Counterperson Training: Air Conditioning Systems

The Air Conditioning parts market is probably the single most regulated service area in the automotive aftermarket.

It is rife with provincial regulations that vary from region to region, all under the larger umbrella of federal laws, regulations, and guidelines. It is notable that the regulations, in Canada are considerably different from those of the U.S. regarding products, containers, and those certified to purchase refrigerant. (For Regulations at a Glance, visit www.autoserviceworld.com, and search our back issues for A/C Special Report. These are updated each April.)

Fortunately, A/C systems have remained largely the same.

The basic components of a typical five-part A/C system are: the Compressor; the Evaporator; the Con-denser; the Expansion Valve or Orifice Tube; and the Accumulator or Receiver Drier. In addition, switches and controllers as well as refrigerant and lubricant are indispensable.

There are four basic types of A/C systems: the basic cycling clutch system; the control valve system; the clutch cycling orifice tube system (CCOT, also known as an accumulator orifice tube system); and the variable displacement orifice tube system (VDOT).

All systems cool by the same principle: converting a liquid to a gas (the refrigerant) pulls heat from air passing through the evaporator.

Although they are a closed loop, A/C systems are divided in two sections: High Side and Low Side, referring to the pressure levels at each. The system area “downstream” of the compressor, including the condenser and receiver/drier, are on the High Side. The orifice tube or expansion valve marks the start of the Low Side, where the pressure in the system is dropped from as much as 250 psi (or more) to, for example, 30 psi.

It is the easy evaporation of refrigerant as a result of this drop in pressure which provides cooling. Low pressure refrigerant is routed through the evaporator.

The Compressor literally drives the system. In any type of system, it provides the system pressure necessary to condense the refrigerant at normal operating temperatures (e.g. 250 psi). The refrigerant is, however, still in a gaseous state, not becoming a liquid until it has passed through the condenser. There are a number of configurations of compressor: two piston, multiple piston, radial and variable displacement.

The Compressor Clutch is only found on systems that cycle the compressor on and off. These are the basic cycling and CCOT systems. Clutches are not found on Control Valve and VDOT type A/C systems. The cycling action comes under the control of a Thermostatic Cycling Switch (or an Evaporator Pressure Switch) in order to prevent the evaporator from freezing. It uses a capillary tube filled with refrigerant and is located at the evaporator. In “clutchless systems”, a Suction Throttling Valve (STV or POA or Control Valve) regulates evaporator pressure, preventing freezing by limiting the pressure.

The Condenser, essentially the A/C system’s radiator, allows the heat collected by the refrigerant (to cool the air, it gains the heat) to be passed to the outside air. The condenser is usually located ahead of the car’s radiator, but is much smaller. The refrigerant enters the condenser as a high pressure gas and leaves as a liquid.

The Receiver/Drier or Filter/Drier is located after the condenser, on the high pressureside of the system. Its job is to remove moisture (using a desiccant) from the refrigerant, as well as remove any particles of contamination. It also serves as a liquid storage device. These are used in basic cycling systems and control valve (STV) systems that use Expansion Valves.

Accumulators are the equivalent of the Receiver/Drier but are located on the low-pressure side of a system and used in CCOT and VDOT systems, which both use Orifice Tubes.

R-134a systems use either XH-7 or XH-9 desiccant in the receiver/drier or accumulator. Some manufacturers recommend routine replacement of the accumulator or receiver/drier to one containing XH-7 or XH-9 during the retrofit procedure. Systems with silica gel should also be switched to XH-7 or -9 desiccant. Others recommend leaving it alone.

Both the Expansion Valve and the Orifice Tube devices serve the same function: to regulate the liquid refrigerant flow to create a low-pressure area downstream between it and the compressor (which also results in a high-pressure zone on the upstream side). Expansion Valves vary the amount of refrigerant flow in response to temperature/pressure changes in the evaporator. Orifice Tubes cannot vary their size but are balanced to the system.

Variable Displacement Orifice Tubes (VDOTs) have entered the aftermarket as a modification over a system’s standard orifice tube in some cases. These are generally used to compensate for some of the situations occurring in R-134a retrofits of R-12 systems, acting in a method similar to Expansion Valves, to regulate pressures.

The Evaporator, located in the heater core area, receives the low-pressure refrigerant (as small droplets) and, when hot air from the passenger cabin is blown through it, the refrigerant droplets absorb heat from the air and boil as they do so. The cooled air is routed back to the cabin through ducts and is regulated by blend doors.

There are of course many Safety Switches and System Controllers; Low Pressure Cutout Switches; Ambient Temperature Switches; High Side and Low Side Safety Switches; and Fan Controllers. These are generally designed to protect the system from damage due to freeze-up, low refrigerant; or refrigerant overcharge.

Many service issues can arise from the number of options available to the technician. In such cases, a counterperson should always refer to warranty details, as in cases such as lubricants, compressors, or receiver/drier replacement, it may be warranty policy that dictates the service approach.

A/C Lubricants

The use of R-134a as both an OEM fit and in retrofits of R-12 systems as mandated in many jurisdictions, as well as the arrival of other retrofit options, has meant that the number of service options has also risen.

One of the least understood issues is that of lubricants. There is much competing information available, but there are essentially two popular types, polyolester (POE or Ester) and polyalkylene glycol (PAG), with a third type, polyalphaolefin (PAO), having entered the market recently.

PAG oils are available in different viscosities:

PAG – 46 (Low Viscosity) was developed for use in automotive systems using a low viscosity PAG in the presence of R-134A.

PAG – 100 (Medium Viscosity) was developed for use as a “universal” in PAG systems using R-134A.

PAG – 125 was specially formulated for General Motors and approved as an aftermarket service lubricant for Harris Thermal System compressors.

PAG-150 (High Viscosity) was developed for use in automotive compressors requiring a high viscosity polyalkylene glycol lubricant when HFC-134A is used.

For retrofitting and top-offs, many companies offer ester lubricants. The advantage of ester lubricants is that they are compatible with the mineral oils found in an R-12 system, and are less hygroscopic (water absorbing) than PAG oils.

PAO oil is non-hygroscopic and will not introduce or carry water into the A/C system. It is considered compatible with all refrigerant types used for mobile A/C systems.

However, it is important to note that for 134a systems, compressor manufacturers may require the use of PAG lubricants to meet warranty requirements.

Counterpeople should be aware of this fact, and other service issues, when recommending A/C lubricants.

Special Thanks:

Special thanks to all those whose resources aided in putting this Counterperson Training section together, including: Affinia Canada, ArvinMeritor Light Vehicle Aftermarket Brake Manufacturers Council, Cliplight, Continental-Teves, Federal-Mogul, General Motors, Greening Testing Laboratories, Hella, Institute for Automotive Service Excellence, Mobile Air Conditoning Society, and Thermofluid Technologies.