Automatic Temperature Control (ATC)

SYSTEM OPERATION
The Automatic Temperature Control (ATC) system provides fully adjustable three zone climate control; Driver front zone, Passenger Front zone and Rear zone. The following is a list of ATC controls and features:
- A POWER button which allows the operator to turn the system completely OFF.
- AUTO HIGH/LOW switch allows the operator to select what rate (fan speed) the system will provide the selected comfort level.
- DRIVER, REAR and PASSENGER rocker switches to select desired temperature for each zone.
- Recirculation button allows cooling air to be recirculated which maximizes cooling ability.
- A/C select button allows the operator to turn the A/C compressor OFF.
- Defroster button allows the operator to turn on the defroster independently during automatic control.
- Fan control rotary switch for selecting fan speed.
- Mode rotary switch for selecting heating/cooling direction.
- REAR SYSTEM rotary switch for activating the Rear ATC Switch allowing intermediate passenger control over rear climate control.

Blower Motor Operation
The Automatic Temperature Control (ATC) front and rear blower control provides continuously variable control of air flow rate to meet occupant comfort requirements. Pulse width modulation of the blower motor power allows the front and rear blower to operate at any speed from OFF to full speed. When front or rear blower operation is required, the ATC sends a PCI Bus message to the Front Control Module (FCM) requesting blower relay ON. The FCM provides a ground for the front and rear blower motor relay coils, activating both relays. The front and rear blower relays provide 12 volts to their respective blower motor power modules. Each module provides a 12 volt blower motor control signal to the ATC. The ATC provides a pulse width modulated (duty-cycle) ground signal to this circuit based on climate requirements. The higher voltage on the signal circuit (less duty-cycle ground) the lower the blower speed request. The lower voltage on the signal circuit (more duty-cycle ground) the higher the blower speed request.

Infrared Sensors
The ATC system uses infrared (I/R) sensors to monitor and control occupant comfort levels. This sensing system replaces interior air temperature and solar sensors which required complex control programs to maintain occupant comfort levels. The front I/R sensor is located in the instrument panel center bezel outlet. The rear I/R sensor is located in the rear overhead mounted ATC switch.

Rear ATC switch
The rear ATC switch is mounted in the headliner. The switch contains a rotary adjustment for fan speed, a rotary adjustment for mode selection, a push button switch for temperature selection and a digital display of the selected temperature.

Cooldown Testing
The ATC provides a feature referred to as a Cooldown Test. This test is initiated using the DRB III and is designed to check the performance of the air conditioning system. The ATC will not perform this test if the ambient air temperature is below 12 °C (53 °F). During the test, the ATC compares the ambient air temperature to the evaporator temperature sensor. To pass the cooldown test, the evap temperature sensor must drop 6 °C (20 °F) below ambient temperature within 2 minutes of start of test.

At the completion of the cooldown test the DRB III will display one of the following messages indicating test outcome:
- Cooldown Test Passed
- Cooldown Test Failed
- Evap Temp Sensor Shorted
- Evap Temp Sensor Open
- A/C Pressure Too High
- Invalid Conditions for Cooldown Test, Evap Temperature Too Low

If a message other than Cooldown Test Passed occurs, refer to the appropriate symptom for diagnosis.

Active And Stored Trouble Codes
The Automatic Temperature Control (ATC) is capable of storing Active and Stored trouble codes. Active codes indicate a current fault in the system. Stored codes indicate that a problem has occurred in the system, however is not currently present. Active codes cannot be erased until the problem causing the code has been repaired. At this time the Active code is converted to a Stored code, which can be erased using the DRB III.