Knowledge Building: Air Conditioning Systems (July 01, 2003)

Air conditioning is both simple and complex. The basic physics of a system–how it can cool in the first place–is simple. The control systems in place, and the reasons they can fail, are not.

It is surprising to many, though, how few people understand how air conditioning works. Here it is in a nutshell:

Refrigerant, which is really just a substance that can evaporate (turn to vapour form) and condense (return to liquid form), is used to carry heat away from the air entering the passenger compartment. When the air from outside is passed over a radiator-like component called an evaporator, it transfers some of its heat to the refrigerant inside, which causes the refrigerant to evaporate and thereby carry heat away. This is basic physics, and is the same principle that causes you to cool down when you sweat.

Of course, you don’t want to keep adding refrigerant to the system, so it is sealed. The evaporated (or partially evaporated) refrigerant then circulates back around to the front of the vehicle, where it travels through the condensor. Because air is travelling through the condensor, it removes heat from the refrigerant, causing it to return to the liquid state. To get this circulation to occur, a pump is required. In an a/c system this is called the compressor, which takes the refrigerant vapour as it leaves the evaporator, compresses it, and pumps it through the condensor.

To regulate both pressure and flow, systems may have either an expansion valve or an orifice tube. Both control flow to the evaporator.

The refrigerant moves through a receiver-drier (or accumulator), which stores, filters, and dries the liquid refrigerant. A receiver-drier, which is on expansion valve systems (often referred to as thermal expansion valve, or TXV), is located between the condenser and the expansion valve; an accumulator, which is on orifice tube systems, is located between the evaporator and the compressor. There are a number of other controls that regulate when the system can operate, but these are the basic components.

You may have heard technicians talk about “high-side” and “low-side” pressures. This refers to system pressure and is a good way of diagnosing functional problems with a system. The portion of the system where liquid refrigerant resides–downstream of the compressor–is the “high-side” because the refrigerant is under pressure.

The “high-side/low-side” differential is not only a function of the system; it is probably the technician’s most useful tool in diagnosing a problem (after his hands, eyes, and ears, that is).

Using a manifold gauge set–two gauges on a common assembly–a technician can check for a surprising amount of problems. After an external check of the components for damage, and to check that items like blend doors that direct the cold air into the passenger compartment are functioning properly, plus an internal check that the lines and condensor are hot or cold, a tech can use the gauge set to diagnose in more detail.

A normal reading in the gauge set would have the low side pressure at about 15 to 45 psi, and the high side at 115 to 300 psi. If this is the case, the a/c system is working, but the cold air isn’t getting to where it needs to be. The problems can be situated in a mechanical failure–like the aforementioned blend door–or in a control system, which can require a scan tool.

If readings are 10 psi and 70 psi for low and high respectively, there is probably a low refrigerant level. It could be a clogged orifice tube or stuck expansion valve (evidence is frost on the inlet of the valve or tube).

If readings are high–60 to 80 psi on the low side, 350 to 450 on the high side–it is likely an overcharge of refrigerant, but it may take a few minutes to show up. It could be an expansion valve stuck open, though, or air flow restrictions that prevent the condensor from operating properly.

If a reading starts out normal, then drops, there is a restriction on the high pressure side of the system. Think of it like a clogged fuel filter that will allow enough fuel to trickle through to allow a car to start after it sits for a few minutes, but does not feed enough fuel to keep it running properly. With a high-side restriction, the refrigerant eventually gets built up behind the restriction, and starves the compressor for refrigerant.

Commonly, however, a technician will find that both gauges read about the same. The high side is low, the low side is high, which means that the compressor is fried. Hopefully for the technician, and you, it wasn’t one just installed.

If it was, you can expect a comeback. Unless you make sure that the system is properly cleaned, flushed and a filter installed, you can probably expect to get the next one back too.

Special thanks to Delphi Product and Service Solutions and Blue Streak-Hygrade Motor Products for their assistance.

A/C Regulations Vary

While the federal government sets the guidelines for mobile air conditioning service, it is up to the provinces to produce the regulations that cover it. This creates some variation from province to province. An A/C Regulations-at-a-Glance chart is published as part of a special supplement each spring. It is also available at www.autoserviceworld.com. Look in Issues Archives under Jobber News “A/C Special Report.”

Some Failure Causes and Diagnostic Clues

R-12 condensors had large tubes in comparison to those in R-134a systems, which are measured in millimetres. They are easily clogged, the refrigerant can get blocked, flow is reduced, and oil can be prevented from reaching the compressor, which can cause it to fail. Coupled with damaged or blocked condensor flow, no amount of system repair short of condensor replacement will return the system to proper cooling function. Inlet and outlet temperature drop can diagnose this condition: measured four inches off the condensor, 20 degrees F minimum, maximum 50 degrees F from inlet to outlet; at the evaporator, plus or minus 5 degrees F.

On a vehicle where the PCM controls the compressor engagement, check the coolant temperature sensor (CTS) values or check for diagnostic trouble codes (DTC). The computer will not energize the compressor clutch if it is receiving an engine overheat condition.

For an intermittent inoperative A/C complaint associated with a driveability concern, check for wide-open throttle (WOT) DTCs. The purpose of this is to prevent excessive compressor head pressure from developing, resulting in compressor failure or refrigerant loss.

On vehicles that use a TXV for the rear air conditioning, if the A/C compressor has developed a loud knocking noise, check for refrigerant flooding the compressor. A liquid refrigerant flood-back condition may occur through the rear air conditioning system. This is caused by a poor contact between the TXV capillary tube and the rear evaporator outlet tube, which allows the TXV to remain open when the rear system is not in use.