Automatic Climate Control

Automatic Climate Control

Automatic temperature control is yet another evolution in the constantly changing climate control system.

The technology of keeping drivers comfortable continues to evolve with each new model year. When automotive air conditioning was offered as a high-priced option back in the 1950s (actually the 1939 Packard was the first car to have factory air), no one would have guessed that half a century later more than 80 percent of the cars and light trucks in North America would be equipped with A/C – or that an estimated 20 to 25 million automotive A/C systems are serviced annually in this country. Those are huge numbers that represent a sales opportunity for aftermarket A/C replacement parts.

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.

Simple manual 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, 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 appeased.

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 and on 2004 Acura TL there’s even a GPS global positioning sensor 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! A number of vehicles even 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, 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. And very few grasp the control logic their ATC system uses to regulate itself and what happens when.

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.

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.

Here’s another glitch that can affect some ATC systems: the systems can forget how to work if the battery runs down or is disconnected! Loss of power to the ATC control module wipes out 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.

Some systems even have built-in glitches right from the factory. On 1998-2003 Cadillac Seville, for example, 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 berserk under certain conditions. The fix here is to reprogram the computer with revised logic from GM using a Tech 2 scan tool.

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, technicians also have to deal with bad sensors, corroded wiring connectors, bad grounds, blown fuses, failed blend door motors, module failures and misguided logic.

Once a fault code has been identified, further testing usually requires a wiring diagram for the vehicle. Bad grounds, corroded connectors and faulty relays are common problems that play havoc with the control electronics, so accurate diagnosis is essential to avoid unnecessary parts replacement and warranty returns.

Compressors and condensers are relatively easy to replace and labor times are typically less than a couple of hours. But evaporators are another story. Most are buried deep inside the HVAC assembly inside the vehicle. Replacing an evaporator often involves tearing the dash apart or even removing it, a job that can take 8, 10 or even 12 hours depending on the application. It’s a time-consuming and difficult job, yet it must be done if an evaporator is leaking refrigerant. The only other alternative is to try an A/C sealer product, which may or may not stop the leak depending on the size and location of the leak. Most OEMs and compressor suppliers do not approve the use of sealants. These products may also gum up refrigerant recovery and recycling equipment and pressure test gauges. But for older vehicles that have limited value or customers who can’t afford a new evaporator, a can of sealer may be the only fix that’s affordable.

Evaporator leaks can be caused by internal corrosion from within or by external corrosion. The underlying cause of internal corrosion is usually moisture contamination of the refrigerant due to leaks or improper servicing procedures. Moisture reacts with the refrigerant and oils in the system to form acids and sludge. Over time, this can lead to evaporator leaks, a plugged orifice tube and/or compressor failure.

When evaporator leaks are caused by external corrosion, the culprit is often an accumulation of leaves and debris in the evaporator case. The debris traps moisture and promotes metal corrosion. It may also jam the fan and cause noise or blower motor failure. Some vehicles do not have screens to keep debris from entering the HVAC inlet. Others have "cabin air filters" that not only stop debris but also dust, pollen, bacteria and even odors from entering the passenger compartment. Many motorists have no idea if their vehicle has a cabin air filter or not, let alone where it might be located or how to replace it.


The only rule about replacement compressors is that there are no rules. Some compressors are shipped dry, others are shipped with assembly oil (which must be drained prior to installation) and others may be shipped with mineral oil for R-12 applications, a specific type of PAG oil for OEM R-134a applications, or polyester POE oil for a retrofit. Hopefully there will be instructions in the box indicating if the unit is oil-filled, and if so with what type of oil – and most importantly, what type of lubricant is required to meet the warranty requirements.

Most compressor manufacturers will void their warranty if the wrong oil is used, if the compressor is installed in a dirty system (contaminated systems must be flushed with refrigerant or an approved solvent and a filter installed), if a non-approved flushing chemical is used to clean the system, or the accumulator or receiver-drier is not replaced. So warn your customers to pay attention to the installation instructions and warranty requirements.

If a vehicle has had a catastrophic compressor failure and there is metallic debris in the system, the system must be flushed to reduce the risk of a repeat compressor failure. Most manufacturers also recommend installing an in-line filter in the liquid line. There are also screen filters for the suction hose to provide added insurance against any debris migrating from the evaporator.

One problem technicians face is that many condensers cannot be completely flushed because of their design. Parallel-flow condensers and those with extremely small passageways are nearly impossible to flush with refrigerant or solvents. On these applications, the condenser may also have to be replaced if there is a risk of contaminating the new compressor.


Nobody knows how refrigeration systems will change in the future, but it appears likely that R-134a’s days are numbered. The Europeans are seriously considering phasing it out starting in 2009. If that happens, the North American OEMs may follow suit.

Europe wants future A/C systems to be as environmentally benign as possible. That means no chlorine-containing CFCs that cause ozone depletion and as little greenhouse gas effect as possible to reduce global warming. The new refrigerant must also be nontoxic and safe – though that doesn’t necessarily mean nonflammable.

The leading contenders that may eventually replace R-134a are plain old carbon dioxide or a close-cousin to R-134a called HFC-152a. Either substance will work as a refrigerant, but both have some drawbacks. CO2 requires extremely high operating pressures (up to 1,800 psi compared to 300 to 400 psi for R-134a.) HFC-152a, on the other hand, is almost a straight drop-in substitute for R-134a and has similar operating and cooling characteristics. In fact, it even cools a little better than R-134a. But HFC-152a is flammable (though not as flammable as propane or most other hydrocarbon-based refrigerants.)

If the Europeans force the auto makers to go to CO2, it will require a major redesign of existing A/C system components as well as all-new service procedures and equipment. If they opt for HFC-152a, it may require adding special safety valves to vent refrigerant away from the passenger compartment in the event of a leak or accident, or using a "secondary loop" cooling system that eliminates the evaporator and substitutes a pair of heat exchangers to circulate chilled water into the passenger compartment. Either way, it will create more parts and service opportunities for the aftermarket.

EPA Ruling: No Restriction of R-134a Sales

Sales of the refrigerant R-134a will not be restricted, according to a new rule finalized last month by the U.S. Environmental Protection Agency (EPA). EPA had proposed in 1998 to restrict the sale of the refrigerant to only certified individuals, but had not finalized the proposal because of significant opposition from the Automotive Aftermarket Industry Association (AAIA) and the Automotive Refrigeration Products Institute (ARPI). R-134a became the standard refrigerant found on all 1994-and-later vehicles when production was phased out for R-12, considered an ozone-depleting substance.

EPA had based its proposal on the fact that car owners converting their vehicle air conditioners to R-134a would vent into the atmosphere the R-12 that still remained in the system. However, AAIA and ARPI opposed the proposal, questioning the authority under the Clean Air Act to take such action and pointing to the fact that the proposal would unfairly impact low- and fixed- income individuals who are forced to work on their own vehicle air conditioners for economic reasons. EPA did reaffirm that sales restrictions continue to be in place for refrigerant blends containing an ozone-depleting substance.

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