A/C System Performance Testing Part 1

Automotive Refrigerants

The refrigerant used in automotive air conditioning systems from their inception until the early 1990s was a chemical compound called dichlorodiflouoromethane, more commonly known as Freon or R12. R12 is a member of the chemical family known as chlorofluorocarbons(CFCs). Although R12 works extremely well as a heat transfer medium in an automotive air conditioning system, CFCs have been found to have a devastating effect on the earth’s ozone layer, which protects all life on earth from harmful ultraviolet radiation from the sun. When a chlorine atom from a CFC like R12 is released, it can rise into the stratosphere and combine with one of the oxygen atoms of an ozone molecule to form chlorine monoxide and an oxygen molecule destroying that ozone molecule. Since R12 was very inexpensive, it was a common practice to release it into the atmosphere during the servicing of an air conditioning system.

The Clean Air Act of 1990 required that the automotive industry stop releasing ozone damaging refrigerant into the atmosphere. R12 was replaced with R134a and Section 609 of the Clean Air Act required that any person performing automotive air conditioning service “for compensation” be certified in the proper recovery and handling of refrigerants. These certifications can be obtained from a number of different sources, and failure to produce EPA certification when asked by any law enforcement representative can result in stiff Federal penalties. Repair facilities have additional requirements to meet to remain in compliance with the law as well. Contact the EPA for full information and details.

In 2006, the European Commission found that R134a was a contributor to global warming, with a GWP (Global Warming Potential) rating of 1300. It banned its use as an automotive refrigerant, and required that all vehicles offered for sale in the European Union starting with new model platform s in the 2011 model year be equipped with a refrigerant with a GWP under 150. The replacement choice is a product called HFO 1234yf, also known as R1234yf. It is important to note that R134 a is still approved for use in all vehicles that came originally equipped with it, and that, unlike the move from R12 so many years ago, there is no plan to “retrofit” any R134a vehicle to the new refrigerant. Additionally, the move to HF01234yf is not a requirement in any other market. Here in the U.S., it is expected the transition will be voluntary as domestic manufacturers take advantage of the CAFE (Corporate Average Fuel Economy) credits that can be earned by switching over to more environmentally friendly materials, including refrigerants. GM was the first domestic manufacturer offering at least two models equipped with HF01234yf systems in the 2013 model year.

HFO 1234yf is classified as a “mildly flammable” refrigerant and as such will require special service procedures and equipment. In addition, evaporators for these systems will have to meet more stringent manufacturing standards and certification. In no case should an R1234yf evaporator be repaired or replaced with a salvaged unit. As of this writing, there are still pending EPA regulations that will likely require additional training and certification for technicians and shops. Regardless of the outcome of these pending rulings, all current Section 609 rules apply equally to HF01234yf as they do to any other approved refrigerant. Those include the need for dedicated recovery and recycling equipment, and the ban on any form of venting refrigerant straight to the atmosphere.

Refrigerants are gasses at room temperature. Refrigerants have a ‘roughly’ one-to-one temperature to pressure relationship. The key word here is “roughly” though, because there are some differences. The one-to-one pressure and temperature relationship are close at low pressures, but as temperatures increase, the pressure to temperature relationships vary. Refrigerants cannot be mixed. Even though they operate very similarly, they are distinctly different chemicals, and they require different approaches.

A mixture of refrigerants can result in higher pressures than normal which can cause serious damage to the system components. This can be such a serious matter that recovery/recycle/recharge equipment certified for use with HFO 1234yf are required to first identify that the recovered refrigerant is pure HFO 1234yf before the machine will begin.

Refrigerant identifiers can be incorporated into the machine or tethered via USB cable. There are numerous refrigerants approved for use by the EPA, many of which contain blends of hydrocarbon compounds like propane and butane. No OEM manufacturer approves of the use any of these substitutes and failure to use an identifier prior to recovering systems charge may result in the contamination of your shops supply. It is considered a “bestpractice” to check every vehicle prior to service with a refrigerant identifier.

Refrigerant And System Identification

Before testing system performance, the type of refrigerant used and the design method used to regulate system pressures and temperatures must be identified. Each refrigerant has unique service fitting designs to help prevent accidental charging with another gas. Check the underhood label for system refrigerant identification, service precautions and specifications. It is also best
practice to check for applicable TSBs (Technical Service Bulletins) issued by the manufacturer that may include modified/updated specifications. However, labels and the type of service port fittings should not be regarded as proof of refrigerant type. There are a number of other refrigerants, as noted earlier, including refrigerant blends available that are marketed as replacements. Be aware that the availability of counterfeit refrigerants is an increasingly common problem, and these products may contain chemicals that are potentially dangerous to both you and your customer. Mixing refrigerants in the same system contaminates the system and can result in damaged system components, reduced system performance, damaged recycling equipment and contaminated refrigerant supplies. In addition, some replacement refrigerants contain hydrocarbons such as propane or butane. These are highly flammable and could ignite during system servicing, possibly causing serious bodily injury and damaging equipment.

To prevent damaging A/C service equipment and contaminating refrigerant supplies, the refrigerant in the system should always be tested with a refrigerant identifier before connecting any equipment into the system. Some identifiers can detect Rl2, R134a, R22, air and hydrocarbons, displaying the percentages of each substance, while others may only give a pass/fail indication of purity. A pass is given if the purity is 98% or greater. Some identifiers are also equipped with an audible alarm if hydrocarbons are detected. The refrigerant identifier is connected into the low side of the system. Before connecting the identifier, inspect the condition of the filter and hoses. Allow the tool to warm up and calibrate itself before operation . When the testing process is activated, the identifier will obtain and analyze a refrigerant vapor sample. If the refrigerant in the system is less than 98% pure, it should be considered contaminated and recovered using a dedicated recovery machine, so it can be reclaimed or destroyed.

As stated earlier, A/C systems can be identified by whether an expansion valve or orifice tube is used to control refrigerant flow into the evaporator and by whether the compressor cycles on and off or runs continuously. Systems using an expansion valve have a receiver/ drier between the valve and condenser, while orifice tube systems have an accumulator between the evaporator and compressor. A compressor that cycles on and off is usually used in conjunction with an orifice tube. Continuously running compressors can be either the conventional kind and used with an expansion valve to control evaporator temperature or have variable displacement. Variable displacement com­pressors use an internal valve to vary the angle of the compressor swash or wobble plate in response to changing evaporator pressures. When evaporator pressure is low, the angle of the plate is changed to reduce the pistons stroke, in turn reducing the compressor’s suction and output. When low-side pres­ sure increases, the valve changes the plate angle to increase piston stroke and in turn raise compressor suction and output. Systems with variable displacement compressors Usually employ an orifice tube but there are some that have an expansion valve.

A/C System Preliminary Inspection

Begin A/C system diagnosis by interviewing the customer to make sure you understand the complaint. Then check system operation and perform a visual and functional inspection.

To perform a functional inspection start the engine and turn the AIC on and off several times, while listening for the sound of the compressor clutch. If the clutch cannot be heard engaging and disengaging, and there is no change in engine rpm when the A/C is turned on, there is a problem with the compressor’s electrical control system or the refrigerant system may be low on refrigerant.With the A/C turned on and set to full cold, check the air temperature at operating and the system has some cooling capability. If the air is warmer than the temperature inside the vehicle, then there may be an air distribution problem. If the air is close to ambient temperature, then there is a problem with the refrigerant system. Slowly move the temperature control from full cold to full hot and back again. The air temperature at the dash duct outlets should gradually become warmer as the control is moved toward hot and gradually become cooler when moved toward cold. Listen for the sound of the temperature blend door closing in either direction. Move the function control from A/C to Heat and then to Defrost. The air flow should change from the dash outlets, to the floor and then to the windshield defrost outlets. If the temperature and/or function do not change properly, there may be a maladjusted or broken cable or a malfunctioning vacuum or electric door motor. Move the blower motor control to each speed position. Listen and feel for a change in air volume according to control position. If the blower motor does not operate at all, there is a problem with the motor or the control circuit. If the motor operates but only on one speed, then the fault is in the control circuit.

Visually inspect the A/C system for obvious problems. Check the compressor drive belt for evidence of cracking, fraying, glazing or other damage and replace as necessary. If the belt is adjustable, check the belt tension by pressing against the belt with moderate pressure at a point midway along the longest span and compare the deflection with specification, or check the tension using a belt tension gauge. Make sure the compressor is attached securely to its mounting brackets and the mounting bushings are in good condition. Check the refrigerant lines and hoses for damage and signs of oil leakage, especially at any connections or unions. If oil has leaked out, most likely the refrigerant has as well. Pay particular attention to connections that are subject to engine vibrations usually those on rubber lines. Check the front of the compressor. Oil here often indicates a bad front seal. Check all fittings and the pressure relief valve for signs of leaks.Inspect the system electrical wiring to the compressor, blower motor and A/C switches for any damage. Make sure all connections are clean and tight. Check the vacuum hoses between the engine and firewall for evidence of cracks, splits, kinks or other damage that could cause a vacuum leak. Inspect the condenser for debris that could obstruct air flow. If too many fins are bent, air flow will be reduced.

Inspect the fan shroud for broken or missing parts, and make sure all air dams and condenser side and air seals are in place as they should be. Also, check the fresh air intakes in the cowl for leaves and other such debris. If equipped with a mechanical fan, check the drive belt for wear and belt tension. A slipping belt will cause the fan to turn too slowly and not draw enough air through the condenser. Check the back of the fan clutch for an oily film, which would indicate that fluid is leaking and replacement is necessary. Turn the fan and clutch assembly by hand, there should be some viscous drag, but it should turn smoothly during a full rotation. Replace the fan clutch if it does not turn smoothly or if it does not turn at all. It should also be replaced if there is no viscous drag when hot or cold. Have an assistant apply the parking brake and then start the engine and turn on the A/C. The clutch should engage and the compressor should turn. If equipped with an electric cooling fan, it should come on when the A/C is turned on. Some vehicles have two fans with one dedicated to the NC system.

Manifold Gauges

A manifold gauge set is used to read the pressures on the low side and high side of the system. The gauge set has a low pressure gauge and a high pressure gauge. The gauges and hoses are often color coded blue for low and red for high. The low side is connected to the service fitting that is located somewhere between the evaporator outlet and the compressor. The high side fitting is located somewhere between the compressor and the expansion valve or orifice tube. A center hose or hoses, usually yellow in color, and often referred to as the service hose. This connects to a vacuum pump or refrigerant source for evacuating or charging the system. On modern automotive air conditioning systems use much smaller system charges (refrigerant quantity), and proper charging is critical to system performance and component longevity. It is also critical that no venting of the refrigerant be allowed during service. For both of these reasons, it is recommended that only approved recovery, recycling and recharging equipment be used to service these systems. Leave the yellow service line alone. All hoses must have a shutoff within 12 in. of their ends to prevent excess refrigerant from escaping during connection and disconnection. A schrader valve or quick disconnect device at the hose end fitting is most common.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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