SYSTEM DESCRIPTION
| CAUTION | When discharging air conditioning system, use only approved refrigerant recovery/recycling equipment. Make every attempt to avoid discharging refrigerant into the atmosphere. |
| WARNING | To avoid injury from accidental air bag deployment, read and carefully follow all SERVICE PRECAUTIONS and DISABLING & ACTIVATING AIR BAG SYSTEM procedures in AIR BAG SYSTEM SAFETY article in the ACCESSORIES/SAFETY EQUIP section. |
The Automatic Temperature Control (ATC) control panel uses a microcomputer to analyze temperature and mode selection through input from in-vehicle temperature sensor, ambient temperature sensor and engine temperature.
The microcomputer determines correct conditions for compressor clutch engagement, blower speed, blend door position, floor/panel door position, panel/defrost door position, and fresh/recirculated air door position.
Electric actuators are operate air doors. Each actuator has a feedback circuit so microprocessor can determine door position.
A Blower Motor Speed Controller (BSC) converts low voltage signals from ATC control panel into high voltage signals to run compressor clutch and blower motor.
AMBIENT TEMPERATURE SENSOR OPERATION
Ambient temperature sensor, located on fresh air intake duct of evaporator case, is a thermistor (temperature sensitive resistor) that measures outside air temperature and inputs signal to ATC control panel.
ASPIRATOR OPERATION
The aspirator, located on evaporator case at blower motor housing outlet, is connected by a hose to in-vehicle temperature sensor. It creates a slight vacuum pulling in passenger compartment air. The in-vehicle temperature sensor inputs the signal to microprocessor.
BLOWER MOTOR SPEED CONTROLLER OPERATION
Blower motor speed controller, attached to evaporator case, downstream of evaporator core, converts low current signals from ATC control panel to a high current, variable ground feed to blower motor.
Variable blower speed is controlled by microprocessor and blend door actuator position. A delay function provides a gradual change in blower motor speeds.
Note. DO NOT operate system with blower motor disconnected or damage to blower motor speed controller may occur.
CONTINUOUS POSITION ACTUATORS OPERATION
Continuous position actuators operate blend door and floor/panel door. These actuators are NOT interchangeable. Actuators operate in both directions. Actuators stop at any position within a 180-degree operating range as directed by signal from ATC control panel.
TWO-POSITION ACTUATORS OPERATION
The two-position actuators operate panel/defrost door and fresh/recirculated air door. Actuators operate in only one direction and stop in two predetermined positions as directed by microprocessor signal. Actuators are NOT interchangeable. Actuators are identified by color code on arm. The panel/defrost door actuator arm is Silver. The fresh/recirculated air door actuator arm is Black.
ATC CONTROL PANEL OPERATION
The ATC control panel consists of 9 push buttons, a Vacuum Fluorescent Display (VFD) and 7 Light Emitting Diodes (LEDs). Set temperature can be raised or lowered in one degree between 65-85°F (18-29°C) with WARM or COOL push buttons. (Scheme 1)
Temperature settings can be changed from Fahrenheit to Celsius readings by depressing MILES/KM button on vehicle message center. The ATC control panel performs the following functions
- Processes data from manual input to ATC control panel, in-vehicle temperature sensor, ambient temperature sensor,engine temperature sensor and 4 actuator position sensors
- When turned on, it remembers temperature and mode selected before last shut-off.
- Provides continuous position control of blend and function actuators.
- Provides 2 position control of panel/defrost and fresh/recirculated air actuators.
- Provides a continuously variable blower signal which is amplified by the BSC to control blower voltage.
- Provides compressor clutch on-off signal which is amplified and delivered to the pressure switch by the BSC.
Scheme 1
IN-VEHICLE TEMPERATURE SENSOR LOCATION & OPERATION
The in-vehicle temperature sensor, located behind right lower instrument panel, is a thermistor (temperature sensitive resistor) which measures vehicle air temperature and inputs signal to ATC control panel.
NO A/C COMPRESSOR CLUTCH OPERATION
- Connect jumper wire between pins No. 4 (Lt. Green/Purple wire) and No. 6 ( Yellow/Black wire) at blower motor speed controller. See WIRING DIAGRAM at end of article. Leave jumper wire connected until it is determined no short exists. If A/C compressor clutch engages, go to step 4).
- If A/C compressor clutch does not engage, go to step 3). If 20-amp fuse blows, A/C compressor clutch is shorted. Check clutch diode in wiring harness. Repair short and retest system to see if ATC control panel will turn A/C compressor off and on. If compressor does not turn on and off, replace blower motor speed controller.
- Measure voltage at pin No. 6 (Yellow/Black wire) of blower motor speed controller connector. If reading is zero volts, check battery circuit. If reading is more than 10 volts, check A/C clutch for damage.
- Set ATC control panel in automatic mode. Measure voltage between pin No. 2 (Gray/White wire) of blower motor speed controller and ground. If reading is 7 volts or less, replace blower motor speed controller. If reading is greater than 7 volts, go to next step.
- Check continuity of circuit No. 321 (Gray/White wire) between blower motor speed controller and ATC control panel connector. If continuity is okay, replace ATC control panel. If continuity does not exist, repair wiring and retest system using self-test.
Note. Repeat blower motor speed controller failure indicates a short in Purple wire between controller and clutch cycling pressure switch in engine compartment or leaky clutch diode.