 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 mode selector is in the OFF, MAX 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 determines air inlet source, except when the airflow mode selector is in the OFF, MAX or DEFROST mode. 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 and heated mirrors. The rear defog button is an integral part of the HVAC module and cannot be installed separately.

The HVAC module is not equipped with self-test capabilities.

Blower Motor Switch
The blower motor switch controls blower motor speed by adding or bypassing resistors in the blower motor resistor in all function selector positions except OFF. The blower motor switch is mounted to the HVAC module and can be separately installed.

Control System Outputs - Electronic Manual Temperature Control

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 HIGH 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 wiper voltage value.

The temperature blend door actuator is located on the heater core and evaporator core housing.

Mode Door Actuators
Electric mode door actuators are used to position the 3 airflow mode doors on command from the HVAC module. The 2 mode door actuators each contain a reversible electric motor. The DEFROST/PANEL/FLOOR door actuator contains a potentiometer to allow 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 door. The air inlet mode door actuator is driven to the full RECIRC or full FRESH air inlet position, and does not require a potentiometer circuit to monitor its position.

Dual-Zone Electronic Automatic Temperature Control (EATC)
The HVAC module analyzes input from the following major sources:
- Temperature, mode and blower selection (made by the vehicle occupants)
- In-vehicle temperature
- In-vehicle humidity (Navigator only)
- Ambient temperature
- Evaporator discharge air temperature
- Sunload
- 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
- LH side temperature blend door position
- RH side temperature blend door position
- Airflow mode door positions

Control System Inputs - Dual-Zone Electronic Automatic Temperature Control (EATC)

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.

Steering Wheel Audio/Climate Control Switch
The steering wheel audio/climate control switch allows the driver to adjust the passenger compartment temperature setting and manually override the blower motor speed setting.

For removal and installation of the steering wheel audio/climate control switch, refer to Radio, Stereo, and Compact Disc.

Ambient Air Temperature Sensor
The ambient air temperature sensor contains a thermistor, which measures the temperature of outside air as a resistance and sends that reading to the HVAC module.

In-Vehicle Temperature/Humidity Sensor - Navigator
The in-vehicle temperature sensor contains 2 thermistors, which separately measure the in-vehicle air temperature and humidity and send those readings to the HVAC module. An aspirator hose and elbow is connected between the heater core and evaporator core housing and the in-vehicle temperature and humidity sensor. The aspirator hose uses airflow from the heater core and evaporator core housing air stream to create a venturi-type suction to draw in-vehicle air through the in-vehicle temperature sensor and across the thermistors.

In-Vehicle Temperature Sensor - Expedition
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.

Dual-Zone Solar Amplifier (Sunload Sensor)
The solar amplifier is a dual-zone type that supplies separate information to the HVAC module indicating LH side and RH side sunload. For information on the location and service of the solar amplifier, refer to Lighting and Horns.

Control System Outputs - Electronic Automatic Temperature Control (EATC)

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.

The blower motor speed control is located on the heater core and evaporator core housing near the blower motor.

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.

Mode Door Actuators
Electric mode door actuators are used to position the 3 airflow mode doors on command from the HVAC module. The 2 mode door actuators each contain a reversible electric motor. The DEFROST/PANEL/FLOOR door actuator contains a potentiometer to allow 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 door. The air inlet mode door actuator is driven to the full RECIRC or full FRESH air inlet position, and does not require a potentiometer circuit to monitor its position.