Electronic Climate Control

Automatic Temperature Control-ATC

The control assembly operating in conjunction with the system's microprocessor, analyzes inputs from five major sources:

- Temperature setting, function and blower speed selections (made by an occupant).
- In-vehicle temperature sensor.
- Ambient temperature sensor.
- Cold Engine Lockout Switch (CELO).
- B+ Terminal (with ignition in RUN position).

From the input derived from these sources, the microprocessor determines the conditions which will produce optimum performance in the following respects:

- A/C compressor clutch engagement.
- Blower speed.
- Blend door position.
- Panel-Floor door position.
- Panel-Defrost door position.
- Outside-Recirc air door position.

Fig.1 is a block diagram showing system inputs and outputs.

FIG.1: ATC System Block Diagram:

A small DC electric actuator is used to operate the temperature blend door. Vacuum motors are used to control the three air distribution doors. A feedback circuit is used in the blend door actuator to supply the control head with blend door position information. The blower motor is controlled by the control assembly through the variable blower speed controller. The blower speed controller converts a low power signal from the control head to the high power signal required to drive the blower motor.

The automatic temperature control (ATC) system is a split case design, integral blower system and will automatically control the temperature, blower speed and redue the relative humidity of air inside the vehicle (Figure 2 and 3). Two control levers are provided to maintain the desired temperature and system functions. The system will deliver heated or cooled air to maintain the vehicle interor temperature and will maintain the set comfort level automatically. The blower speed is controlled automatically with the blower control lever in the AUTO position or can be set (LOW, MED, HIGH) for more or less air as desired. In the AUTO position the blower speeds will automatically be reduced as the vehicle interior temperature approaches the set point temperature selected.

Automatic control of passenger compartment temperature is maintained in all function control settings (figure 4), except when the system is turned off. In hot weather, it will cool the vehicle to the preset comfort level and then modulate the temperature blend door as required to maintain the desired temperature. In cold weather, provision is made in the FLOOR position of the function lever to delay the operation of the system until engine coolant has warmed enough to minimize the discharge of cold air from the heater outlet. The system will then heat the vehicle to the desired temperature and level out to maintain the preset temperature.

During system operation, outside air is drawn from the cowl vent just below the windshield at all times, except during maximum cooling when recirculated air is used.

FIG.2: ATC System-Lincoln Town Car, Grand Marquis:

FIG.3: ATC System-Crown Victoria:

FIG.4: ATC Control Functions:

The system uses what is called a reheat method to provide conditioned air to the vehicle interior. With this method, all airflow from the blower passes through the evaporator core, where it is cooled and dehumidified. Temperature is then regulated by reheating a portion of the cooled dry air and blending is varied by the temperature blend door which controls the amount of cooled air that flows through or around the heater core, where it is mixed and directed into the distribution plenum. The air is finally directed to the heater ducts, defroster nozzles or instrument panel registers according to the selected function lever position.