24 - 58 HEATING AND AIR CONDITIONING					Ä
AUTOMATIC TEMPERATURE CONTROL (ATC)
		INDEX
	page			page
Computer Aided Diagnostic Tests . . . . . . . . . . .	60.		Failure Code Service Procedures . . . . . . . .	. . . . 62
Erasing Failure Codes . . . . . . . . . . . . . . . . . . . .	71.		General InformationÐComponent Operation	. . . . 58
Failure Code Definitions . . . . . . . . . . . . . . . . . .	62.		NonÐComputer Aided Diagnostic Tests . . .	. . . . 60

GENERAL INFORMATIONÐCOMPONENT OPERATION

The Automatic Temperature Control (ATC) system lets the operator change the passenger compartment environment. A computer, built into the control panel (Fig. 1), regulates the desired temperature, air flow direction and blower speed. The operator may also select an AUTO mode feature in which the computer would select the blower speed and air flow direction. Refer to the Owner's Manual for proper operation.

Fig. 1 Automatic Temperature Control Computer

Panel

The system goes into a maximum cool recirculated air lock-in mode:

²After selecting a comfort temperature setting of 18°C or 65°F.

²With the A/C compressor turned on (snowflake illuminated) (Fig. 2).

²With the system not in the defrost mode.

²When the temperature button is held in for ten seconds.

This will not regulate the temperature until the system is turned off or the comfort setting is raised.

Vacuum is not used to control any of the ATC system or components. It is a totally electronic unit.

COMPUTER CONTROLLER

The ATC computer controller (Fig. 3) manages all of the systems electronic functions. It provides logic and/or power to operate the power module, blend-air

Fig. 2 Automatic Temperature Control Panel Display

Symbols

door actuator, the mode door and fresh/recirc actuators. It remembers the operators control panel selected settings when the vehicle is not running. Then it measures return inputs from the sun sensor and various temperature sensors. After measuring all input information, the computer will ground the output circuits. This provides logic signals for automatic system control. The computer control panel is not serviceable except for the illumination bulbs.

Fig. 3 ATC Computer ControllerÐAspirator Air

Intake

Ä

For bulb replacement, refer to A/C-Heater Control Lamp Replacement in Group 8E Instrument Panel.

POWER MODULE

The power module (PM) (Fig. 4) receives pulse width modulated logic signals from the computer control. The PM then supplies varied voltage to the blower motor ground circuit for different blower speeds based on the logic signal. The power module is not serviceable and must be replaced if found to be defective.

Fig. 4 Power ModuleÐAmbient Temperature

SensorÐFresh/Recirc Door Actuator

AMBIENT TEMPERATURE SENSOR

The ambient air temperature sensor is located in the A/C housing above the glove box. This is also a thermistor and will pick up on the environmental ambient temperature. The computer control uses the information provided by the ambient sensor to:

²Regulate the low blower speed

²Temperature offsets

²Mode control

The ambient sensor is not serviceable and must be replaced if found to be defective.

FRESH/RECIRC DOOR ACTUATOR

The fresh/recirc door actuator (Fig. 4) is an electric servo motor. It (with the use of linkage) mechanically positions the A/C unit door in the open or closed position. Actuation of the servo motor will occur when drive signals are supplied to the actuator from the computer control. This actuator does not contain a feedback strip therefore can not communicate the fresh/recirc door position back to the computer control. The fresh/recirc door actuator is not serviceable and must be replaced if found to be defective.

BLEND-AIR DOOR ACTUATOR

The blend-air door actuator (Fig. 5) is an electric servo motor which mechanically positions the A/C

HEATING AND AIR CONDITIONING 24 - 59

unit temperature door. Actuation of the servo motor will occur when drive signals are supplied to the actuator from the computer control. A feedback strip in the actuator allows the computer control to know the exact position of the temperature door at all times. The blend-air door actuator is not serviceable and must be replaced if found to be defective.

Fig. 5 Blend-Air Door ActuatorÐMode Door ActuatorÐView from bottom of A/C Housing

MODE DOOR ACTUATOR

The mode door actuator (Fig. 5) is an electric servo motor. It (with the use of linkages) mechanically positions the A/C unit panel/bi-level door and the floor/ defrost door. Actuation of the servo motor will occur when drive signals are supplied to the actuator from the computer control. This actuator also contains a feedback strip which allows the computer control to know the exact position of the mode door at all times. The mode door actuator is not serviceable and must be replaced if found to be defective.

IN-CAR TEMPERATURE SENSOR/ASPIRATOR ASSEMBLY

The ATC system uses various sensors which return electrical signals to the computer control. The in-car temperature sensor (Fig. 6) is part of a motorized aspirator assembly (Fig. 6) that is mounted in the instrument panel. A small fan (in the aspirator) draws air through an intake on the instrument panel. This air flows over a thermistor which picks up temperature variations. The computer control then makes system adjustments to maintain a constant passenger compartment temperature. The in-car temperature sensor/aspirator assembly is not serviceable and must be replaced if found to be defective.