Contrary to popular belief, air conditioning systems are not complicated, or very difficult to work on. Not very many tools are required for in-depth service, and it is perfectly safe if the proper knowledge and information is applied carefully and thoughtfully.

All automotive air conditioning systems use a closed-loop refridgerant system, recirculating the same refridgerant throughout the system, using a pump/compressor to provide a temperature differential between two seperate sides. One is cooler than ambient temperature, while the other side is hotter. Obviously the cooler side is the cabin, while the hotter side is the engine compartment where most of the equipment is located, so temperature does not matter there in reference to passenger comfort.

Types of refriderant may vary by vehicle age, but the basic functionality is the same, as it is in most refridgeration or air conditioning systems. The AC compressor sucks low pressure refrigerant gas from the evaporator, compressing it and forcing it into the condenser. A byproduct of this compression is that the gas rises in temperature. Airflow across the condenser cools the high pressure gas, causing it to condense into a high pressure liquid. The high pressure refrigerant liquid flows through a regulator system, usually either an orfice tube or expansion valve, and pressure drops as the liquid flows to the evaporator through this restriction. Airflow across the evaporator causes the liquid refrigerant to boil and turn into a gas again. This physical change in the refriderant makes it cold, which in turn cools the evaporator it flows through. The air blowing through the evaporator thus becomes cold, cooling the cabin. The warmed gas refridgerant then flows back to the compressor and the cycle starts again.

The restriction of the regulator system provides a way to meter the temperature of the system. The higher the restriction, the less liquid can pass and the less the system will cool. The less the restriction, the more the system can cool. The total cooling capacity of the system is based on ambient air temperature and the performance of the system itself. In order to cool effectively and efficiently, every part of the system must operate correctly.

Most modern systems will have one or more pressure sensors in the system to prevent damage. A switch in the low pressure side, called a 'low pressure switch', can turn off the system if refridgerant level (and thus pressure) is too low. A 'high pressure switch' on the high pressure side of the system can turn off the system if pressure rises too high due to a restriction such as frozen lines.

The oil for the compressor is usually mixed in with the refridgerant in modern compressors. A lack of refridgerant thus means a lack of circulating oil through the compressor. If the compressor runs with low refriderant, this can cause damage if the compressor does not have its own independant oil supply. Some compressor types have their own onboard oiling supply and system, and do not rely on refridgerant for oiling. These compressor types are generally more expensive and are becoming rarer in automotive use.

Advanced computerized systems can use the pressure sensors to control the performance of the system, cycling the compressor according to the readings to help maintain temperature more effectively. Simpler systems merely use the pressure sensors as cut-off points to prevent system damage. There are also systems that use a temperature probe on the evaporator to turn off the system if it reaches freezing temperatures, as this can cause a system restriction and damage.

An orifice tube is a refrigerant metering device to control the rate of flow of refrigerant into the evaporator. The tube causes a restriction to the high pressure liquid refridgerant, and this liquid changes to a gas as it enters the evaporator due to the pressure drop caused by this restriction. As cabin air blows across the evaporator, the gas heats and its temperature rises. The temperature differential (colder air) ends up in the cabin.

Orifice tubes are always located in the refrigerant tubing that connects to the high pressure port on the evaporator, or in the evaporator tubing itself. Some are changeable, but some are integral to the line and the entire line must be replaced.

It is a simple system, with not much to go wrong. There is no reliance on temperature sensing or regulation, only the restriction of the screen.

Orifice tubes fail due to a clogged screen. A clog can be caused by oil deterioration, acid formation in the system due to excess moisture, or metal particles from component failure.

Most commonly, orfice tube systems use an accumulator/drier after the evaporator to collect any residual liquid to prevent a liquid slug from damaging the compressor. Incoming gas/liquid goes into the unit, and the outgoing tube is positioned so that only gas can leave. The drier function will absorb residual air to prevent system contamination, usually through the use of a dessicant pack.

An expansion valve is a refrigerant metering device to control the rate of flow of refrigerant into the evaporator. It is located directly before the evaporator, and will have a temperature probe against the fins of the evaportator or the refriderant line. As the temperature of the refridgerant line changes, the valve, operated by a diaphragm, will close or open accordingly to meter the flow of refridgerant into the evaporator.

The valve causes a restriction to the high pressure liquid refridgerant, and this liquid changes to a gas as it enters the evaporator due to the pressure drop caused by this restriction. The nature of the valve is that it can control the amount of restriction, unlike an orfice tube that is fixed. The position of the valve is controlled by a diaphragm, which is regulated by the temperature of the refridgerant entering the evaporator. As cabin air blows across the evaporator, the gas heats and its temperature rises. The temperature differential (colder air) ends up in the cabin.

Expansion valves can get stuck or fail, and this will be evident in the pressure readings on the system. An open valve will allow too much refridgerant to pass while a stuck closed valve will allow none. They are more complicated than an orifice tube system, but can better meter the flow of refridgerant for optimum system efficiency.

There are two main types of valves used in automotive systems: internal sensor types and external sensor types. The internal types are often call 'block' valves because they basically look like a block. The external types will have a thermal sensing line with a probe that will connect to the line coming into the evaporator and only connect directly to the outgoing line. The internal type will connect to both pressure lines, incoming and outgoing.

Most commonly metering valve systems use a receiver/drier between the condensor and evaporator to provide a liquid resevoir. The drier function will absorb residual air to prevent system contamination, usually through the use of a dessicant pack. Some units also include a filter screen to collect debris.

Internal sensor:

External sensor:

Almost all internal combustion engine automotive air conditioning compressors rely on an electrically activated clutch to engage a belt-driven pulley with the compressor shaft. This voltage is generally applied manually through a user-operated switch, or through a control computer. Both systems usually have regulatory feedback circuits of some kind to protect the compressor and prevent system damage.

There are two main types of regulation: thermal and pressure. Pressure switches can monitor both low and high refridgerant pressure points of the system to deactivate the clutch, or activate it when pressures are within proper operating ranges. Thermal switches can monitor system temperatures at select points and deactivate the clutch to prevent icing or overheating, or give feedback on system performance.

The main goal of system regulation is to prevent compressor or system damage when conditions fall outside standard operating parameters. If the system loses too much refridgerant (and thus oil), pressures will fall too low and can trigger a low pressure switch to disengage the compressor clutch and stop the system. Likewise if pressure rises too high on the high pressure side, the system may be disengaged. If the system ices up and restricts flow, a thermal switch can disengage the compressor clutch to prevent damage.

A secondary goal of clutch control is efficiency. In some computer controlled systems, at wide open throttle the compressor may be deactivated to free up more engine power. Some systems use a method of clutch cycling so that the compressor doesn't pump when unnecessary, disengaging the clutch under stringent pressures or temperatures to closely regulate the output.

Some systems such as those using the Fridgedaire A6 axial compressor allow the compressor to run continuously when the air conditioning is activated by the operator, except when conditions would endanger the compressor. Some of these older systems use a superheat switch for protection that will blow a fuse or open the clutch circuit if the compressor head gets too hot with abnormal operating parameters.

A note on superheat switches: these are usually mounted on the compressor head (in the case of an A6 the rear of the compressor), and usually have only one wire which is live system voltage. In the case of an overheat condition, the switch contacts connect and create a short circuit between the live wire and the grounded compressor, blowing a fuse. Some systems use a thermal circuit breaker rather than a fuse.

Some Frigidaire A6 compressors use a one-wire pressure switch on the back of the compressor which looks like a superheat switch but rather monitors pressure and grounds the wire when the system has enough pressure to operate. These one-wire switches are no longer manufactured and must be retrofitted with a different system. Using a superheat switch mistakenly in place of the one-wire pressure switch will result in the system not operating properly as one is NO (normally open) and the other NC (normally closed).

Not all systems have any or all of these feedback devices. Some have combined high/low pressure switches.

Almost all air conditioning system troubleshooting will start in the same place: checking the system pressures. All systems are standardized to have two ports to perform this test; one on the low pressure side and one on the high pressure side. These readings will give a good indication of what is going on inside the system, and what parts are functioning.

Compressor clutch: A visual check may be preformed if the compressor clutch is suspect. If the clutch spins along with the engine belt, then the clutch is ok. If it does not engage when it should (or ever) then it is bad. It is basically an electromagnet that pulls the clutch together to engage it, and it can weaken or go bad over time. An audible 'click' noise will also usually happen at the point of engagement as the electromagnet turns on.

Compressor: Compressors wear and go bad over time, sometimes gradually. Although not particularly hard to replace other than pure mechanical skill, the rest of the system should be thoroughly checked out when replacing a compressor as it is usually one of the most expensive parts of the system, and failure can sometimes damage other components. Wear or damage in the compressor can mean metal particles through the rest of the system.

Adding refridgerant: Some notes before adding anything to the system. If the system diagnosis determines that more refridgerant is necessary, before adding more the source of the leak should be found and repaired. The system is a closed-loop system, meaning no refridgerant is lost during normal operation. Thus a lack of refridgerant means there is a leak. Also if the refridgerant is completely depleted or the system is being newly recharged from zero, ensure that the proper quantity of compressor oil is added along with the refridgerant. Running straight refridgerant without oil will kill the compressor.
There are several methods to add refridgerant. It can be added in liquid or gas form, as a system charge is measured by weight. Each system will have an exact, most efficient refridgerant quantity. This amount will be listed on the vehicle, or in the vehicle manual, usually by weight. This weight can be added exactly to an empty system using either a metering canister that measures the liquid as it goes in, a pre-charged canistor with the proper amount, a scale under a larger canister to measure how much enters the system, or expensive a/c charge machines will measure this amount automatically (of course most of us don't have access to those machines). There is also a less exact but still effective method of adding refridgerant to a running system until the proper pressures are realized on both sides of the system. This last method requires no special equipment other than a guage set and a refridgerant canistor, and the proper knowledge obviously.
Adding refridgerant by running system pressure measurements should only be done on the low pressure side. The high pressure valve should remain closed, as liquid refridgerant flowing directly into the compressor will damage it. Liquid cannot be compressed. When using this method, not the ambient air temperature beforehand, then cross-reference the pressure chart below to find what pressures are necessary. Carefully add refridgerant to the system until these pressures are met and maintained. However, pressures on both the low and high side should be monitored, as just one side cannot show the whole picture of what is going on.

Optimum pressure ranges (r134a refridgerant):

Finding a refridgerant leak: One of the most efficient tools for this is a refridgerant detector, sometimes called a 'sniffer'. It samples the air and checks for the presence of refridgerant. If found, it will trigger a warning to the operator. By moving this tool over portions of the system, a leak can sometimes be found in a pressurized system without touching the system itself. Another method is adding a dye to the refridgerant that fluoresces under ultravoilet (UV) light. As this dye leaks out of the system along with the refridgerant, leaks can sometimes be visually spotted in a pressurized system that have the dye added. Some compressor oil also comes with the dye already added for future troubleshooting. There are several downsides to using dye, one being that previous leaks that were not cleaned up correctly may still show up as current leaks. Another is that the dye obviously must be present and well circulated in all parts of the system, so it has to be added in a fully running system before the test.

Fixing a refridgerant leak: The only proper way to do this is to replace the leaking part, seal, or o-ring. Almost always this will involve discharging the system to open it up. Generally it is good practice to replace all the seals if one is found bad while the system is discharged, as this can be a time-intensive process. If replacing o-rings, ensure that the exact replacement size is used, as any size difference will create a leak. Keep in mind that the system eperiences extreme temperature changes, and this results in shrinkage and expansion regularly. Do not use any 'leak-fixers' that require adding anything to the system. Anything other than refridgerant or oil will shorten the life of the compressor or damage it, or clog orfice tube screens or other parts.
Before recharging the system: If the system has been discharged for a long period, the drier should be replaced. Long-term exposure to air will damage its capability. Short periods of discharging for a few minutes will be fine, regardless of what the manufacturer says. Before recharging, the system should be vacuumed out and kept under vacuum for a period. This ensures that there are no leaks before filling with refriderant, as well as ensuring there is no air in the system.
It is not hard to vacuum the system. A good air compressor can do it with the proper attachement, which is just a venturi which creates a vacuum as air passes through it. This in turn creates a vacuum in the hose leading to the system. It can easily be hooked to the fill hose of the guage set, so the vacuum can also be measured as it is created and held. Once satisfactory, the guage valves can be closed and the system should hold the measured vacuum. Obviously this will not recover any refridgerant or oil in the system, but for the average shade-tree mechanic this will be fine. Remember that it is illegal to vent refridgerant into the atmosphere, so this should only be used with a recovery system or if the system has already been emptied.

If refriderant without added oil is being used, the oil should be added first to make sure it circulates through the system and mixes with the refriderant before the system is activated. A quick way to do this if you do not have a professional refriderant machine is to use the fill hose on the gauge set, completely fill the hose with oil (a syringe works best) and and then connect the refriderant. Bleed off any air at the gauge valve until oil comes out, and then the system will suck in the oil first when the gauge valves are opened, with the refridgerant directly behind it. As long as care is used to bleed off ALL air from the hose and gauge manifold, this method works well. Alternatively oil-adding attachments may be purchased which are less likely to introduce air accidentally.

Things to watch out for: When adding refridgerant to a live system, never add to the high pressure side. If raw liquid gets into the compressor instead of gas, the compressor will be damaged because liquid cannot be compressed.
When measuring a live system with a gauge set, never open the valve to the high side between the gauges. This will create a second pathway for high pressure from the compressor, and unbalance the system. Parts could be damaged or destroyed if the pressure is too high in areas where it was not intended to be so.
When working on a refridgerant system, always go slowly and carefully to ensure no air is accidentally allowed in. Any air content will unbalance the refridgerant and cause less efficient cooling.