slowcivic2k

10/21/2017 EDIT: The scope of this post is for R134a systems, and retrofits from R12 to R134a. Eventually I plan to either create or add R1234yf related content to this post. Please be aware that due to Photobucket stupidity, some images may be missing or not load correctly. I am currently working on a resolution for this.

For those of us that live in warmer climates, A/C is your best friend. It cools your car, your home, and also functions as a dehumidifier so you stay dry. Today I will describe in complete detail, the modern vapor-compression refrigeration cycle that is used in the automotive industry today and also in most stationary A/C applications as well. Summer is fast on the horizon, so evaluate your A/C system now before you have to suffer the intense summer heat without it!

This will be divided into two sections/posts: A theory section and a diagnostic/repair and advice section. Be advised, this information will be very thorough and detailed. I will make every attempt to make the information presented as clear as possible for the average do-it-yourselfer the first time around. But, you may still have to read it or reference this information many times to gain real understanding. Let’s first look at the basic components that are familiar to any A/C system in general.



The Compressor:

The compressor creates the flow of refrigerant inside the A/C system. It is a positive-displacement style pump that sucks in low pressure vaporized refrigerant from the low side of the system, and discharges it to the high side. Most automotive A/C compressors currently in service use an axial piston arrangement. This type of construction uses pistons that are stroking in line with the axis of the A/C pulley shaft as pictured above. This creates a more compact compressor package with more displacement than comparable vertical piston systems, such as those used by air compressors. Most axial A/C compressors also utilize a fixed angle swash plate, and in some designs, a variable pitch swash plate can be used. A swash plate is like the crankshaft in an engine. It determines the timing and stroke of the pistons, and thus determines the compressor displacement and compressor efficiency. Many compressors on newer vehicles today are being equipped with an electronic solenoid-valve to tilt the swash plate to increase or decrease the cooling capacity and control compressor efficiency dynamically. There are also many other styles of A/C compressors in use: Scroll compressors that use wound scrolls to compress refrigerant, Variable-vane style compressors that use a star and vanes to pump refrigerant similar to the way a power steering pump operates, and screw-type compressors which operate similar to the way a blower/supercharger works. The style of compressor used depends a lot on the cooling requirements, the environment in which the compressor will spend its life, and the overall package size required for the application.



The Condenser:

The condenser has the critical function of cooling the refrigerant in the system. The condenser’s ability to dissipate heat impacts the amount of actual "cooling" that can be performed inside the vehicle. The condenser is very similar in design to a traditional radiator and will always sit in front of it due to its need to dissipate extreme heat during operation. The refrigerant temperatures inside the condenser can/will reach over 300 degrees F during normal operation. Within the cooling circuit schematic, the condenser is placed just after the discharge port on the compressor, and before the receiver or metering valve. The cooling (or condensing) process will transform the high pressure, high temperature gas refrigerant entering it into a high pressure liquid. In order to achieve this, the surface area of the condenser must be quite large (like that of a radiator or larger).



The Evaporator:

This component does the actual "cooling" within the vehicle interior. It absorbs heat from the passenger compartment, and by doing so lowers the air temperature inside the vehicle. This process will usually bring the air temperature inside the vehicle below the dew point. Humidity from the air will then condense onto the evaporator coils as the air is cooled and discharged into the vehicle interior. This accumulated moisture will then run off into a built-in drain tube to the vehicle's exterior and onto the pavement. In the cooling schematic, low pressure liquid refrigerant leaves the metering device and enters the evaporator, before it returns to an accumulator as a low pressure gas, or the compressor. Since the air in the cabin is warmer than the low pressure liquid refrigerant, the refrigerant absorbs the heat from the air blown over it and cools the interior. This causes the low pressure liquid refrigerant to boil, turning it into a low pressure gas. The evaporator can be physically much smaller due to the density of the liquid refrigerant flowing into the evaporator coil. Many evaporators use a "snake" type plumbing design, or single input, single output. This contrasts with a condenser, which will normally have many paths for refrigerant to flow into and out of, like a radiator does.

These above components are universal for any vapor-compression A/C system. But the way the refrigerant is metered/controlled, or restricted will vary from manufacturer to manufacturer and system to system. Metering/restricting the refrigerant gives the compressor something to compress or work against, which is where the A/C compressor gets its name. No A/C system can operate effectively without this type of control implemented in some form or fashion.


Dynamic Metering Device(s): (TXV, H-Block, Variable/Electronic metering devices.)

These Devices are capable of increasing or decreasing the flow of refrigerant into the vehicle's evaporator based on some input, be it mechanical or electronic.



Thermo-Expansion Valve (TXV)

These devices dynamically control the flow of refrigerant into the evaporator by sensing the discharge temperature (superheat) on the evaporator outlet tube (the compressor suction line). It features a diaphragm-controlled valve and a remote-mounted and calibrated sensing bulb connected to the diaphragm via a copper thermal tube as pictured above. A return spring inside the diaphragm called a superheat spring is used to calibrate the efficiency of the refrigerant and control the valves' base position. Some expansion valves may also implement a pressure equalizing tube (called a ballast tube) which is used to balance the pressures on both sides of the diaphragm which establishes a neutral valve position. Operation is quite simple: The cooler the evaporator outlet is, the more the valve shuts, to a point. The higher the outlet temperature, the more it opens, to a point. This method of control is purely mechanical, requiring no electronics to control the valve movement itself. Expansion valves are typically located within the same housing as the evaporator. These two parts are generally serviced as a unit because of age and brittleness of the connections. Expansion valve systems are almost exclusively used on import vehicles, with a few domestic exceptions.



H-Block

Very similar in design to the TXV, but this is an integrated unit. All the metering components, including the sensing bulb, are all within a small modular housing. This method of control is also purely mechanical, requiring no electronics to control the valve itself. This part is often situated near the firewall entrance of the A/C lines, making them much easier to service if they become faulty. These are normally seen on GM vehicles and later Chrysler (1996-present) vehicles, with some import makes included as well.



Electronically Controlled Cooling Valve (ECCV)

This valve is controlled directly by the Body or Climate Control module in the vehicle using sensors and switches as inputs. Based on requested air temperature, blower speed, and in some cases humidity level, the module can adjust the valve position in real time to reach the requested settings. Many of these cooling valves are solenoid-valves or stepper motor-controlled valves that are duty-cycled on and off, or stepped up or down depending on the requested cooling settings and sensor inputs. This valve can be fitted in many locations on the vehicle, but is often found in the engine compartment near the evaporator inlet. Few vehicles use this type of control valve in practice, but many of those that do are higher-end luxury vehicles made by Cadillac, BMW, etc. The control specifications for this type of cooling valve are software programmed, and are capable of being updated to suit different climates and applications, making them a very versatile control method.

Static Metering Device(s):

Static metering devices can only control refrigerant flow with a fixed sized opening or orifice.



Orifice Tube: (Fixed Bore Metering Device)

This device meters the amount of refrigerant through a small fixed hole, or orifice, which regulates the refrigerant flow within the system. This is not a dynamic system, meaning that it will always allow refrigerant in and out of the tube without any other control other than the tube's physical size, and to some degree, the amount of refrigerant inside the system. As such, an orifice tube system requires that the compressor be cycled on and off to produce to correct refrigerant flow to reach the desired level of cooling in the vehicle. These systems are usually referred to as Clutch-Cycling Orifice Tube systems, or CCOT systems. If a variable displacement compressor is not used, switches and other control devices must be used to cycle the compressor to maintain the correct level of cooling without damaging the compressor. Orifice tubes come in many different sizes, and are indicated by the body color on the tube assembly. These orifice tubes can usually be field serviced and replaced with another size tube depending on the climate the vehicle operates in. In some instances, the tube may be molded into the refrigerant line, requiring the line containing the tube to be replaced during service. Transit vehicles especially (Express 1500 and above, E-150 and above) will use over-sized orifice tubes (or selective size orifice tubes) to provide extra cooling capacity due to the increase in vehicle size.

Each of these metering devices will also have a unique refrigerant reservoir or storage device based on how the refrigerant is managed. These are present to prevent liquid refrigerant from being sucked into the compressor (accumulator), or to prevent gas refrigerant from flowing into the metering device and evaporator (receiver/dryer). They look very similar physically, the main differences are in the location in the system and the type of metering device that is used.



Receiver: (Receiver/Dryer)

This device is placed after the condenser and before the metering valve. Its purpose is to prevent un-condensed refrigerant from flowing into the evaporator. The receiver is constructed with an internal tube that runs to the bottom of the canister so that only liquid refrigerant can flow into the metering device and into the evaporator. This is similar to a glass with a straw running directly to the bottom. The un-condensed gas will lay above the liquid until it also cools off and liquefies. This type of container is used in conjunction with a dynamic metering device like a TXV, H-Block, or electronically controlled cooling valve. There can also be a non-serviceable filter installed in most of these containers as well. A sight glass is a common addition to some receivers on older vehicle years between 1994 and 2000. This sight glass can be used to determine if a system is operating with a sufficient level of refrigerant, and also to aid in system diagnosis. The sight glass should appear clear when a normal charge is flowing. If bubbles are detected in the glass during operation, this means the system is not being properly cooled, or the system could be undercharged. Oil streaks can also indicate excessive oil flow in the system. A greenish tint in the sight glass indicates a dye was installed in the system previously. Dyes are quite rare on virgin systems, but some manufacturers (Nissan in particular) use A/C dye from the factory on certain models. If you observe this coloration in a sight glass, take extra precautions to find leaks.



Accumulator: (Usually a dryer as well, and can be equipped with a separate filter element.)

This device is used with orifice tube systems only, because of the way the refrigerant is controlled. Because a fixed sized hole can permit too much liquid to enter the evaporator, the compressor could suck this un-vaporized liquid up. This will cause severe damage the compressor due to hydro-lock. Accumulators are positioned after the evaporator and before the compressor inlet. Its job is to allow the compressor to suck vaporized refrigerant only by sucking refrigerant from the top of the accumulator instead of the bottom as in a receiver system. The rest of the liquid at the bottom of the container will eventually absorb heat and enter the compressor. There is usually a filter element installed in this container as well.



Filtering Device: (Dryer/Desiccant)

Every A/C system will have at least one primary filter, and sometimes more if the system has suffered a failure of a hard part in the past. The filters job is to remove any suspended debris and also remove any moisture circulating within the system. Any time the system is opened for service, this part (or the part containing it) should be replaced to prevent internal corrosion damage due to moisture and acid formation from the system being exposed to the atmosphere. Generally, this means replacing the receiver or the accumulator assembly, depending on which part your vehicle is equipped with. Some newer vehicles today may be equipped with an integrated receiver/filter within the condenser itself, requiring that the condenser be serviced as a unit to replace the filter. Some of these condensers may contain an access plug that allows for filter replacement without the need to replace the entire condenser assembly. If you are unsure about your configuration, consult a service manual for your application. Always replace a refrigerant filter with the same type that came out.



Switches/Sensors: (A/C system I/O components)

A/C systems use several inputs/outputs (I/O) to control the A/C system. In the event of a malfunction and/or to more precisely control the delivery temperature, these inputs will determine the commanded outputs based on a programmed table.. There are low pressure switches, high pressure switches, low pressure sensors, high pressure sensors, evaporator temperature sensors, sun load sensors, ambient air sensors, switches, relays, and many others. Low pressure switches/sensors are designed to perform two basic tasks. One is to prevent the evaporator from icing up due to a system malfunction (usually a leak). The other is to prevent the compressor from running due to a severe refrigerant undercharge which could damage the compressor. The high pressure sensors/switches are designed to prevent over-pressurization of refrigerant which could blow a hose or seal. This could be caused by an inoperative cooling fan, refrigerant overcharge, air in the system, or poor condenser cooling efficiency (missing or bent fins, missing air dams, internal blockage, inoperative cooling fan).

Most vehicles will use a combination of low and high pressure switches which are either normally open or normally closed in conjunction with a control panel and fuse panel equipped with switches and relays respectively. Many higher-end vehicles use sensors instead of switches to detect actual system pressures and temperatures in real time with a compatible diagnostic scan tool. Most vehicles will use a temperature sensor on the evaporator itself (called an evaporator temperature sensor) to determine the cooling load placed on the system. At low blower motor speeds the evaporator will get very cold, and liquid refrigerant will flow more prominently through the evaporator without boiling because there is no heat being blown over the coils to boil the refrigerant. This temperature sensor will report the evaporator temperature to a module and cycle the system off and on to prevent hydro-lock events and maintain a sufficient cooling load. This is where variable displacement A/C compressors really provide an advantage: They can change compressor output on demand without ever turning the compressor "off". All of these switches and sensors will normally run to a controller/module, or are a hard-wired part of the compressor circuit itself to ensure the compressor does not run if an unsafe operating condition exists (bad fan, undercharge, overcharge, etc). If these switches or sensors were to fail, serious damage to the system can occur because there is no intelligent logic to disable the system. In these instances, a climate control module-equipped vehicle can respond and disable the system and trigger a DTC. Because these inputs are not capable of catching every failure that can occur within an A/C system, a thorough inspection of the system for problems by a competent technician is still required.



Refrigerant Oils:

PAG oil, or Polyalkylene Glycol, is the refrigerant oil used in native R134a refrigerant systems. The oil used in an A/C system is unique must have several properties that most conventional oils do not possess. The oil must be miscible in the refrigerant. This means the oil and refrigerant can mix and move together as a unit. This ensures that the compressor has proper lubrication at all times. It must also be able to cope with the very high and low operating temperatures of an A/C system without degrading or slugging the system. Being a Glycol-based oil, they readily absorb moisture the same way that brake fluid does. Only use refrigerant oil from a sealed or "fresh" container when servicing. These oils are commonly sold in three primary viscosities: 46, 100, and 150. Many variations exist. Some oils include dyes and sealers while some include cooling enhancers. Be advised: Leak sealers, and refrigerant and/or oil enhancers, may damage the compressor and refrigerant seals over time, and are not recommended by any manufacturer. Whenever the system refrigerant is recovered, oil must be measured and replaced in specified amounts or damage to the system could occur due to a lack of lubrication (The refrigerant moves with the oil!). Likewise, if a hard component fails or leaks, like the evaporator or condenser, a measured amount of oil must be added to the system to replace what was lost. Refer to your service manual for specifications or consult a professional. Under or over-oiling a system can damage the compressor at worst, and will also insulate the evaporator from the refrigerant which will reduce overall cooling performance.

Now that I've introduced the system components, its time to start putting things together, so you can see how the system works. I will now start with the theory behind vapor-compression refrigeration.

Theory:

The whole point of air conditioning is to move heat from one area, to another. This is accomplished with a refrigerant medium. There are several scientific concepts to understand so that you can fully understand how the system works. You may need to read over this many times, or research the principles covered if you are not familiar with them.

It is very important to understand the Ideal Gas Law, which for the sake of simplicity states: For any given drop of pressure on any matter in a fixed space, a proportional drop in actual temperature will result. For any given increase in pressure in a fixed space, a proportional increase in temperature will result. This principle is paramount to the way a vapor-compression A/C system operates.

This phenomena can be observed in a lot of everyday activities. For instance, if you remove the valve core from a fully inflated tire on a summer day, frost will begin form on the valve stem as the tire decompresses. If there is water or humidity in the tire, it may turn into small ice pellets as the tire decompresses and cools off. Another instance of this would be a 2 liter of carbonated soda. You'll notice as you open the cap, bubbles will start to form in the soda and travel to the top. By releasing pressure by opening the cap, you have lowered the boiling point of the contents. This causes the CO2 in the soda to boil off. This explains why soda goes "flat": All the CO2 in the soda will boil off and leave over time, leaving you with sugar water.

Another very important concept to understand is the concept of Latent Heat. Most people understand that as you heat an object up, the temperature of that object will also increase according to how much heat you add to it. During changes of state from solid to liquid, or liquid to gas and vis-versa, heat can be added to or removed from an object, but no net change in temperature can be seen on a thermometer. This principle is called latent heat, and it cannot be measured with any thermometer. Here is an example: If you take a glass of ice that is 10 degrees F and set it outside in the summer heat, the temperature of that ice will rise until it reaches its melting point. This is exactly 32 degrees F for water. You will notice at this point that the ice will begin to melt, but it's temperature still remains 32 degrees F. Energy is being absorbed by the ice to melt it, but only to change its state from solid to liquid, not its actual temperature. Once the change of state is completed from solid to liquid, the temperature will begin to rise again on a thermometer again as one would expect.

You may also notice at this point that it requires a large amount of energy to change the state of water from solid to liquid, or from liquid to gas and vis-versa. This energy is called the latent heat of fusion or vaporization. To quantify this energy, the same amount of energy used to change states could raise the temperature of already liquid water by 50-60 degrees. A/C systems work by utilizing this principle of latent heat. By condensing and cooling high pressure gas refrigerant, into high pressure liquid refrigerant of near the same "actual" temperature, latent heat is removed due to the change in state. After this liquefaction occurs (releases latent heat/cools off), we introduce it to a metering device which reduces the pressure on this high pressure liquid, into a low pressure liquid. The temperature will also drop as well (Ideal Gas Law). Reducing the refrigerant pressure will also reduce the refrigerant's boiling point. Now, when hot air is blown across the evaporator coils, the cooler low pressure liquid refrigerant in the evaporator picks the heat up causing it to boil into a "hot" low pressure gas. This "hot" low pressure gas refrigerant then is sucked up by the compressor and the cycle repeats over and over.

Now let’s brief over super-heating and sub-cooling. These two parameters are usually not available in the automotive space, because the system design never changes. This basically means that as long as the system is operating to specification, you should not have to worry about these principles. These two measures however, can help a technician pinpoint the source of a problem that may be intermittent. These readings are normally used with stationary A/C units, where the system components can be chosen individually and must be sized appropriately.

Dynamic metering valves such as a TXV or H-Block use the principle of superheat to control the valve's position and the refrigerant flow. If the discharge temperature is higher than the inlet temperature, the valve will unseat and allow more refrigerant in. When the desired level of cooling is reached, the pressures on both sides will be equal, keeping the valve in its current position. When cooling is no longer needed in great amounts, the discharge will be colder than the inlet line due to the compressor "sucking" the refrigerant into a boil. This moves the valve towards the closed position. These dynamic valves will change positions based on the superheat that the sensing bulb sees. Let's now define superheat, and subcooling.


Super-Heat is the rise in refrigerant temperature observed once all refrigerant in the end coil of the evaporator has boiled completely, and can rise in measurable temperature again. This value is equal to discharge temperature of the evaporator, minus the boiling point of the refrigerant at the pressure indicated on a manifold gauge set. This principle is similar to the ice scenario I mentioned earlier. Dynamic metering devices utilize these principles for two main reasons: One is to prevent liquid refrigerant from getting into the compressor. Two, by preventing liquid from getting into the compressor, it is ensuring all the refrigerant within the evaporator is absorbing heat efficiently. Too much refrigerant can cause cooling on the return line of the compressor, and will cause a dynamic valve to close. Excessive superheat can result due to a low refrigerant level, oversized evaporator, or air in the system. Not enough superheat indicates that liquid refrigerant may reach the compressor and damage it. This can be caused by too little airflow over the evaporator, excessive condenser cooling, etc.

Sub-Cooling is the drop in temperature observed once all refrigerant in the condenser has liquefied completely, and begins to drop in measurable temperature again. This would be analogous to freezing water. After water has completely frozen, it will continue to drop below 32 degrees F. Sub-cooling is considerably more desirable in most cases, as this indicates excellent condenser cooling performance. It can also indicate that the condenser is too large for the application. Over-sized condensers are common in climates where prolonged high temperatures (90-100F) can impact the amount of heat that can be transferred to the air. Excessive sub-cooling can indicate that the refrigerant level is too low (cooled too fast, not enough mass), or the condenser is too large for the application (again, mostly stationary or aftermarket condensers). Too little sub-cooling can in some cases indicate excessive refrigerant levels, a condenser that is too small for the application, or a condenser cooling fan that is inoperable or any condition that does not allow the condenser to cool refrigerant properly.

Basic System Operation:

The entire vapor-compression A/C system operates in a big loop with two distinct sides: A high pressure side, and a low pressure side. It also has two distinct phases: Liquid and gas. To illustrate, here is a typical A/C system which can be divided in such a manner.



Once the compressor engages, it starts moving the refrigerant to the high side of the system creating pressure against the metering device. The metering device will allow a preset base level of refrigerant flow to get the cooling process started. The high pressure gas refrigerant is first pumped through the condenser which then cools due to airflow over it. This converts the high pressure gas into a high pressure liquid.

This now liquid high pressure refrigerant flows into the receiver/dryer (TXV, H-Block systems) or directly into the metering device (Orifice Tube systems). If the system is equipped with a receiver, only liquid refrigerant can flow past this point. Any gas in the receiver that has not yet cooled enough to condense will remain there until it cools off and liquefies. The flow is then obstructed by a metering device. This only allows a controlled amount of refrigerant to pass through it. Once the refrigerant passes through this device, it is now being sucked back towards the compressor’s suction port. This suction of the compressor causes the pressure on the refrigerant to drop dramatically. Since this decrease in pressure also results in a corresponding decrease in temperature, the refrigerant becomes cooler than the ambient air.

The heat from the cab is then blown over the cold evaporator coils by the blower motor, transferring heat into the refrigerant. This causes the refrigerant to boil into a low temperature, low pressure gas. This is a lot like adding heat to boil water on a stove. Once the evaporator has absorbed this heat, this low temperature low pressure gas refrigerant is sucked into the accumulator (Orifice Tube systems) and then into compressor. The heat that the refrigerant absorbed in the evaporator is then compressed into a high pressure gas and then discharged into the high pressure side of the system, on its way to be cooled by the condenser again. This completes the entire vapor-compression heat exchange cycle. This cycle repeats until the system is turned off.

The amount of work a particular system can perform is dependent on many factors. These factors normally refer to the size and efficiency of all the components, the amount of refrigerant the system contains, and the temperatures the system works in. The type of refrigerant used will also impact cooling efficiency.

Other Useful Information:

Climate control is a modern type of air conditioning that can control temperature, air flow, and in most cases humidity in a very precise matter. These functions are provided using the same techniques, but with much greater precision and control. Higher-end luxury vehicles may also have additional sensing equipment to monitor high/low side pressures, high/low side temperatures, and additional controls for the compressor and metering device, which is usually electronic or variable to a high degree. This allows for a precision humidity and temperature-controlled environment with just a touch of a button. These types of systems in general are becoming more commonplace on newer vehicle makes with every passing day. These systems, being much more controls-based than previous systems, will require the use of scan-tools or special test equipment to fully evaluate them in the future.

Retrofit Information:

Retrofitting an R12 system to R134a is a complex process. Performing a proper retrofit for longevity and performance requires the consideration of a lot of factors. Most technicians will understand to some degree that R134a operates at higher pressures than R12 does. The oils required by an R134a system are also significantly different from those used in a native R12 system. In many instances, replacing the condenser alone for one that is R134a compatible may be required for reasonable cooling performance in addition to any other upgrades. R134a is also a much smaller molecule, and will permeate R12 hoses that are not barrier style. These hoses will produce leaks that are not repairable except with a newer R134a compatible hose. Most R12 gaskets will also leak due to this retrofit, and will require service to prevent refrigerant leaks.

A decent, minimal “Retrofit Kit” will normally include these items:
1 R12 to R134a High side pressure port with service port cover.
1 R12 to R134a low side pressure port with service port cover.
An installation wrench. Thread sealing compound.
Warning labels indicating a retrofit has been completed.
POE (Polyolester) refrigerant oil.

With this type of retrofit kit, the compressor should be removed and drained of existing oil thoroughly. The receiver should be replaced with one that includes an XH-9 desiccant, or is listed for use with R134a. The evaporator and condenser should be flushed to remove residual system oil, and refilled with an appropriate level and viscosity of POE or PAG oil. The expansion valve or orifice tube should also be replaced for one calibrated to work with R134a refrigerant. Performing these tasks will require opening nearly every seal on the system. These seals, gaskets, and o-rings should be replaced with R134a-compatible parts to prevent parasitic leaks. Performing a retrofit to this level of detail should produce reasonable performance and great system longevity.

Elaborating much beyond this required application-specific information, as each retrofit is specific to a particular application and situation. If you require additional information regarding retrofitting an R12 vehicle to R134a, message me or create a thread and message me directly. Each and every situation is different.

A/C Diagnostics and Inspection (particularly for Honda vehicles, but suitable for any make or model) is covered in the next post.

slowcivic2k

Diagnostics:

Diagnosing and repairing any A/C system requires detailed training on the subject. Diagnosis and system repair should never be attempted by anyone not trained in mobile A/C repair and/or certified to handle or recover refrigerant to EPA specifications. Not only could harmful gases be vented to the atmosphere, but personal injury by contact frostbite, lung tissue damage by inhalation, and personal injury could result. Even if you meet these basic criteria, repairs should not be performed unless you fully understand the nature and procedure of the repairs that are required. Be aware that venting refrigerant is against US federal law, and most states and localities also have legislation in place prohibiting these actions as well.

Most over-the-shelf refrigerant kits may contain butane, propane, or isomers of these two, and leak sealers and other enhancers which are stated to improve cooling performance in addition to the base refrigerant. Most people readily realize that propane and butane are combustible gases. Neglecting proper service and repair procedures could result in a potential explosion or serious injury. R134a alone is known to be explosive with certain percentages of oxygen at certain pressures. Let’s also not forget some of the worst stuff: Some refrigerants, if combusted, may produce very toxic gases such as Phosgene (Mustard) gas. Always wear proper personal protection, use nitrogen gas to identify large (fast) leaks if you need to perform leak testing, and always follow service procedures outlined by the manufacturer. Always refer to a service manual for procedures and safety information. The information above is only a basic snippet of the precautions you should take.

System Evaluation:

Evaluating an A/C system with an "A/C Performance Check" like most repair facilities do should include a basic function check of all the available HVAC controls as well. Noting which controls work, and which ones do not and so on. It is important to make sure that all A/C components and related systems function because if part of the A/C system doesn't work properly, these problems may also interfere with the normal operation of other related components and systems. I must also note that many service facilities (dealerships included) RARELY complete this type of inspection with the level of detail I am about to describe (even though they should!). Most facilities will take a basic pressure reading with a set of manifold gauges, produce a diagnosis, and that’s it. Taking the time to compile this information will get you very close to a suspected problem. It is to your advantage to be thorough when diagnosing your suspected problem!

Here is a basic list of tools you will need (and know how to use properly) to complete this type of inspection correctly:

A service manual for your particular make and model.
Access to vehicle and model-specific TSB and recall information, as it applies.
A pad of paper and writing utensil.
A pair of mechanic's gloves that insulate heat/cold.
A 0-100C (32-212F) thermometer or equivalent. This must have fine temperature gradients. down to 1 degree.
A UV inspection lamp.
An A/C manifold gauge set (or recovery machine with gauges).
A tape banner to measure air flow levels out of the vents.
A good set of eyes with safety glasses.

Some optional equipment that is more precise can also be used:

A multi-meter and type-K thermocouple for temperature measurements.
An electronic refrigerant leak detector or halide torch.
A face shield.

If you plan on recovering refrigerant:

A recovery/recycle machine. Optionally, you can use:
A refrigerant pump, trash cylinder, and vacuum pump.
Publications regarding acceptable methods of recovering and storing used refrigerant.

So let's get started.

First, it is highly recommended you speak with the owner of the vehicle about previous repairs, and any problems that the owner may have about the system. You may uncover information that will be helpful in diagnosis, and you may uncover additional service-related problems, such as parts not being installed correctly. For instance, if the compressor was recently replaced on this vehicle, take the extra time to inspect all the compressor gaskets, and lines that run to the compressor. Most components do not leak until they are serviced, so it is to your benefit to validate previous system repairs when necessary. At all times, you should be looking for signs of previous repairs and damage to mounting brackets, bolt threads, gaskets, etc.

Interior Controls Inspection

All vehicles have at least four standard sets of controls, equipped with A/C or not:

1: Blower speed control (1, 2, 3, Hi, Etc.)
2: Air direction control (Defrost (Hi), Panel (Med), Floor (Lo)
3: Temperature control (Cold, Mix, Hot)
4: Ventilation control (Recirculation Mode, Fresh Mode)

Perform the following tests with the engine fully warm, at idle and in park or neutral position.

1: Place a suitable thermometer into the center vents of the vehicle, turn the heat to maximum, the blower to its highest setting, and in recirculation mode. If your vehicle has dual zone climate control in the front, measure the driver and passenger center vents accordingly. If you have rear A/C, do the same for the rear system with the same settings. Note the temperature on the thermometer.

2: Place the thermometer in the defrost vent on the dash and record the readings again. If you have access, take a temperature reading in the floor only mode as well. A heater core should be able to provide a temperature of at least 100 Degrees F but can reach temperatures up to 210 Degrees F depending on the condition of the engine cooling system and engine run time. Ensure that the airflow out of all positions is adequate or are similar to one another. Also verify that the temperatures are also similar (+/- 5 Degrees). There will be some discrepancy as the ducting length will play a role in the temperatures and air flows that are delivered. If the airflow seems weak or the blower motor seems to make an excessive amount of noise, make a point to inspect the cabin filter (if equipped) or remove the blower housing to inspect for debris later. Note if airflow is occurring in any other position other than the desired position.

3: Change the air temperature to its middle setting. If the vehicle is equipped with manual A/C, turn it on now by pressing the A/C button. If the vehicle is equipped with automatic climate control, ensure the A/C indicator is illuminated. Listen for the compressor clutch to engage when you power the A/C system. It will generally create a metallic clank as the compressor engages. This ensures that the electronic end of the A/C system is still functional. Place the thermometer into the center vent again and measure the temperatures. Generally, it should remain around 65-80 degrees F. If it is abnormally low or high after 1 minute, make a note of this. Measure the defrost ducts and foot ducts as well, and any rear components as before. If your vehicle has adjustable numerical temperature increments on a dial or display, make sure they are reasonably close. If not, make a note of this.

4: Change the air temperature to its coldest setting. With the A/C on, place the thermometer again in the center vent. After 3 minutes, measure the temperature and record it. Do the same for the floor and defrost. Generally, the evaporator should be able to produce 58 degree air on a hot day. (~95 Degrees ~80% humidity) Note that not all systems can operate in this temperature range, and a lot of factors including humidity, ambient temperature, and system size play a role. Reference factory information if your readings fall outside of this range for accuracy: Not all A/C system are built to the same specification.

5: Verify operation of the recirculation door. Press the recirculation button, or turn/push the dial/button in the vehicle and listen for a change in air frequency. The air flow from the blower motor will produce more noise in the recirculation mode and also produce extra airflow. This helps indicate proper operation of the door. If you can smell exhaust or other mechanical odors while the airflow is in the fresh air mode with the hood closed, inspect the hood seals near the firewall. Note the condition or if they are missing. Also take a moment to inspect the firewall wire harness entrances into the car. Aftermarket accessories will often create less than perfect holes in the firewall that can contribute to exhaust getting into the car.

6: After taking all of these readings and writing them down in an organized fashion, reference your relevant service manual for actual specifications. The ones I have provided are very close to every manufacturers specifications, and can be used as a general guideline.

Under-hood Inspection

1: Raise the hood with the engine running and A/C on, doors closed and windows up. Inspect the compressor and ensure the compressor is engaged and is rotating. If it is not engaged, the clutch assembly in front of the compressor will be stationary. Note if it is engaged or not. With the A/C still on, inspect the A/C condenser fan assembly and ensure that the fans are rotating and pulling air into the engine bay. Make sure the fan shrouding is intact and functional. Note if the cooling fans are running or not or if the speed/airflow produced seems insufficient. Some systems will use a dual cooling fan arrangement. You may need to research service information or a electrical schematic to determine if both fans should be operational when the A/C is on.

2: Inspect the upper and lower radiator hoses and ensure that there is firm pressure on both of them. They will be warm to very hot, so be cautious and use gloves. Note if the hoses have no apparent pressure or squeeze readily. Also note if the engine begins to overheat during the test. If so, abort any further testing until the engine cooling system has been repaired.

3: Inspect all the visual A/C lines and service ports including the caps. Note any missing service port caps or frost on any of the lines. Condensation or sweating is normal on the low side lines. Light frost indicates a refrigerant level problem so note this condition if it applies. Inspect the front of the vehicle grille and condenser assembly for road damage, debris blockage, or signs of oil leakage.

4: Use a black light and go over all the visible A/C components from compressor to evaporator, and look for a fluorescent green color. This indicates a system leak may be present and that is was serviced in the past for a low refrigerant level with a dye to identify the leak. Inspect the ground for condensation puddles from the evaporator case. If it is humid outside and there is no condensation from the vehicle drain tube, the evaporator drain may be plugged up and will require service. If a water pool is present under then car, inspect it along with the drain tube with the black light for traces of green dye. Using caution, feel the high and low side lines for temperature change. The high side hot and the low side cold. Ensure you do not have cold spots where there should be heat, or hot spots where it should be cold. This generally applies to vehicles with rear air conditioning, where the hoses are more prone to collapse and damage.

5: Turn off the engine completely. It is now time to install your manifold gauge set. Ensure the couplers are closed, and then remove the service port covers from the A/C system. As you loosen the caps, listen for a hiss or for pressure under the cap, like a soda bottle. If you notice a pressure release, you should replace the valve cores after you recover the refrigerant due to a leak. If your service port caps are missing, inspect the port for debris or corrosion. If any debris or damage is present inside the port, servicing the A/C system may render the valve in the port inoperable after service and may leak profusely, requiring replacement of the valve or worse, the line that contains the port and the valve. If your service valves are clear, you can safely hook up. This part is absolutely critical: You could recharge a car after a repair, only to have it blow refrigerant out of the service ports after you disconnect!!!

6: Snap the couplers onto the service ports, and then turn the valve ***** on the manifold gauge set, low side first until pressure begins to register on the gauge. Then turn 1/3 to 1/2 a turn more. Do the same for the high side valve. DO NOT bottom out the service valve by over-rotation!!! You may damage or destroy your service port!! Let the vehicle sit for about 10 minutes or until the gauges register near equal. Expansion valve systems may take a while to equalize, as the valve will be closed preventing any refrigerant flow. If this is the case, you can turn the ***** on the gauges themselves to equalize the pressure. If it equalizes very fast before you do this, suspect an internally leaking expansion valve if the vehicle has one. Orifice tube systems will equalize very quickly and is normal. Once equal, note the pressure on the high and low side. This is your system resting pressure, and this is used to determine a few basic problems:

1: If air is trapped in the system/overcharged (Resting pressure is WAY too high.)
2: If the system is severely undercharged (Resting pressure is very low, or zero.)

Compare your pressure readings with this chart:



NOTE: Gauging the amount of refrigerant based on resting system pressure alone or any pressure readings is not possible!! A pressure test can’t determine the exact composition of the A/C system being tested, nor can it determine the amount. Anything that deviates from resting R143a system pressures should be considered contaminated until proven otherwise!!! Also, if you use recovery equipment without an attached filtering element, you may contaminate the entire tank of refrigerant you are using. This will make it dangerous to use!!!

Diagnosis and Root-Cause

After taking this reading, measure the ambient air temperature in and around the engine bay. Reference your pressure-temperature chart for the proper pressures in the rest state. Your results will be influenced by the heat in the engine compartment, and may vary slightly due to inaccuracy. It is best to use the resting pressure method when the vehicle has sat overnight and temperatures have equalized on all the parts. This allows for a more accurate assessment. Next, restart the vehicle and turn the A/C on with full blower speed out of the center panel and recirculation on. Record the high side pressure and the low side pressure at idle, and record the vent temperature after 5 minutes of operation. Raise the engine RPM to 2500RPM and record the high side, low side, and vent temperatures after 5 minutes. Your readings will change depending on the ambient air temperature and humidity, so every time you make a reading, record the ambient air temperature and humidity as well.

Using two different colored markers, mark your idle readings and your 2500RPM readings in the chart below or one from you manufacturer's service manual. The 2500RPM readings I have specificed may or may not fit in the chart, but these readings can help you determine if A/C problems exist while the vehicle is in motion. I use 2500RPM for most 4 cylinder vehicles, as they cruise around the 2500RPM mark. Larger engines can use a lower second rpm reading such as 1800RPM.



If your markings fall inside the double lines indicated, your system is operating to specification at this time. If your system falls outside the lines, then further inspection/service based on your initial inspection is required. There are also several situations where readings may fall out of these markings normally. For example, most aftermarket A/C condensers are built more efficiently than OE condensers. These will yield a lower high side pressure, but will still meet the low pressure and discharge temperature specifications. Be aware that situations like these can occur.

After these measurements are documented, refer to the service information for more comprehensive service information, technical service bulletins, and potential recalls. This information may point you in the direction of repairs by the manufacturer that were not yet completed, or only completed upon complaint by the customer.

This concludes what a comprehensive A/C inspection should cover. Using the data gathered above, you should now be able to gauge what type of service your system will require, or at least present it to Honda-Tech in a way that we can help you with. Now we will move on to some common A/C problems, and try to put the data you have compiled to good use.

Common Problems:

These are some common problem scenarios that can occur when a mobile A/C system is not operating correctly. These should only be used as a reference! They do not apply to every situation, but in most cases, you may see many of the symptoms you are experiencing in a listed scenario that seems to fit your situation more closely than others. Use these only as a starting diagnostic point.

If your A/C system operates, and your high side pressure is low, and your low side pressure is low, and your A/C output is lukewarm, your system will require refrigerant recovery and full recharge followed by a system pressure evaluation. Be sure to add a dye injection to help detect leaks if they are not currently present. This condition is usually caused by a leaking system or an undercharged condition. You should be looking for signs of refrigerant leakage and proceed from there.

If your high side pressure is too high, and your low side is normal to moderately high, your system is most likely overcharged, or has air contamination. The condenser fins may also be clogged with debris or bent, or restricted internally. The cooling fan may also not be pulling enough air over the condenser to cool the refrigerant (inspect fan shroud/fan assembly).. Air is non-condensing, and will increase system pressures which may result in operating problems. This type of condition usually manifests itself with poor cooling output, but can vary from vehicle to vehicle depending on system size.

If your high side pressure is high initially and tapers off, and your low side pressure is very low, there may be an excessive restriction in the A/C system, normally a defective metering device, collapsed hose, or an internally clogged condenser. Inspect all the lines for signs of leakage/cracking/deformation. A/C hoses are barrier-style, so the inner portion of the hose may collapse and may physically still look okay from the outside. Generally speaking, restrictions are easy to locate, as there will generally be a large temperature drop across the suspected area, confirming that a restriction exists. A restricted expansion valve system that runs for a long enough time will usually produce a lower high side pressure and a low side vacuum, even with a full charge of refrigerant. Expansion valve problems tend to produce erratic pressure readings depending on how long the system has been operating, so you may need to operate the system for a longer period of time to uncover more symptoms.

If you observe erratic pressure gauge needle movement, consistent fluttering, and/or wild jumping, generally seen on the high side gauge, you could have a bad compressor valve or a partially blocked metering device or hose. Replace the offending component, inspect the system for debris, and flush as required. If internal debris is found, replace the filter assembly, metering valve and recheck the system. In most of these, cases the compressor has become faulty. This type of failure is likely contaminate the whole system with metal/rubber debris, and will require complete system flushing for longevity.

If you observe the low side pressure gauge going into a vacuum, and the compressor still continues to run, you may have a faulty low pressure cutoff switch as well as a potential blockage, normally a metering valve problem. Recover refrigerant, note the amount removed, and if low, recharge with dye and retest. Research relevant information on low pressure switch operation and replace if required. There are conditions where a restricted metering valve can also be a culprit.

If you observe ice or frost on the lines, you have a damaged line, a low refrigerant charge, or an expansion valve that cannot compensate enough for the entire operating range. Inspect for low refrigerant level and correct as required and retest. Then replace the expansion valve or orifice tube, or the damaged lines if the problem persists. Damaged lines will generally have a temperature drop across the affected area.

If your gauge pressures barely change with changes in engine speed and are near resting pressure, the compressor is either not engaging, or the compressor is engaged but the piston seals or valving have completely failed internally. This results in no net refrigerant flow in the system. Diagnose the cause of the A/C compressor not engaging (Low refrigerant will cause a compressor to not engage at all due to low pressure cutoff). If your compressor clutch is still operable, replace the compressor, dryer, metering valve, and flush the system if it engages with a full charge of refrigerant and the pressures still do not move. This type of compressor failure will contaminate the entire system and is the most catastrophic to repair, requiring the system to be thoroughly flushed out for system longevity.

If you observe noise while the compressor is in operation, check proper belt tension and retest. If the noise persists, attempt to turn the compressor clutch by hand. If it does not spin easily (it will have slight resistance, it is a pump after all), replace the compressor, dryer, and flush the system and retest. If the front end turns easily and still makes noise, remove the drive belt and attempt to turn the driving pulley. If it has movement (or slack) or is hard to turn, replace the compressor clutch assembly. NOTE: Some compressors, such as the Ford FS-10, 10 cylinder compressor will have pre-load on the clutch shaft preventing it from turning easily and is normal. Consult factory service information if you encounter this type of situation. Many clutch noise problems are the result of the clutch pressure plate on the compressor failing, and rubbing the drive pulley. With the system active the noise usually disappears. Sometimes the compressor drive pulley bearing may fail and produce noise at different times. It is important to not just check for pulley bearing looseness or seizing, but also pulley and belt alignment as well.

Diagnostic Tips:

As a general rule, A/C systems that are overcharged may perform adequately at idle. But when engine RPM is increased, the system may feel like it turns off and on repeatedly. A/C systems that are undercharged may not function very well at idle, but when moving, performance may increase. These types of charge problems can manifest with many systems, and the symptoms you experience are also dependent on the condition of the rest of the system. Most automotive A/C systems in use today include a pair of safety pressure switches (High and Low) to prevent compressor damage in the event of extreme pressure increases (400+psi, overcharged, air in the system, condenser fan failure, etc) or sudden system vacuum or low pressure (10psi and lower, undercharged, blocked metering device, iced evaporator, etc). The 2500RPM test I posted above can help determine if your system is going into a vacuum or into an excessive pressure situation while driving. This test is not provided by most manufacturers, but it can aid you in detecting a system problem in the future that may not be readily apparent.

Care must also be taken when inspecting the lines and service fittings when service is performed. If debris or contamination is evident inside the lines, the system should be flushed completely, along with replacement of defective service components. An evaluation will then need to be performed to determine the source of the material. Lack of oil due to repeated evacuation/recharging is one of the biggest causes of failed compressors and system contamination. Improper lubrication oil grade, and excessive oiling would be a close second. Repairs where contamination is present are the most cumbersome and costly to correct on average.

Sometimes the lack of cooling or heating performance has no actual mechanical systemic problem. The problem may be caused by something else outside the physical A/C system circuit itself. A blend door that is jammed may not allow full heat or A/C operation. If the vehicle was not equipped with a cabin filter, the evaporator coils could be saturated with debris, preventing air flow and causing the system to freeze up or to cycle excessively. Vehicles older than model year 2000 are more prone to this type of problem as cabin air filters were not widely adopted by most manufacturers until model year 2001-2004. Nearly all vehicles built today feature at least a basic paper filter, and many feature activated carbon filtering or HEPA-style air filtering media.

If a vehicle with dual zone front climate control has a low refrigerant charge, it will generally manifest itself as reasonable A/C performance on the passenger side, and warmer A/C on the driver side or vis versa. Confirming this type of problem is quite simple. Adjust the driver side and passenger side temperature control towards hot in increments, and then back to cold. If the vents change temperature output accordingly, the blend door is functional and is most likely okay. The problem is an undercharged system 9/10 times. This behavior occurs because the air partition near the blend door cuts the evaporator airflow in half. In this case, the passenger side gets the first refrigerant into the evaporator. After it boils, there is not enough mass left in the rest of the evaporator to cool the driver side. You should be looking for signs of system leakage when this problem is present.

If during your inspection you noted that the temperature mix control was binding or tough to turn, this is a problem and will require service/repair. A stuck or mis-adjusted heater control valve will heat up the same air the evaporator just cooled, resulting in poor cooling output overall. Inspect the controls for the heater core/air blend door and if your inspection warrants it. This is where if you noticed a huge variation in actual temperature vs what a digital temperature display (on systems with a digital temperature control) reports, this type of blending door problem would be a suspect cause.

In many vehicles, even those with cabin air filters, sufficient debris can in some cases (depending on the system layout) skirt past the filter, and block up the evaporator drain tube. If the evaporator drain is clogged, this may cause water to pool inside the evaporator casing. In severe instances, water may overflow onto neighboring components such as the blower motor and resistor. This may cause these parts to short to their ground circuit. In many situations the corrosion that is produced can destroy the resistor and the motor, and will require replacement. Defective/deformed plastic cowls can also cause water intrusion from the outside resulting in the same condition. The hood cowls on some vehicles (such as the 2000-2005 Ford Focus) are notorious for this type of problem. Deformed exterior parts can indicate a previous collision, in addition to the water leaks that can form when exterior parts do not seal properly.

If the recirculation control is binding or hard to move, this may suggest that debris from the hood cowl is clogging the door seals. This type of damage is common where the vehicle is parked under trees that bear fruit, sap, or shed a lot of foliage. In addition, environments where pests such are rats and mice thrive may result in a nest buildup on this door, and the blower motor housing if no filter is present.

Many problems with sticking or defective blend or control doors in my experience are due in large part to collision damage. Especially on domestic vehicles, air control housings are an integral part of the firewall itself. If the firewall becomes damaged or distorted, it can result in air leakage between the housing and the firewall. This can result in exhaust or engine fumes entering the engine compartment, or cooled air escaping the HVAC housing to the exterior. Imports are not immune to collision damage either. Some cars in a collision event may result in the fragile temperature control arms breaking off their drive motors or servos, resulting in a similar situation.

In most cases the HVAC assembly must be completely exposed to service the doors, the controls for them, the heater core, and the evaporator. This may require that the entire interior dashboard be removed from the vehicle to access, so this is no real easy repair even from a professional standpoint. Some systems have a more modular fit, like most Honda and import models. These models allow for servicing of certain components like the evaporator without removing the dashboard. How your doors are controlled and where they are located will determine how far you need to go to at least find out what is causing the problem. Some vehicles use traditional cable controls, in which a cable is connected to the dial directly to adjust the door position. Some use vacuum-operated servo doors, (mostly domestic vehicles) and most vehicles today use electronic encoder motors or stepper motors on each door, which allows for troubleshooting with a factory scan device to determine desired door position vs. actual door position.

Evaporator Odors:

This type of problem is VERY common on older vehicle years, but can occur even on a new production model. In order to understand how evaporator odors become a problem, we must examine the two functions the evaporator performs and how they relate to the problem itself:

1: The evaporator is designed to cool the car. As such, this part will get very cold during operation.

2: By reducing the temperature, the ambient dew point is also reduced. This produces condensation that has to be drained off the evaporator coils during operation.

3: Because the air entering the car from the exterior may contain pollen and other microbials, these can easily attach themselves to the evaporator core because of the moisture that is produced.

4: Because the evaporator may still harbor some moisture once the vehicle is turned off, it serves as a breeding ground for microbial life.

Even in vehicles equipped with a cabin air filter, evaporator odors are still very common. There are a few things that can be done to prevent or correct this smelly nuisance. Several newer vehicles have an "after-blow" feature, which runs the interior blower motor for a specified amount of time. This is designed to dry off the evaporator core to prevent mildew and bacterial growths from forming on the evaporator and its insulating material when the vehicle is parked. This after-blow feature is very similar to an engine cooling strategy that prevents heat from pluming the engine compartment in hot climates with the engine off. On these vehicles, the engine cooling fans run for a short time with the engine off to dissipate heat. This helps prevent the engine cooling system from over-pressurizing when the engine is shut off. This also has the added benefit of taking the surface charge off the battery, which can increase its life span as well.

Many vehicles do not have any after-blow feature, and not having this feature is the primary cause of mildew buildup and the rank odors they produce. Just like your shower, the resting water droplets foster the growth of all kinds of spores and/or bacteria. These odors are more prevalent when the system is turned on for the first time after a cold start. These odors are typically emitted for 15-30 seconds, and will diminish as the evaporator cools and covers the growth with moisture. When the cooling system cycles on and off, the odor may return periodically.

Correcting this problem on vehicles without an after-blow feature is done primarily in two ways: The easy way, and the hard way. In this case, the more effort you exert, the better your results will be in general. There are a lot of over the shelf "odor-b-gone" products that claim to eradicate the musty odors emitted by your A/C system. Most of these require you to hook a hose to the evaporator drain tube and spray a foaming agent that kills mold and bacteria on the evaporator and the surrounding parts. While this is easy enough for most do-it-yourselfers with some skill, it is not an effective long-term solution because these products rarely kill everything on the evaporator itself. The musty odor will usually return after only a few weeks. This would require repeated of the product use which is far from an ideal long-term solution.

The hard way is to remove the evaporator core from the vehicle, and thoroughly clean the unit using bleach or another soft cleaning agent. This includes all the foam and any seals that are around the evaporator. Your not done yet, this next step is very important. Several manufacturers and aftermarket companies create a silver-based spray-on paint that is designed to prevent and kill bacterial growths before they can spread to other areas. Applying this type of coating on a clean evaporator is very successful in preventing microbial growths from recurring in the future, and is a dealership authorized repair in most cases of evaporator odor.

Cabin filters are both a blessing and a curse. They trap airborne particles so they don't end up on your dashboard or in your lungs. But a filter that is neglected can be more dangerous to you health than not having one. One these filters become clogged, they can act as a breeding ground for bacteria and other pathogens which can then spread to other components.

If you do not see removing the evaporator as a feasible solution, simply turning off the A/C for 15-30 seconds and operating the blower motor on high speed to dry it off before you reach your destination will help manage the odor. This strategy will not kill the growths, only manage the odors they can produce. Some vehicle models have service bulletins that can add an after-blow feature to help mitigate the problem for you. Always check with the dealer to see if a modification is available to help manage these odors.

Repair:

Undergoing A/C repairs, as stated at the beginning of this post, are quite complex. If you do not understand the basic fundamentals at the beginning of this post, do not attempt to repair or diagnose your system!!!

Let’s start with one repair that tends to get messed up a lot: Fitting o-rings to replacement parts. Fitting o-rings is both art and science. Reusing an o-ring seal will almost certainly result in a system that leaks right away or will leak over time. In most replacement parts kits, such as a compressor kit, an o-ring seal kit will usually be provided for the parts you are servicing. Many of these kits may not supply the correct size o-ring seals or gaskets required to make the system you’re working on leak free. In many cases you’ll be confronted with 3 or 4 different o-ring seals, all of which seem to fit fine on the part, but you don’t know which one to use.



Typical A/C system O-ring connections.

On most A/C connections, there is a machined down section that an o-ring seal will fit in. This is where the o-ring seal will rest inside the part. There may be one, two, or even more seals in some of these parts. They also come in various diameters and thicknesses. So how do we choose the right part?

Method 1: The Reference Method

This method is used by most technicians because there is an o-ring installed on the old part that can be used as a reference for finding the right sized replacement. The problem with this method is that if the previous o-ring was incorrect or was heavily distorted or damaged, choosing an o-ring that is close to what the original looked like may still produce a leak. This method is generally safe to use if the part did not leak before, and the o-ring appears to be in good enough condition to make a comparison.

Method 2: The Dimensional Method

This method requires two tools: A measuring caliper, and a size specification from a service manual. Using this method is more difficult to complete because o-rings are flexible, and depending on how hard you press on the caliper when measuring the o-ring, may yield the wrong size replacement o-ring. There are some cases (such as automatic transmission pistons) when the o-ring cannot retain its shape because it is physically too large. The diameter of these larger o-rings is generally done by stretching them to their maximum length, similar to how technicians measure drive belt length for a vehicle where two belts are very close, but not quite the same. The basic measurements of an o-ring are: The inside diameter (ID), the outside diameter (OD), and the thickness. In most cases, the thickness is the diameter of the rubber itself. In square cut o-ring seals, the seal may be rectangular. To get the second dimension to form the rectangle, subtract the OD from the ID, and divide by 2 (because the diameter covers twice the radius). Where measurements of an o-ring are taken, measuring at least 3 times and taking an average is the best way to remove human error from the measurement.

Method 3: The Fitness Method

There are some cases where you do not have an o-ring to make a comparison, or manufacturer specifications for what size of o-ring to install. This is somewhat similar to a guess-and-check fitment. The main difference is the installer will need to use their knowledge of o-ring fitments to estimate the correct size o-ring to use. In most applications, an o-ring sits in a machined relief in the tube or device where the seal will function. The goal is to fit an o-ring that almost covers the entire machined groove, and also protrudes from the relief to create a positive seal with the connecting part. If the o-ring sticks out too much, it may pinch during installation, damaging the seal. If it is too narrow, the seal will move around in the groove as it works, distorting it and usually leaking as well. It may also be a very tight fit across a tube, for instance, but will install too easily over the connecting part, creating an outside diameter leak. If the o-ring is a loose fit over the tube, it may leak from both the inside and outside, as the seal cannot stay stationary.

Regardless of the method you choose to use, there is another factor that must be considered: Longevity. Depending on the type of system (we will focus on A/C here), incorrectly choosing the wrong seal size may result in a functional system initially, and as the system works and degrades the o-ring, may begin to leak. This is the hardest part of the process to achieve, as only time and experience will tell. Some seals warp under heat, some swell with oiling, and can also harden over time as fuel, rotting, or material corrosion takes its toll as well.

Compressor Installation:

Installing an A/C compressor, like many other parts, is not as simple and removing and installing bolts. Most A/C compressors are three-piece assemblies, with a front housing, a body, and a rear housing. Improperly torquing an A/C compressor can lead to body seal leaks, housing-to-housing leaks, and physical damage to the compressor assembly itself. Most A/C compressors have a specified bolt torque sequence to prevent any one part of the compressor housing from distorting during installation. Compressor failures within the 1-6 month range are generally caused by excessive or inadequate torque applied to the housings or bolts. Many of these parts may not have been assembled correctly in the first place. Most compressors use long, thin bolts to sandwich all the parts together. If this sandwich was not completed on an assembly jig referencing the application in question, the housings will twist, potentially damaging the unit or distorting the seal. Always reference a torque sequence if available, or follow an X shaped pattern in the lack of any specification where this applies.

Cynical 1

you have a lot of free time...

nice write up!

slowcivic2k

I do this for a living, if I didn't know all of this like the back of my hand, I would be out of a job lol.

deschlong

iTrader: (8)

Bookmarked for a future read! Love write-ups like this. Thanks!

OldSchool Honda

That was properly put together,
For the first time I was actually able to firmly implant this knowledge in my mind.
Other articles have always left me with questions.

Former User

Great write up.

Just an FYI -- The Honda Civic service manual states that refrigerant pressure readings for the chart should be made with the engine running at 1500 rpm not 2500 rpm. Maybe you could comment on your choice of 2500 rpm.

A couple of general questions:

1) If a decade or more A/C system is generally working okay though slightly less well than when new and the low and high side refrigerant pressures are just below spec ranges, can you just add some refrigerant rather than recover and recharge? In other words, how do you decide when to add some refrigerant versus recover, leak test, and recharge?

2) Common problems that I notice pop up in the 92-00 Civic forum but are not really addressed here are a bad A/C thermostat and a compressor clutch clearance that is too large. Do you have any tips for replacing/fixing these issues?

3) What would be some potential diagnoses of a system with normal high side pressure but a below spec low side pressure?

05foresterxt

iTrader: (1)

Great write up. I have a 99 Si and the heater and AC works. The AC was recharged during a summer or two, but it still didn't get cold. It does throw cold air, but not enuogh to be consider a proper cooling AC. What should I check first noting that I did have it recharged and was not informed of anything being bad with the system. Thanks.

slowcivic2k

I use 2500 RPM for smaller cars because this is generally the range in which the system operates most of the time. (IE moving) I will always compare to manufacturer's specification, but I evaluate higher RPM performance for flow problems and compressor valve problems.

1: A system should always be recovered when serviced or inspected, this cleans the refrigerant, removes air and moisture, and simply doing this can increase efficiency marginally. "Topping off will introduce air into the system since the valve has to seal from the outside air to your service fitting, this will result in reduced performance, corrosion, and possibly higher than acceptable pressures.

2: Most of the time the thermostat will not fail, sometimes they do, but generally it is a buildup of debris on the evaporator that causes poor airflow, which over-cools the thermostat. My 1991 Civic has this problem, when the air position is in recirculation, the clutch cycles too frequently (getting coil too cold, prevents freezing/icing) but in fresh air mode, it stays cooler, and does not cycle nearly as frequently.

Adjusting clutch clearance generally requires shims to correct, and like any clutch it will wear and require service eventually. The best preventive maintenance is to maintain and check the system bi-yearly. Just like a car clutch, if it cycles on and off more than normal, it will shorten the life of the unit.

3: Generally lower than normal low side pressure will indicate poor heat transfer in the cab. As heat passes over the evaporator, it will raise the pressure to some degree because of heat, low pressure indicates low transfer of heat. Generally this is caused by the aformentioned thermostat problem, and will also cause rapid clutch engagement in moderate climates.

Former User

Your particular use of 2500 rpm makes sense, but it seems to me that the average H-T user should restrict their A/C performance tests to 1500 rpm as this is the only way to make interpretable comparisons with the service manual temperature/pressure chart you posted. I say this because the refrigerant pressures will differ substantially between 2500 rpm and 1500 rpm.

I'm confused. You are apparently recommending here that nobody should ever top off their system with refrigerant even if the system is just slightly low and does not have a significant leak. And if done properly, how and why would air get introduced into the system? Please clarify.

Is it worth the work to remove and clean the evaporator or would you just replace it? Or do nothing if cooling by the A/C is adequate?

slowcivic2k

The pressure will vary a little bit, but if there are issues with the system as a whole, they can be seen a lot easier. Say a partially debris blocked condenser, pressure will shoot up quite high, that can also indicate air in the system. Higher loads also stress the valving more, exposing a weak reed valve, or a noisy valve under load. (very common for me to see this) And higher engine speed will be more likely to ice up an orifice tube or expansion valve on a hot day, which may not be seen at lower refrigerant speeds.



Topping off a system is futile and dangerous, as you don't know how much is already in there, the ONLY way to do it correctly is recover, vacuum, and recharge. The service ports are exposed to outside air all the time, as is your service equipment, when you connect your service lines to a car, you are trapping a small amount of air between the fittings, and when opened, gets into the A/C system, which is why air is so common.


If the air flow is poor and is not equipped with a cabin filter, I would remove mine and replace it, its 20 years old, probably corroded and brittle by now, on any car that is 4-5 years old, I would not replace it unless it leaked, personal preference.

Former User

Even without knowing exactly how much refrigerant is in the system, can't a top off be done safely if you carefully use the temp/pressure chart you posted?

Why can't this introduction of unwanted air be avoided by purging the low side, high side, and vacuum/supply hoses prior to adding the refrigerant?

slowcivic2k

No, because as a technician, you don't "know" what is in the A/C system. If air is in the system, it may be interpreted to have a full charge. Using a temp and pressure chart will reveal that something other than refrigerant is present in the system if the pressure is too high for the temperature, and a lot of factors go into that. The heat that pours from the radiator and even direct sunlight will eventually reach the A/C components and warm them, thus inflating the inaccuracy of the temp/pressure method of determining refrigerant level. Refrigerant is only dispensed by weight, any other method will be inaccurate to say the least.

I assume you mean purging the hoses by venting refrigerant from an open port and then tightening the leak. To keep this legal, venting any kind of refrigerant is illegal, and improper. Most A/C machines use a purge cylinder to separate air from refrigerant, while some use another means. Either way, the equipment has to be able to separate air, inevitably some refrigerant may be released. But the equipment must be certified for use by the EPA, and if so, the losses that are incurred are considered "De Minimis" and are exempt. (Good example is the release of refrigerant by removing the couplers after service, it is inevitable that some refrigerant will escape)

mistertwoturbo

Great write up, will be referencing this constantly as my civic is in need of AC.

Former User

My original question was about a system somewhat low on charge, which I believe would be pretty straightforward to diagnose by a test done in a shady area with a manifold gauge set.

Your answer as stated all but eliminates the possibility of a DIYer doing any A/C work. For example, based on what you've said, just hooking up the manifold gauge set to the high and low side ports risks introducing unwanted air into the system.

Considering that any amount of refrigerant release is illegal, could a DIYer wanting to diagnose a problem or to top off a low system circumvent introducing air from the manifold gauge set into the A/C system by vacuuming the air out of the gauge set with a vacuum pump prior to connecting the gauge set to the high and low side ports?

DCFIVER

It cannot. Gauge readings alone are insufficient to test an AC system.

This is correct. The AC system was NEVER designed for DIY'ers. The cans of refrigerant sold at autoparts stores often contain sealer in addition to imitation refrigerant. They also rarely have the proper viscosity of oil and or the proper amount of oil that is lost from a system with a low charge. So along with introducing air and moisture into the system, when you use these cans, you will also introduce contaminants. And possibly too much oil and or the wrong oil. Too much oil will cause the compressor to hydrolock. Too little oil will cause the compressor to lock up. This stuff will eventually wreck havoc on the entire AC system. Trying to save a buck now by installing the junk that is sold at autoparts stores instead of having the vehicle properly serviced via an RRR machine will eventually end up costing much more in the end.

How will you achieve vacuuming out air, but not refrigerant and oil?? You seem insistent on cutting corners. There is no ideal short cut. Air and moisture eventually enter the sytem through the connections and service ports. The same way refrigerant eventually seeps out of the AC system and why vehicles need a periodic recharge. Moisture is also generated by the very nature of how the AC system works. At some point in the vehicles life you want to evacuate and cleanse the AC system of these contaminates. And to return lost oil that has also seeped out of the system via the refrigerant. There is no way to do this with out the proper machine.




The irony is that professional technicians are required to be certified in the handling and disposal of refrigerant prior to doing any AC repairs. Any technician that is caught doing ANY type of AC repairs and is not 609 certified, can fined big money.(up to 25k here in CA) there are no such provisions in place for the DIY'er. Mainly because most of the refrigerant you can purchase is not real R134 or R12.
http://www.epatest.com/609/

SpoonyDC5

1998 Honda Civic LX Stock here, my AC works just fine the majority of the time but out of nowhere it will just stop blowing cold, if i keep it on and give it a few minutes it will start to blow cold again and other times, all i have to do is shut off the AC and turn it back on and it will blow cold again. any guesses as to what would be causing this?

Former User

When it stops blowing cold, does the condenser fan stop blowing, does the compressor clutch disengage, or both? Does the A/C button light remain lit or turn off?

Any recent car work done?

SpoonyDC5

fan doesnt stop blowing, clutch stays engaged and the ac light remains lit.

Former User

Are you sure? If these are true, then the next step would be to hook up a manifold gauge set to measure the high and low side pressures when the A/C blows warm.

fcm

Most likely an expansion valve problem, I have done a few Honda/Acura with the same symptoms, it has always been the expansion valve.

It may also be the evaporator freezing up.

Does the compressor cycle "normally" run for a bit then turn off and then run for a bit again and turn off? 94

SpoonyDC5

compressor does cycle normally, and where is this valve located? easy fix?

slowcivic2k

Hook up an manifold gauge set and see what the system is doing, it could be a number of things, restriction in the system, blocked condenser or evaporator, blocked or defective expansion valve, or a compressor that has a bad valve in it that intermittently works. There are a lot more.

Most cars with low refrigerant have poor cooling at a near stand still, and perform adequately at speeds above 25mph.

slowcivic2k

Please, to keep this clean and technical, I would advise all to not post problems in here. I would like to keep this as technical as possible. If you do not understand a concept or feel that something is missing, please post your concerns, otherwise create a new thread.

Former User

Assuming an old A/C system has no problems that require repair, what would be a reasonable price range for a shop to recover, evacuate, and recharge a R134a system (e.g., a Civic)? Should this work also include replacement of the receiver/dryer?

How should a consumer intelligently choose a shop for A/C work like this? Do you have any general or specific recommendations?