Air-Conditioning Systems

Chris Mc Loone   Christian P. Koop

Those who have been around for a while probably remember when fire apparatus did not have air-conditioning (a/c).

That was when most fire trucks still had open cabs in the rear or jump seat area. The requirement to fully enclose cabs for safety reasons also created the need to air-condition the cabs, particularly for those in the warmer climates. In these areas, a/c service and repair for emergency vehicle technicians (EVTs) is pretty much a year-round requirement compared with those in the north, who may only need it in the summer months. Depending on the vehicle type and where and how the a/c evaporator and condenser are installed and mounted, access and service can be time-consuming and difficult. Those involved in developing specifications for new apparatus should keep in mind the importance of designing a/c systems from the onset to make it easier for EVTs to service and repair them. This will ultimately result in reduced downtime and save dollars over the long haul-a very important item with today’s tighter budgets. In this article, I will briefly cover some of the history involving development of refrigerant and automotive a/c systems. I will also discuss several service and repair tips for technicians to identify common causes of a/c system failures.


Today there are many different types of refrigerants for different applications. The first person credited for developing a process to synthesize chlorofluorocarbons (CFCs) is Frederic Swarts in the late 19th century. The refrigerants in use at this time were very dangerous as they were flammable, highly toxic, and deadly. To find a better refrigerant, Charles Kettering, from General Motors, formed a team that developed a more stable, nonflammable, moderately toxic refrigerant in the late 1920s using Swarts’s process. Many may remember Freon 12 or R-12 (dichlorodifluoromethane). This is the refrigerant General Motors and DuPont jointly produced and patented that was used in a/c systems in automobiles, trucks, and various other refrigeration applications for more than 50 years in the United States and many parts of the world. This refrigerant was eventually determined to deplete the atmospheric ozone layer and was phased out in the mid 1990s under the Montreal Protocol. It may still be in production, although illegal, in some countries. It has a global warming potential (GWP) of 2,400.

Although GWP figures are known to be controversial, I think they are worth mentioning to provide a perspective on the environmental problem associated with these refrigerants. R-134A (tetrafluoroethane), which is also harmful to the ozone layer, has a GWP of 1300, and has a 10-year life span, replaced R-12. Because of its high GWP, there is a worldwide push to phase out any refrigerant that has a GWP higher than 150. There are various refrigerants that may replace R-134A. From what I understand, R-1234yf (GWP of 4) may end up being the refrigerant of choice because of its very low GWP and much shorter atmospheric life. In Europe, some manufacturers are already using R-1234yf. In the United States, the Environmental Protection Agency (EPA) will require the phase-out of R-134A over the next few years. Currently there are several replacement refrigerants being proposed to replace R-134A, and it is expected that by the 2021 model year, all vehicles in the United States will be sold with a replacement refrigerant.

Most EVTs won’t have to worry about these major changes for some time but should keep in mind that there are other refrigerants sold as cheaper “alternatives” to R-134A that could be dangerous and can contaminate their a/c recovery and charging machines. Most of these refrigerants are manufactured in other countries and don’t meet the purity standards required by Society of Automotive Engineers Standard J2776. Some of these refrigerants may actually be flammable as they contain hydrocarbons. If you are using some of these so-called R-134A alternatives, keep in mind that a collision that ruptures the evaporator and ignites the refrigerant gas places the occupants in danger of being burned. If you are not sure about the quality of the refrigerant you are using, it pays to invest in test equipment to check the purity of the gas before you run it through your charging station and into an a/c system. There are several of these refrigerant testers available from different manufacturers and are reasonably priced.

Packard was the first automobile manufacturer to offer a/c as an option back in 1940, which it called the “Weather Conditioner.” These were huge trunk-mounted systems that took up a considerable amount of space. At the time, the Packard a/c compressor did not have a clutch and was on all the time. If you did not want the a/c running and needed to turn it off, you had to remove the compressor belt. Imagine having to do that today. It was not until 1953 that a/c systems started to become more widespread and various car manufacturers offered them with the electromagnetic compressor clutch so the belt would not have to be removed. As I have mentioned in previous articles, history seems to repeat itself; clutches are slowly being eliminated in automobiles. This is mainly to reduce weight and reduce the size of the compressor in the engine compartment, which ultimately allows more aerodynamic design and improves fuel economy to help manufacturers meet Corporate Average Fuel Economy requirements. They are now being slowly replaced by continuously variable compressors or electric compressors that are computer-controlled based on temperature settings selected by the driver.

The Compressor

The heart of the a/c system is the compressor. Most found on apparatus and rescue trucks are mechanical and belt-driven and can be piston, scroll, or vane type. The oil that lubricates the compressor is carried with the refrigerant as it circulates in the system. Excessive heat and low refrigerant will lead to the early demise of any compressor. Having the proper oil and correct amount in the system is crucial. During compressor replacement, it is very important to follow the manufacturer’s instructions to ensure the correct amount of oil is added. Too little oil and the compressor will fail; too much oil and system pressures will be higher than normal, and the system will not cool properly. Both of these issues will lead to compressor failure. In fact, most replacement compressors that fail do so because of improper oil balance and not because of a defective compressor.

The compressor clutch has essentially remained the same for many years. In normal use, it cycles the compressor on and off many times. It is composed of an electromagnetic coil; the pulley, which is rotated by the drive belt (either micro V type or the older standard single V belt); and the driven plate. When the a/c is turned on or power is applied to the clutch coil, a strong electromagnetic field is generated that pulls the driven plate tightly against the flat surface of the pulley, connecting the two members and rotating the compressor shaft. If the coil does not have sufficient voltage, this will lead to slippage and a damaged burned clutch. A quick diagnosis is to ensure proper voltage at the coil. Also measure the resistance of the coil, which is generally around three to four ohms.

System Leaks

The most common cause of an a/c system that does not cool well or at all is usually a leak in the system. Because of the large cabs and large rescue modules that need to be cooled, the average fire or rescue truck is equipped with front and rear a/c systems. This basically means longer a/c hoses and more fittings than the average vehicle, so you have more possibilities for leaks because joints and fittings are traditionally the weak points.

The first thing the EVT needs to do is see if the compressor is turning on. If not, install gauges and check pressures. If pressure is very low or there is no gas in the system, there is most likely a leak that will need to be located and repaired to continue the diagnosis. First, look for telltale signs of leaks such as refrigerant oil stains that are visible at fittings, the condenser, around the compressor hose fittings, and the shaft seal. Any area that has evidence of refrigerant oil is a possible leak. If the leak is not easily visible, a leak check using dye or an electronic leak detector is the next step. Keep in mind when using dyes that there are special ultraviolet (UV) lights designed specifically for use with the type of dye being used. These will actually cause the dye to shine much brighter when the UV hits the dye (leaking area) because the light wavelength is matched to the dye, resulting in more fluorescence. Yet another way to detect leaks, which is gaining more popularity, is by listening for the sound of leaking gas with ultrasonic leak detectors. After locating and repairing the leaks, the final step before charging the system is to pull a vacuum and make sure it holds vacuum with no movement of the gauge needle for at least five minutes. I like to go least 15 minutes for good measure.

The Evaporator

Many years ago, the a/c evaporator was considered self-cleaning. As humidity condensed on it, the condensate was thought to wash away dirt and dust particles that would adhere to it through the drain. Because of this thought process, most automotive a/c evaporators were not equipped with filters. The reality is that they do get clogged with dust, dirt, hair, lint, and other debris. These will cling to the evaporator’s cooling fins and reduce air flow, which equals low or no cooling, thus requiring intake filters. Because of the dark environment they are in and the cycle they are subjected to (hot, cold, damp), they can be a haven for mold and bacteria. These not only can smell bad but also can lead to allergies and make occupants sick. Although the filters are not 100 percent effective, they will cut down on the debris that can clog the evaporator fins and reduce cleaning cycles.

If the evaporator does clog, it will need to be cleaned. Its accessibility will determine the degree of difficulty and how time-consuming the job will be. If your units do not have filters for the a/c evaporator, consider installing them. I have found that adapting filters to units that did not have them is fairly simple. It is a lot easier and quicker to clean or replace the filter than it is to clean a dirty evaporator and should reduce the chance of mold and bacteria growing on the evaporator. For those who are involved in specifying apparatus, if the evaporator filter does not come standard, specify one. You will be glad you did. Today, most modern cars and trucks are factory-equipped with a/c filters called cabin filters.

Yet another important item in the evaporator housing is the blower motor. This is subject to failure depending on equipment usage and should also be easily accessible for efficient replacement and less downtime. Another important concern with evaporators hung from the roof of fire apparatus, typically mounted between the driver and officer positions, is condensate drainage. This is particularly important in very humid areas. Some manufacturers have used different types of pumps to remove the water from the collection tray. In my experience, these have proven to be problematic and unreliable. Another system involved using truck air pressure through a venturi-style electric-over-air valve arrangement to evacuate the water, which also proved unreliable. If the valve remained on or stuck on, it would tax the truck air brake compressor, reducing air compressor life and lowering air pressure in the system. When the pumps or evacuation system quits, the water in the tray will typically be slung out of the vents during cornering, soaking one of the occupants, computers, switch panels, and other items in the cab. To give another perspective, just remember that electronics and water are not a good mix. The most reliable system, and the simplest I have seen by far, for removing condensate is by gravity. Although they still require filter maintenance and evaporator cleaning, there are fewer system items that can fail.

A lot has changed since a/c systems began appearing on fire apparatus. However, the common causes of poor a/c system performance and failures that EVTs need to be concerned with to diagnose and repair have essentially remained the same. I still remember when our first fully enclosed cabs equipped with a/c were ordered in the late 1980s and placed in service in 1990. Those early systems, depending on the climatic conditions in which the equipment was operating, may have been adequate or not. Of course, the overall design of the system also had significant impact on how efficient it was. This includes cab insulation, compressor size, proper condenser sizing, and evaporator sizing. I think it is important for manufacturers to build and install a/c units that perform adequately, have reliable and durable service life, and have adequate access to all components to ensure efficient service and repair by EVTs. This will result in less equipment downtime and ultimately a cost savings.

CHRISTIAN P. KOOP is the fleet manager for the Miami-Dade (FL) Fire Department. He has been involved in the repair and maintenance of autos, heavy equipment, and emergency response vehicles for the past 35 years. He has an associate degree from Central Texas College and a bachelor’s degree in public administration from Barry University and has taken course work in basic and digital electronics. He is an ASE-certified master auto/heavy truck technician and master EVT apparatus and ambulance technician.

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