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What does automatic temperature control mean in your car’s A/C system?

ATC systems contain what we’ll refer to as three major component groups. These are:
The Controller – The “brains” of the system;
The Sensors – Supply the controller with pertinent input data;
The Controlled or Output Devices – Vacuum solenoids, electric motors, etc. that eventually turn electrical signals into mechanical motion.
Of course, an ATC system contains many other components, such as the refrigeration system, the blower motor, ductwork, wiring, etc. These components essentially work the same way in an ATC system that they do in a manual temperature control (MTC) system.
As noted, the ATC controller is the “brains” of the system. It processes the data it receives from its sensors and issues output commands to the various devices it controls. Really, it has only one job: to do whatever it takes to keep the interior of the vehicle at a stable set temperature as selected by the vehicle occupants.
ATC controllers are essentially microprocessor devices, mini-computers very similar to the type that control electronic fuel injection systems. They gather input data from sensors, make decisions based on the sensor data, then issue appropriate commands to controlled devices, with the end result usually some type of mechanical motion. Examples of this could be a heater control valve opening or closing, a vacuum control solenoid passing vacuum through to a vacuum actuator, an electric motor moving a temperature blend door, etc. The result of a controller command may not always be motion, however. Sometimes, controllers issue electrical commands to other components, such as blower motor power modules.
ATC controllers go under a variety of different names. In many domestic vehicles, ATC controllers are often referred to as “Programmers.” In many imported vehicles, ATC controllers are often referred to as “Amplifiers” or “A/C Amplifiers.” (Note: Many non-ATC equipped imports also use a module commonly referred to as the amplifier or A/C amplifier. In these cases, it is strictly a simple electronic device that more or less only controls compressor clutch, cooling fan and engine idle-up functions, somewhat of a “helper” to the PCM or ECM.)
Some vehicles are equipped with an electronic device called a “Body Control Module” or “BCM.” BCMs can control many different functions, such as power window and seat operation, audio systems, in some applications, even windshield wiper and instrument panel illumination. The list of components and systems that can be controlled by a BCM can be quite lengthy. Sometimes, control of ATC system operation is also a function of the BCM. Where this is the case, there will be no separate “stand alone” ATC system controller.
The control head is the “control panel” mounted in the dashboard, that allows selection of the various system functions. In cases where the ATC controller is located in the control head, it is often referred to by the industry slang term as being a “smart head.” In situations when the control head serves only as an interface between the occupants and a remotely mounted controller, it is referred to as a “dumb head.”
In systems that use smart heads, all system operational decisions originate directly within the control head, and in most cases, all of the controlled devices, such as vacuum solenoids and electric motor actuators will be remotely mounted. Often, all of the vacuum control solenoids will be packaged together in a “solenoid box,” but sometimes they may be mounted individually in the appropriate locations on the case/duct assembly.
In systems that use dumb heads, the ATC controller is often located in the programmer or amplifier. The programmer/amplifier is usually located under the dash, most often directly on the case/duct assembly. The programmer will often contain vacuum solenoids and the temperature blend door control motor.
The ATC controller usually communicates with other electronic devices or modules. As mentioned previously, two of these could be the control head and/or BCM, but other common ones are the fuel injection system’s Electronic Control Module or Powertrain Control Module (ECM/PCM). Why is the ECM or PCM involved with ATC system operation? The refrigeration system/A/C compressor, when operating, can represent a significant increase in heat load and also rob the engine of power. There may be times when it would be undesirable for the refrigeration system to function, so the ECM or PCM always has final say concerning compressor clutch activation or deactivation.
Very often, the various electronic modules in a vehicle will be connected together by a communication line, often referred to as the “data bus.” They can share data back and forth many, many times per second. The process whereby many different devices can communicate over the same data line is often referred to as “multiplexing.” The ATC controller, BCM, PCM etc. “talk” to each other over the data bus.
Dual-Zone Climate Control Systems

In many vehicles, the ATC system is capable of supplying discharge air of more than one temperature to different areas in the vehicle. Most often, systems that incorporate this type of function are referred to as Dual-Zone Climate Control Systems and will allow the driver and front seat passenger to set their own desired temperature. Dual-zone systems contain two separately commanded temperature blend doors. The temperature variance range between the driver and front seat passenger can be up to 30° F, usually with settings between 60° F minimum and 90° F maximum, which also happens to be the temperature range that most single-zone systems span. There are also “Tri-Zone” and “Quad-Zone” ATC systems. These systems provide for up to three or four different discharge air temperatures. Tri-zone and quad-zone systems are usually found in passenger vans, sport utility vehicles and luxury cars, and allow the passengers in the rear section of the vehicle to control the temperature in their location.
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.
You can E-mail us at or visit to find a Mobile Air Conditioning Society repair shop in your area. Visit to find out more about your car’s mobile A/C and engine cooling system.

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