A/C Update: The Logic Behind Automatic Temperature Controls
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A/C Update: The Logic Behind Automatic Temperature Controls

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Many new cars and trucks now have automatic temperature control (ATC) systems that not only regulate cooling but also heating for year-round passenger comfort. Most of these systems have their own computer that may be built into the control panel head, located elsewhere or integrated into the body control module. One thing’s for sure: the more sophisticated the system is, the more complex are its control electronics and operating logic — which increases the odds of something going wrong.

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Simple manual temperature controls are being replaced with digital push button settings and computer logic. Today’s new car buyers want separate controls for the driver and front seat passenger, and triple-zone rear A/C in minivans and SUVS with a separate control head in the rear to keep the kiddies or mother-in-law comfortable.

ATC systems require a complex array of internal and external sensors including ambient air temperature sensors, interior temperature sensors, outlet duct and evaporator temperature sensors, pressure sensors, humidity sensors, blend door position sensors and sunload sensors. On the 2004 Acura TL there’s even a global positioning sensor (GPS) that allows the control module to determine the vehicle’s orientation to the sun so it can increase cooling on the side facing the sun and decrease cooling on the shaded side. Some vehicles also use passive infrared sensors mounted in the dash or an overhead console to monitor the body temperature of the vehicle’s occupants. This allows the system to fine-tune heating and cooling so everybody stays comfortable.

All this technology is great when it works the way it is designed to work, but sometimes things go wrong. One of the biggest problems auto makers face today is that many motorists don’t fully understand their ATC systems. They may not understand which buttons do what or why. This can sometimes lead to complaints about poor cooling when in fact the system is operating normally. When a vehicle is first started, the system logic may prevent it from blowing hot or cold air until the engine has run a certain length of time or reached a certain operating temperature. The operating logic will choose which ducts the hot or cold air blows out of as well as the fan speed and the position of the recirc door. Most ATC systems have a manual override mode or semi-automatic mode that allows the driver to control more of its functions, but even some manual control modes must pass through the computer and be acceptable to the operating logic.

To make matters worse, every auto manufacturer develops its own operating logic — and that may vary from one model to another and from one year to the next. The logic may dictate that the system go into the recirc mode to maximize cooling when a vehicle is first started on a hot day, or it may not. Most systems will not turn the A/C compressor on if ambient temperatures are below freezing. Most will run the A/C compressor when the system is in the defrost mode to dehumidify the air.

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One of the most common mistakes that’s made with manual A/C systems is choosing recirc during cold, wet weather. When the humidity inside the vehicle is higher than outside the vehicle, cold damp weather causes the windows to fog over. The defrosters may not do a very good job of clearing the glass unless recirc is discontinued or the windows are opened. ATC systems don’t make that mistake because most are programmed to let in fresh air when in the defrost mode. Semi-automatic A/C control systems maintain a constant air temperature inside the vehicle by monitoring existing operating conditions and anticipating changes that are needed to keep the cabin at the preset comfort level. This includes controlling the compressor (cycling the clutch on and off or changing the duty cycle of a variable displacement compressor), changing the position of the blend air doors and adjusting the blower speed as needed.

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Fully automatic A/C control systems go even further and turn on the defrosters when needed, which may also include electric defrost for the rear window and heated mirrors. Some luxury vehicles even have heated seats that are controlled by the ATC system, so it’s important to know what the ATC system controls and what inputs it uses before you try to diagnose it.

Dumb Heads and Smart Heads
Automatic temperature control systems may have either a “dumb” control head with no built-in electronics (typically four to eight wires in back), or a “smart” head where the ATC module is in the control head itself (larger wiring harness with up to 20 wires in back).

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When the driver turns on an ATC system with a dumb head, the head passes its input to a separate ATC control module, or in some cases to the powertrain control module (PCM). The ATC module or PCM or both then work together to control the compressor. But the ATC module is solely responsible for maintaining the preset temperature inside the vehicle by controlling the blend doors using inputs from its various sensors.

If the ATC system has a smart head, the control module inside the unit processes the driver’s command, turns on the system and does whatever is needed to regulate conditions inside the vehicle. Because of the built-in electronics, smart heads can be very expensive (up to $2,000!) to replace. That’s why accurate diagnosis is so important with today’s high tech, high dollar ATC systems.

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Temperature Sensors
To maintain a preset air temperature, an ATC system will typically have one or more interior air temperature sensors, an ambient (outside) air temperature sensor, and possibly one or two sunload sensors.

Interior air temperature sensors are usually simple two-wire thermistors that change resistance with temperature, but some are infrared sensors that detect heat from the vehicle’s occupants. The interior air temperature sensors may be located in the ATC control head, instrument panel, overhead console or even the seats. The thermistor type usually has an aspirator tube that pulls air through the sensor when the blower fan is running. Others use a small electric fan for the same purpose. A plugged aspirator tube or inoperative fan will slow the sensor’s response to temperature changes.

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Most air temperature sensors have a “negative temperature coefficient,” which means they lose resistance as the temperature goes up. A simple way to check this type of sensor is to use a blow dryer to heat the sensor. The resistance should drop as the sensor warms up.

To monitor the outside temperature, an ambient air temperature sensor is located outside the cabin. Some vehicles have two, one in the plenum at the base of the windshield and a second in the front of the vehicle so it can monitor the air temperature when the vehicle is moving. Ambient air temperature sensors typically have a slow sample rate to even out variations in readings 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 module into thinking it was getting hotter outside. So most ATC modules only look at the ambient sensor input every couple of minutes instead of continuously. On some applications, the ATC module may even ignore input from the ambient sensor when the vehicle is not moving.

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Many ATC systems also make use of a photo diode sunload sensor on the dash. This sensor allows the ATC system to compensate for increased cooling needs when the cabin is being heated by direct sunlight. On vehicles with dual-zone systems, there is often a separate sunload sensor for each side. Sunload sensors are supplied a reference voltage by the ATC module, and pass current when the light intensity reaches a certain threshold.

Some ATC systems have additional temperature sensors located on the evaporator and/or compressor to prevent evaporator icing and to regulate the operation of the compressor. Some Asian vehicles also have duct temperature sensors and heater core temperature sensors to further refine temperature control. These are usually found on the dual-zone ATC systems.

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Door Control
ATC systems have to be able to keep track of air door positions, so they use door position sensors for this purpose or feedback signals from the door position motors. Some three-wire “smart” motors have a separate feedback circuit inside that keeps the ATC control module informed about their position. On other applications, the ATC module counts commutator pulses to determine the motor’s position. Other ATC systems have a separate module for each air door motor position to keep track of what’s going on in the system.

Heating, cooling and defrost problems are often due to inoperative air doors, which in turn is caused by a broken door motor. Older ATC systems typically use vacuum-operated doors while newer systems mostly use electric motors.

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On older vehicles with vacuum motors, loose or leaky vacuum lines are a common fault. Motors and hoses can be checked by applying vacuum with a hand pump. With electric motors, loose wiring connectors or internal motor faults can prevent a motor from moving. Checking voltage, ground and wiring continuity at the motor, and checking motor resistance with an ohmmeter will help you find these kinds of faults.

Also, don’t overlook the possibility of debris inside the plenum causing a blockage or preventing a door from moving.

Glitches & Fixes
One problem that afflicts some automatic temperature control systems is that they may “forget” how to work if the battery runs down or is disconnected! Loss of power to the ATC control module wipes its memory or causes it to forget blend door settings. To reboot the system and restore normal operation, a special start-up procedure may be required using a scan tool. On others, the driver has to press certain control buttons in a specified sequence to reset the computer.

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Component Snapshot: Water Pumps

On Chrysler minivans with three-zone automatic temperature control, for example, the A/C system can be reinitialized by turning the ignition key to the run position (engine off), and pressing and holding the PWR button and RECIRC switches on the front HVAC control head for five seconds. This will start a self-calibration test that runs about three minutes. The LEDs on the head will flash until the calibration procedure is complete.

You also may encounter automatic temperature control systems that have a glitch from the factory. On 1998-2003 Cadillac Seville, the A/C outlet temperatures may fluctuate rapidly blowing hot air then cold air, or it may oscillate between panel and floor mode. The problem, says GM, is the control logic allows too much variation in the temperature of the evaporator, which may cause it to behave oddly and go into the “berserk” mode under certain conditions. The fix here is to reprogram the computer with revised logic from GM using a Tech 2 scan tool.

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On 1993-’98 Ford Taurus and 1998-2002 Lincoln Continental with Electronic Automatic Temperature Control (EATC), erratic or noisy operation of the cool/warm function is covered in Ford TSB 02-23-6. Codes in the system can be read by pushing the floor OFF buttons together, then the AUTO button. This will put the system into a self-diagnostic mode. Codes 022, 024 or 025 indicate a blend door control failure, but this is not the real cause of the problem. The underlying cause is a warped “stratification/secondary blend air door” that is binding and sticking in its case. This is a second door that moves in conjunction with the main blend air door to help mix heated and cooled air before it exits the outlet ducts. It is connected by a nylon rod to the main blend door. Ford redesigned the door and added reinforcing ribs so it will not bend and jam. The new part number is 3F1Z-19D842-AA. Replacement requires removing the dash and opening the plenum case.

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Regardless of what kind of vehicle you’re working on, just remember that today’s ATC systems can experience both electronic and mechanical failures. So in addition to troubleshooting and repairing refrigeration circuit problems such as refrigerant leaks, compressor noise, compressor failures, plugged orifice tubes, condenser and evaporator leaks, you may also have to deal with bad sensors, corroded wiring connectors, bad grounds, blown fuses, failed blend door motors, module failures and misguided logic.

Trouble Codes
Most ATC systems will generate trouble codes that can help you pinpoint problems in the electronics. On many vehicles, the codes can be read manually through the ATC control head, but on others you’ll need a scan tool to access the codes. If you find a code, write it down, clear it, then test drive the vehicle to see if the same code reappears. Follow the diagnostic chart to isolate and repair the problem. There are few shortcuts here, so don’t skip any steps that might lead you to the wrong conclusion. Always refer to the service information for the vehicle before proceeding.

If there are no codes, you need to verify the customer’s complaint and make sure he isn’t using the wrong control settings or misunderstands how the system is supposed to operate.

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Do a visual inspection to look for obvious faults like loose wires, debris in the plenum (listen for noise when the blower runs), compressor engagement when the system is on, disconnected or missing aspirator tube for an air temperature sensor, etc.

Check for any technical service bulletins (TSBs) that may be directly related to the problem. TSBs can save you time and frustration, and prevent you from replacing parts needlessly.

Finally, once repairs have been made, verify the correct operation of the ATC system before you return the vehicle to your customer. This can save you the embarrassment of having to deal with a “hot” customer if you failed to fix the problem correctly.

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