A/C System Performance Testing Part 2

A/C Performance Testing

A performance test can determine whether the A/C system is functioning properly and if it is not can indicate what is wrong so the system can be repaired.Different vehicle manufacturers specify different methods to conduct system performance checks, but as a guideline, the following procedure may be used.Connect a manifold gauge set as previously described. Hang the gauge set or place it where the gauges can be seen, making sure the hoses will not contact any hot or moving engine parts. With the engine off, the high and low-side pressures should be equal and the pressure should be correct for the ambient temperature. For example, at 70°F the pressure in a system with R134a should be slightly higher at 71 psi. Check your service information system for these specifications and DO NOT rely on this as an indicator of proper system charge.

Place a thermometer at the center dash panel duct outlet to indicate the system temperature and place another thermometer at the center of the grille to register ambient temperature. Start the engine and turn on the A/C. Set the temperature control to the coldest setting. Turn on the blower motor and make sure it works on all speeds. Generally, if one of the blower speeds doesn’t work, the problem is a faulty resistor. If the high speed doesn’t work, the problem could be the high speed relay or fuse. If the blower is too slow at all speeds, perform a current draw test to see if the windings or brushes are bad. At this point, if the A/C system is operating properly, there should be cool to cold air coming from the instrument panel air ducts. When the temperature control is moved from the coolest to the warmest setting, the air temperature should change to warm. When the function control lever is moved from A/C to heat, the air flow should move to the floor ducts. Place the function control in the ‘MAX AC’ or ‘RECIRC’ position and set the temperature control to the coldest position. Close all of the vehicle’s doors and windows and make sure that all dash panel duct outlets are open. Run the system on high for about five minutes, and then reduce the blower speed to low and allow the system to stabilize for about five more minutes. The thermometer at the center dash panel duct should read 35-55°F , depending on ambient temperature and humidity. Some vehicle manufacturers may recommend that you place a large fan in front of the grille to provide sufficient airflow over the condenser while performing this test. Check the vehicle manufacturer’s specifications for proper system operating pressures, especially on R134a systems. Normal high side gauge readings depend on the ambient temperature, so it is best to consult a temperature/pressure chart. Normal low side readings vary depending on what is happening in the system, particularly in the evaporator, and what it is. The system controls (clutch cycling, expansion valve opening and closing, etc .) affect the readings and affect the evaporator temperature since there is roughly a one-to-one-relationship between temperature and pressure. Always look up the specifications
for the system being worked on.

Here are some examples of gauge readings and what they may indicate:

  • Low side normal, high side normal (but poor cooling) air or moisture in the system blend door stuck open (admitting too much heat) defective or misadjusted thermostatic switch defective pressure cycling switch.
  • Low side low, high side low low refrigerant charge (possible leak), expansion valve stuck closed or a restriction on the high side.
  • Low side high, high side high damaged compressor (bent or damaged valves), refrigerant overcharge, condenser restriction,
    inoperative cooling fan or expansion valve stuck open.
  • Low side high, high side low damaged compressor (bent or damaged valves), loose drive belt or slipping clutch (if there’s no compressor noise).
  • Low side low, high side normal to high expansion valve stuck closed or clogged orifice tube.
  • Common causes of high pressurerefrigerant overcharge restricted condenser (leaves, paper, bugs, etc.), clogged receiver/drier or plugged orifice tube.

A/C Diagnosis By Sight, Sound, Smell & Touch

By Sight:

If the system has a sight glass, observe the refrigerant as it passes by. On older vehicles still equipped with Rl2, a clear flow indicates a proper charge. If there is foam, the charge is low, but if there are only a couple of bubbles, it is probably OK. A few bubbles when the compressor cycles on are normal. Oil streaks in the sight glass usually indicate a low charge and that the compressor is pumping oil from its pump. If the sight glass is cloudy, the desiccant bag has probably burst. Systems with orifice tubes and accumulators usually don’t have sight glasses. While rare, if an R134a system has a sight glass, do not use it to try to perform diagnosis, as it is normal to observe bubbles in this type of system. If the refrigerant has color like yellow or green, someone has probably added refrigerant with a leak detection dye; this is no reason for concern. Look for frost buildup on the A/C lines and components. If there is a restriction in the system, frost will accumulate at the point of the restriction. A restricted receiver/drier will have frost buildup. A flooded evaporator, which can be caused by a restricted TXV, will have frost buildup at the evaporator outlet. Keep in mind that it is normal for light frost buildup at the outlet of the refrigerant metering device.

By Sound:

Squealing noise can be caused by loose or glazed belts. Sometimes belt noise sounds a lot like bearing noise. Spray some water on the belts and see if the noise goes away. If it does, the tension may need to be corrected or the belts may have to be replaced.Bad compressor or clutch bearings may also make noise. Start the engine and engage the compressor clutch . Defective compressor bearings won’t make noise until the compressor is operating, but bad clutch bearings will most often make noise without the clutch engaged. To verify defective pulley bearings, remove the belt(s) and turn the pulley by hand. If there is roughness, the bearings are bad. If there is no roughness, the compressor shaft bearing or thrust bearing may be the cause. Make sure you observe the proper clutch air gap specification if you need to replace a clutch or field coil assembly. A clicking or buzzing noise coming from the compressor is a sign that the system is overcharged and liquid refrigerant is entering the compressor. Unless some refrigerant is removed, severe compressor damage may result. This noise could also result from air in the system.Knocking or rattling sounds usually indicate internal compressor damage, especially on piston type compressors. However, knocking or rattling noise can also be caused by loose compressor mounting brackets and/or bolts. Noises from the blower motor could point to a bad bearing or debris, such as leaves, in the blower plenum. A customer may voice concerns about noise that is normal for the A/C system, such as the random clicking of the compressor clutch when it engages, or a hissing noise when the A/C is turned off The hissing noise is normal and caused by high-side pressure going through the refrigerant metering device as system pressures equalize.

By Smell:

Foul odors caused by bacterial growth can originate from the evaporator and evaporator case. As explained earlier, the evaporator removes moisture from the air as it condenses and collects on the cool evaporator surface. The moisture then drips from the evaporator and drains from a tube in the bottom of the evaporator case.If the drain at the bottom of the evaporator case becomes clogged and water collects and stagnates, or if the surface of the evaporator remains too moist due to high humidity, bacteria can grow in the stagnant water or on the evaporator surface and cause an odor. This odor is then sent into the passenger compartment by the blower motor. To prevent stagnant water from collecting, make sure the evaporator case drain is clear. In some instances, a clogged drain can cause the evaporator case to fill up and leak water into the passenger compartment. To prevent bacterial growth on the evaporator surface, the system should be run periodically on vent or heat with the A/C off, to dry the evaporator. If equipped, make sure the passenger compartment air filter is clean. Chemical fungicides can be used to kill the bacteria, but they must be applied to the evaporator’s surface. Depending on the vehicle, it may be possible to gain access to the evaporator through fresh air vents, by removing the blower motor resistor or by drilling holes in the evaporator case. In some cases the evaporator may have to be removed for cleaning.

By Touch:

Feel the temperature of the A/C lines and hoses during A/C operation. It is normal for the high-side components to be warm or hot and the low­-side components to be cool or cold. If you feel along the high-­side components and suddenly the surface turns cold, you found a restriction in the system at the point of temperature change. On humid days frost may even form at this point. If frost forms on the outside of the TXV, it may be stuck or clogged with ice. A receiver/drier should be warm and an accumulator should be cool. The evaporator outlet should be cool, if not the system may be low on refrigerant or the proper amount of refrigerant may not be entering the evaporator. The line from the condenser to the TXV or orifice tube should be warm to hot. Feel the compressor itself. It may be warm, but if is very hot it may have internal failure. This is particularly true of a piston type compressor if the head is hot.

Diagnosis Refrigerant Leaks

The presence of oil residue at a line connection is usually where the leaks is cause oil will escape along with the refrigerant. Check all line hose connections and fittings. To find leaks there must at least 50 psi of pressure in the system. Connect your manifold gauges set and inspect the pressures, add enough freon to achieve 50psi for testing. A solution of soap and water is the simplest and cheapest leak detector. Simply apply the soapy water to the suspected area with a brush or spray bottle and look for the presence of bubbles or foam, which would be caused by a leak. The problem with this method is that it is only useful for a large leak. Electronic leak detectors and fluorescent dye detectors are the preferred methods for finding refrigerant leaks.

A/C Electronic Leak Detectors

Electronic leak detectors signal the presence of refrigerant by some type of audible indication, usually a beeping, clicking or buzzing. The more rapid the beeps, or the louder the tone, the larger the leak. Electronic leak detectors certified to the latest SAE standards are lab-quality tools, and capable of detecting leaks as small as 4 grams per year. Treat them with care. Perform the leak check in a location free of wind and drafts. If the area is contaminated with refrigerant or other gases, use a fan or compressed air to blow the excess refrigerant away. Make sure you pass the probe around the bottom of components and hoses as refrigerant is heavier than air and tends to settle. Don’t move the probe too quickly and try to keep the probe tip no farther than 1/4″ away, as this may cause you to miss small leaks. No faster than one inch per second is the general rule. Never allow the probe to contact anything to prevent contamination of the sensing tip. Concentrate your testing at line connections and fittings as these are most likely your leak sites. Also check the front of the compressor and compressor crankcase, as these are common areas for seal leaks. Evaporator leaks can be hard to detect because of the lack of accessibility. It may be possible to insert the probe through the blower motor resistor-mounting hole or through the evaporator case drain tube. If using the latter, take care to prevent contaminating the sensing tip with moisture. Leak detectors are different for different refrigerants. Be sure the tool you select is compatible with the system you are working on. When purchasing a new leak detector, be sure to check for the SAE certification label.

A/C Leak Detection With Fluorescent Dye

Another way to find leaks is by injecting a special fluorescent dye into the system, operating it for a short time, and then passing an ultraviolet light over all of the components. This is a good method to find very small leaks, or leaks in inconvenient places. In fact, some vehicles come from the manufacturer with dye installed in the system. There is a difference between the dyes to be used in different systems, so be sure to use the proper type for the system being worked on. Also be sure to follow proper procedures for adding dye. Too much can be as bad as overfilling the system with oil, and adding a PAG-based dye to an electric compressor can cause a high voltage leak that may require total system replacement to correct.

Refrigerant Recovery

Unless the refrigerant has completely drained from the A/C system due to a leak, before replacing any components that carry refrigerant through the system, the refrigerant must be removed. On a typical 80°F day, a fully charged system will have over 80 psi of pressure. Opening a fitting or union will allow the refrigerant to escape into the atmosphere. Such a procedure is not only unsafe, it is illegal and environmentally harmful. All refrigerant must be extracted and recycled using approved recovery/ recycling equipment. Most units operate similarly. Connect the service hoses to the vehicle’s A/C system, and activate the equipment to draw out the refrigerant. The machine will weigh the amount of refrigerant that is removed from the system. Allow the system to empty until there is no pressure or a vacuum. Measure the amount of oil that is removed from the system so the proper amount of new oil can be added to the system during recharging. Once the refrigerant is safely stored, repairs to the A/C system can be performed. Make sure the recovery/recycling equipment is operated in accordance with the manufacturer’s instructions. Recovery/recycling/recharge machines meeting SAE standard J2788 are more capable of removing all of the existing refrigerant charge than those made before this standard was enacted. These machines are also more accurate In measuring the amount of refrigerant recovered and the amount recharged to the system. Regardless of the machine used, it is considered best practice to allow the machine to sit for at least five minutes before completing the recovery process. Check the pressure in the system at the end of five minutes. If the pressure rises above “0”, there is still refrigerant outgassing from the oil. Repeat recovery until stable system pressure remains in a vacuum or rises no higher than 0 psi.

A/C System Evacuation

After repairs have been performed on the NC system, or any time the system has been opened, it must be evacuated. When repairs are made, air will enter the open system, and if not removed, the air can cause higher than normal operating pressures, leading to poor cooling performance. Moisture from humidity in the air can form destructive chemicals inside the system when mixed with refrigerant and oils, and can cause a breakdown of the newer synthetic oils used in R134a systems. Also, water in the system can ice up in the expansion valve and block refrigerant flow. On systems with R12, water can combine with the chlorine in the R12 to form hydrochloric acid. These chemicals can corrode metals and attack rubber parts. During evacuation, as the pressure inside the system lowers, so does the boiling point of the water in the system. At 29 in/Hg, water can vaporize and be withdrawn by the vacuum pump.

While stand-alone vacuum pumps are commercially available, if you are using a charging station, the vacuum pump is most likely incorporated into the station. After properly connecting the service hoses, start the vacuum pump and open both sides of the system, exposing it to vacuum. Once the vacuum level reaches specification, continue pulling the vacuum for at least 30 to 60 minutes. The more humid the air, the longer the vacuum pump should be allowed to run. If the pump is turned off too soon, some of the moisture in the system will be left behind. Close all valves and turn the pump off. Wait for 5 minutes and note any vacuum loss. Loss of vacuum within 5 minutes indicates leakage.

Before turning off the pump, close off the A/C system to the atmosphere to prevent air from being drawn back into it. Although evacuating removes moisture, it won’t remove debris. To help prevent any debris remaining in a repaired system from reaching the compressor, filters can be installed, usually in the liquid line. These filters are usually capable of trapping all debris normally encountered while allowing the refrigerant to pass freely. A system may also be flushed in accordance with the manufacturers recommendations, either with liquid refrigerant using specialized equipment or an approved solvent. A common misconception is that any oil recovered during the recovery/evacuation process should be measured and added back into the system. The truth is that very little oil should be lost during this procedure. Typically, oil collected in the oil recovery bottle of the machine is a result of lack of machine maintenance and/or oil trapped in the internal lines after use of the onboard oil injection feature.

Refrigerant Charging

Although charging the system can be performed through either the high side or low side when the engine is not running, it must be done only through the low side when the engine is running. This safety measure prevents high­side pressure from entering the refrigerant container and possibly causing an explosion. Obtain the refrigerant capacity specification for the system before charging. This is often printed on a sticker located somewhere under the hood, or in a service manual. The most accurate method to assure that the proper amount of refrigerant has been installed into the system is to use some type of charging station that can be programmed to deliver the exact amount. It is crucial that the exact amount of refrigerant called for is used. Overcharging can cause system damage, and undercharging will cause poor performance. With reduced system capacities, especially in R134a systems, charge amounts are more critical than ever. When using a charging station or recovery/recycling equipment, follow the manufacturer’s instructions for its use. Recharging equipment should also indicate when the refrigerant oil should be injected. Allow the system to stabilize for a short period of time after charging. Then performance check the system, observing proper operating gauge pressures.

Refrigerant Oil

The main purpose of refrigerant oil is to provide compressor lubrication. The oil travels with the refrigerant throughout the entire system on some systems. On others, the system is designed to allow oil to stay in the sump of the compressor. The oil has a secondary purpose, to lubricate seals and 0-rings and keep them pliable, and also to keep expansion valves moving freely. Refrigerant oils are specific to the type of refrigerant used and to system design. Older R12 systems used a mineral oil, while modern R134a systems use various weights of PAG (polyalkyline glycol). HF01234yf and systems using electric compressors use different oils still. It is imperative that the right oil be used in the right application, or serious system damage can result. Be aware that some refrigerant oils are hygroscopic. That means they absorb water readily, even straight from the surrounding air. It is important these oils remain tightly sealed when not in use, and that contaminated oil be disposed of properly and NOT installed in the vehicle’s air conditioning system. Proper oil quantity is critical, just as it is in the engine. Too little oil will result in lack of lubrication to components and early failure of the compressor. Too much oil coats the inside of the heat exchangers (evaporator and condenser), reducing their efficiency. When replacing a component or servicing the system, always follow the OEM procedure for balancing the system charge. Typically, this is simply a matter of removing the oil from the replaced component, measuring the amount recovered and adding a like amount back to the system.


Many newer vehicles are equipped with passenger compartment air filters to trap dust, pollen and other pollutants. They are beneficial for vehicle occupants, and help protect the HVAC system downstream components. The filter should be replaced at the intervals specified in the manufacturer’s scheduled maintenance information. Filters that are not changed regularly can cause an airflow restriction resulting in decreased heating and air conditioning performance. On some vehicles the filter is located under the cowl screen and accessed from outside the vehicle, while on others it is installed in the evaporator case and is accessed from under the dash. Refer to the vehicle service manual for specific procedures.

Air Bag System

Servicing the Heating, Ventilation and Air Conditioning (HVAC) system may require working around the SRS (Supplemental Restraint System) or air bag system. For example, procedures like heater core and evaporator core removal may require instrument panel and air bag module removal. To prevent accidental deployment and personal injury, the SRS must be properly disarmed. In general, to disarm the air bag system, disconnect the negative battery cable from the battery terminal and tape the cable end to prevent it from accidentally contacting the battery terminal. Then, wait at least 10 minutes for the backup power supply to discharge. However, always consult the vehicle service manual for the exact disarming procedure.When working on the air bag system, wear eye protection and follow all safety precautions. After an air bag module has been removed, carry the module with the cover pad facing away from the body. Store the module with the cover pad facing up, so that accidental deployment does not launch it into the air.

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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