It’s hard to believe how complicated some of today’s automatic temperature control systems have become. Simply turning on the A/C is not so simple anymore.

On older "manual" A/C systems, the A/C button usually energizes a relay that engages the magnetic clutch on the compressor. The temperature setting is controlled by a slide switch that uses cables or vacuum motors to operate the HVAC blend air doors, and the fan speed is controlled by the driver.

Except for a few Korean econoboxes and other entry-level models, most A/C systems now have some type of automatic temperature control (ATC) system. The driver picks a temperature setting and the ATC system does the rest. It controls the operation of the compressor, the setting of the blend air doors and even the blower speed. And it is able to maintain a relatively consistent interior temperature by monitoring the temperature inside and outside of the vehicle, and many also compensate for sunload.

DUMB HEADS AND SMART SYSTEMS
Automatic temperature control systems may have either a "dumb" control head (no built-in control electronics) or a "smart" head (the ATC module is part of the control head). When the driver selects a temperature or turns the system on, a dumb head will send the input signal on to the ATC module or directly to the powertrain control module (PCM) on some applications. If the unit has a smart head, the control module processes the input and forwards a request on to the PCM, which then looks at inputs from the throttle position sensor, vehicle speed sensor, ABS/traction control module and other sensors to determine if it’s okay to turn on the A/C.

When the compressor engages, the ATC system adjusts the blend doors and blower speed to achieve and maintain the desired temperature setting. If the system has "dual-zone" controls, the driver and passenger can each select his or her own temperature settings, and if the vehicle is a minivan or SUV with rear air, there are usually separate controls for the rear-seat occupants, too.

All this technology is wonderful, as long as it is working properly. When something goes wrong, it can create a real challenge for the service technician to diagnose and repair it.

Most automatic temperature control systems will generate fault codes that can help a technician pinpoint problems in the electronics. On some vehicles, the codes can be retrieved manually through the ATC control head itself, but most require a scan tool.

ATC systems rely on feedback from various temperature sensors (ambient air temp, interior air temp, inlet air temp) as well as feedback on the position of the air control doors (from motor or door sensors) and blower speed. Without this vital information, the ATC system can’t operate properly.

Dumb control heads (which typically have four to eight wires in back) have fairly simple electronics. The switches activate solenoids that operate vacuum motors to open and close mode doors and the blend door. A separate ATC control module handles the information processing and manages the system.

Smart control heads (which typically have about 20 wires in back), by comparison, have the control module as part of the control head. This eliminates the need for a separate control module, but also makes the control head very expensive to replace if something goes wrong with it. Some of these heads can cost upwards of $2,000.

ATC SENSORS & INPUTS
Most of the temperature sensors used in today’s ATC systems are simple two-wire thermistors that change resistance in response to changes in temperature. Most are "negative temperature coefficient" sensors that lose resistance as the temperature goes up. A simple way to check this type of sensor is to use a blow drier to heat the sensor. The resistance should drop as the sensor warms up.

Ambient (outside) air temperature sensors are usually in the front of the vehicle near or under the condenser. They typically have a slow sample rate to even out variations in air temperature that may be sensed at different vehicle speeds. When the vehicle stops moving, heat can build up quickly around the sensor. This could mislead the ATC control module into thinking it was getting hotter outside. So, the ATC control module only looks at the sensor input every couple of minutes instead of continuously. On some applications, the ATC module may even be programmed to ignore input from the ambient outside air temperature sensor when the vehicle is not moving. Consequently, if a technician is reading the outside air temperature sensor with a scan tool, the reading may not change when the vehicle is sitting still. Why? Because the ATC control module is programmed to ignore the reading and substitute a fixed value until the vehicle is moving.

Most ATC systems also have one or two "sunload" sensors located on the dash. Most of these are photo diode sensors. Sunlight changes the sensor’s output voltage. On dual-zone ATC systems, the right and left sunload sensors usually have shades so each sensor will only read light input on one particular side. Therefore, the right and left sensors are not interchangeable. Sunload sensors are important because up to 60 percent of the heat load inside a vehicle can come from sunlight shining through the windows.

To monitor the temperature inside the vehicle, one or more interior air temperature sensors are used. Some of these are "fan style" sensors that use a small fan to pull air through the sensor. If the fan dies, the sensor may not read accurately. One way to test the fan style sensors is to see if smoke from a match or cigarette is drawn into the sensor opening.

Some in-car temperature sensors are connected by a hose to a venturi on the blower motor duct. When the blower is running, it creates a vacuum to pull air through the sensor. If the hose gets plugged up with debris, it can affect the operation of the ATC system.

Some Asian vehicles also use duct temperature sensors and heater core temperature sensors to further refine temperature control. These are usually found on the dual-zone ATC systems.

ATC systems also have to know the position of the air control doors, so it may get this information from door position sensors or the motors that control the doors. Some motors have a separate feedback circuit inside that keeps the ATC control module informed about their position. On some applications, the ATC module can count commutator pulses to monitor the motor’s position.

Other sensor inputs that may be used by an ATC system include an A/C pressure transducer (usually mounted in the high-side line) to monitor refrigerant pressure, an evaporator temperature sensor (to prevent evaporator icing), a compressor temperature sensor (to turn the compressor off if it gets too hot) and/or a compressor rpm sensor (to monitor belt slippage). Mitsubishi, for example, uses a "belt lock controller" to disengage the compressor if the drive belt slips or the compressor seizes.