Control Assembly, HVAC
Control AssemblySystem Response Chart
The electronic automatic temperature control (EATC) assembly is located in the instrument panel. It consists of 11 push buttons, a blower speed override thumbwheel for manual input, and a vacuum fluorescent display (VFD) for displaying set temperature, ambient temperature, functional selection and diagnostic trouble codes.
When the system is operating under AUTOMATIC control, the VFD display will show the preferred or target temperature to which the elements of the EATC system respond. Blower motor speed adjusts automatically to the existing conditions. To control the fan speed yourself, use the thu mbwheel which will cancel the automatic fan speed control. Temperature selection may be raised or lowered in one degree increments between 18°C (65°F) and 29°C (85°F) by pressing the red button to raise or the blue button to lower the desired temperature. The system will respond accordingly.
Other control assembly features include:
^ A 16°C (60°F) setting for maximum cool and a 32°C (90°F) setting for maximum heat.
^ Six manual override buttons. These buttons control manual override for the system's airflow direction only. These manual override buttons are located along the bottom of the Electronic Automatic Temperature Control (EATC) assembly. The system will remain in a manual mode until the automatic mode button is depressed. While the system is in any of the manual override positions the blower motor will still be controlled automatically.
^ A rear fan oft button to control operation of the rear seat blower.
^ An OFF button which, if pressed, will apply vacuum to close the heater and A/C air inlet duct door, shut oft blower motor operation, and discontinue climate control functions through the system.
When the thumbwheel control for the blower motor is rotated out of the position it occupied under automatic control, it will remain under manual control until the automatic button is again pressed. (Under automatic control, blower speed varies as required to accommodate the total automatic functions in the system. Under manual control, blower speed is constant based on the thumbwheel setting.)
Whenever the headlamp switch is engaged and the panel dimmer rheostat is rotated, the backlighting on the control assembly, as well as in other instrument panel locations, will dim.
The control assembly, operating in conjunction with the system's microcomputer, analyzes inputs from five major sources:
1. Temperature function and blower speed selections (made by the vehicle occupants).
2. Automatic temperature control sensor.
3. Engine coolant temperature sensor (ECT sensor).
4. A/C sunload sensor.
5. Cold engine lock out (CELO).
From the input derived from these sources, the microcomputer determines the conditions which will produce optimum performance in the following respects:
6. A/C clutch engagement.
7. Blower speed.
8. A/C air temperature control door position.
9. Heater air damper door position.
10 Windshield defroster door position.
11. Heater and A/C air inlet duct door position. A small DC electric motor or A/C electronic door actuator motor is used to operate the A/C air temperature control door. Vacuum control motors are used to control each of the three remaining air distribution doors. A feedback circuit is used in the A/C electronic door actuator motor to supply the control assembly with A/C electronic door actuator motor position information. The blower motor is controlled by the A/C control through the A/C blower motor speed control. The A/C blower motor speed control is required to react to low power signals from the A/C control to provide the high current ground required to drive the blower motor. The following table shows the control assembly response to the function control selections.
Input Sensors
A/C Ambient Air Temperature Sensor
The A/C ambient air temperature sensor is located in front of A/C condenser core on LH side of vehicle, and contains a thermistor which measures temperature of outside air.
Automatic Temperature Control Sensor
The automatic temperature control sensor is located behind the instrument panel, above the glove compartment, and contains a thermistor which measures temperature of air inside the passenger compartment.
A/C Sunload Sensor
The A/C sunload sensor is located in the RH side of the instrument panel near the defroster grille assembly, and contains a photovoltaic (sensitive to sunlight) diode.
A/C Cycling Switch
The A/C cycling switch is located in the engine compartment on the top of the suction accumulator/drier, near the low side manifold gauge access port. It senses pressures within the air conditioning system, and energizes or de-energizes the A/C clutch to maintain designed operating pressures within the air conditioning system.
Pulse Width Modulator
The pulse width modulator works with the dimmer control on the headlamp switch to control instrument panel and A/C control illumination intensity.
A/C Blower Motor Speed Control
The A/C blower motor speed control is located in the A/C evaporator housing, upstream of the A/C evaporator core.
The function of the A/C blower motor speed control is to convert low current signals from the electronic control assembly to a high current, variable ground teed to the blower motor. Blower motor speed is infinitely variable and is controlled by the electronic control assembly software and A/C electronic door actuator motor position. A delay function provides a gradual increase or decrease in blower motor speed under all conditions. There is also a high blower relay integral with the blower speed controller which provides power for maximum airflow in the high blower position.
CAUTION: The system should not be operated with the blower motor disconnected. Damage may occur to the A/C blower motor speed control if cooling air is not provided by the blower motor.