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Air Conditioning:
A Cool Necessity in a Hot World

Gear case (3)At Karmakanix, we agree with the vast majority of the world’s scientists: Climate change is not a hoax. Every year seems to set a new world record for average temperatures. We also seem to set a record each year for vehicles getting their air conditioning serviced or revived after some years of disuse.

Let’s take a moment to explain the Air Conditioning Cycle for those who might need a little coaching. Automotive air conditioning works on the same principal as refrigerators for houses. First a gas is compressed, then cooled into a liquid. Then when the liquid sprays through a hole and evaporates into a gas, heat is removed from the system. Some liken an air conditioner to a swamp cooler, which cools by evaporating water, but there are major differences. A swamp cooler is adding humidity to the air, whereas an air conditioner is actually removing humidity from the air, hence the puddle of water that forms under a vehicle when idling with the A/C on. Swamp coolers therefore work and feel best in drier environments. Air conditioners work anywhere, and feel even better when the air is East-Coast humid.

Air Conditioning System
Freon is a gas at room temperature, and a liquid when at pressures above around 80 psi at room temperature. A typical A/C system consists of a compressor, a condenser, a receiver/drier, an expansion valve, and an evaporator. Almost all compressors are driven off the crankshaft by a serpentine belt. The pulley driven by the belt houses an electrically activated clutch on the compressor. When engaged, the compressor uses engine power to compress Freon gas up to a high pressure, typically 90 to 160 psi, and much higher in older systems.

The typical air conditioner compressor is has rods and pistons in cylinders that are arranged like parallel pipes set in a circle around the center shaft. The bottom of each rod connects to a swash plate which is attached at an angle to the center shaft. As the center shaft rotates, the rod and pistons rise and fall in the cylinders, driven by the angle of the swash plate (some other car models have a wobble plate). A pair of calibrated reed valves allows each piston to suck in Freon, then push it back out under compression.

Older cars from last century, have the swash plate at a fixed angle, and thus pump the same amount of Freon with each turn of the shaft. Newer cars have a variable swash plate that changes angle so it can vary the pressure and volume of Freon pumped according to the system requirements. These modern A/C compressors are called Variable Displacement Compressors.

The Freon is now a high pressure / high temperature gas. The condenser in front of the radiator uses air movement from driving and/or the radiator fan to condense the Freon into a liquid, which is now a high pressure / high temperature liquid. The temperature of the Freon drops while going through the condenser, but the liquid Freon is still much hotter than outside air temperature. The Freon then goes to a receiver/drier which has a filter and a desiccant bag which remove any debris and water.

The high pressure / high temperature liquid now sprays through the expansion valve into the evaporator. The expansion valve has a tiny adjustable orifice which is the hole through which the Freon sprays. The evaporator is built somewhat like a radiator. It has long folded tubes with many metal vanes to transfer heat rapidly. The orifice may change size according to evaporator temperature, or in some systems, the orifice is a fixed size. As liquid sprays through the expansion valve orifice, the pressure drops, and it evaporates into a low pressure / low temperature gas. The evaporation process removes calories of heat from the evaporator; i.e. cools the evaporator.

A blower fan forces air across the evaporator and into the cabin of the vehicle. In older vehicles from last century, once the evaporator cools down to the temperature set by the thermostat on the dash, the clutch shuts off the compressor. When the evaporator warms back up, the clutch is turned back on. In newer vehicles, the clutch only engages once, then the swash plate varies its angle to the center shaft to change how far the pistons move up and down. The compressor runs continuously, and the amount of Freon pumped is varied to keep the evaporator at a constant temperature.

Variable Displacement Air Conditioner Compressor

Variable Displacement Compressors

A YouTube Video by Valeo which helps visualize how a variable displacement compressor works. You may want to mute the music.

Basic Air Conditioning Diagnostics

A quick touch on the pipes / hoses at the compressor will tell one if the system working. The high side should be about boiling hot, and low side should be freezing cold. Older air conditioners that did not have variable displacement compressors would run pressures between 20 psi to 35 psi on the low side, and 150 psi to 250 psi on the high side whenever the compressor was turned on with the engine idling. Generally raising the rpms will make the low side go down and the high side go up. A modern system with a variable displacement compressor has completely different pressures. The best generalization is that the difference between the pressures starts out high, then gets more equal as the system cools down. There will always be some difference between them, usually at least 10 psi to 25 psi. At very high heat loads, the pressures will resemble an older system, but will quickly equalize.

Just some advice about temperatures and pressures. Modern systems with variable displacement compressors can have wildly varying normal pressure gradients from car to car. And from day to day. The differences between systems, the changes in the amount of heat that needs to be removed from a vehicle’s cabin, and the changes in temperature and the humidity of each day will make pressures vary around the scale. Most of the information about A/C pressures currently found on the internet deals with older systems, and is completely inaccurate and useless with modern A/C systems.

Almost every car air conditioning system seeps some Freon. Just a handful will make it 10 years without needing a recharge. Most systems need the Freon recharged every 5 to 8 years. The main reason is that the compressor shaft has a seal between the Freon and the outside world, and it is the nature of a seal to seep very slowly to allow lubrication of the seal. All of the oil seals in an engine seep slowly, hopefully so slowly as to not leave even a visible trace. As a gas, Freon is much more capable of getting past a seal, as evidenced by the damp clutch face of many older compressors. A/C systems have a few ounces or more of lubricating oil, and a tiny bit of oil comes out wherever the Freon seeps. A long term Freon seep or leak will likely leave some damp trace.

When contained, Freon has a normal pressure in psi just slightly higher than the degrees of ambient temperature in Fahrenheit. When the level of Freon drops near empty, the pressure drops until the damp ambient air can get back in wherever the Freon came out, especially when cooling off at night after the engine gets everything hot. Corrosion begins where the system is the coldest, in the evaporator. Evaporators are built like radiators, many hundreds of tiny internal passageways. Even slight corrosion starts to choke off and insulate the walls of the evaporator, and the system does not cool as well as designed. Normal A/C outlet temperatures are between 38 °F and 42 °F.

Some cars have air conditioner compressors that have no clutch, and are permanently engaged. All our models have a clutch for two main reasons. First, should a compressor fail drastically and there was no way to disconnect it, the car could not be driven. And if the Freon simply went low, the compressor would overheat. Secondly, if one is trying to be as efficient as possible, lowering the compressor load is not as good as shutting it off completely.

Some other cars have compressors in which the swash plate angle is automatically controlled by the internal compressor pressures. All our models have electronic control of the swash plate angle by varying the on/off ratio of the voltage to an actuator. That form of voltage is called a duty cycle, or PWM signal, meaning Pulse Width Modified signal. Electronic control allows for a fast and accurate response to temperature changes, faster cool down times, and the ability to barely idle the compressor just to keep the oil lubricating the seals, pistons and valves.

The evaporator of a working A/C system will generate some water as it pulls moisture out of the air, and that water should drip out under the vehicle after just a few minutes of running. Higher humidity air will yield more water, and less in desert conditions. No water dripping out means that either the drains are clogged, which is rare, or the system is not working.

All of the A/C systems in our models can vary the actual outlet temperature using a blend door to mix in a bit of hot air from the heater core to adjust the temperature upwards. When the system is used for defrosting the windshield in the winter, it is the dry aspect of the air that contributes to defogging more than the actual temperature. That said, if one finds that the outlet temperature is not low enough, first check the temperature setting on the controls. If the controls are already at the lowest setting, yet the temperature feels warm, it is possible that the blend door is stuck open or is leaking.

Automatic climate control introduces other variables. For the most part, they have proven reliable, but with tiny motors and potentiometers, temperature sensors and interfaces to other control units, the odd problem can occur. The challenge can be the diagnostics of a modern climate control system gone awry. The systems have diagnostic trouble codes (DTC’s) just like other systems, and like other systems, the DTC’s are symptoms to aid in diagnosis, and do not necessarily provide a firm answer as to the problem. This is not a system where your average service station can tackle a problem.

Occasionally we get an A/C system that is not cooling well caused by incorrect adaptations. Pretty much always, the issue has been the result of trying to start the car when the battery was dying. The A/C was in the process of self-tests when the voltage dropped, and some or all of the flap motor position potentiometers have large differences between the actual values and specified values. Yeah, sounds complicated. And it is. But the fix is fairly simple.

R-12 Refrigerant: Systems Up To 1992

This article refers to older systems from last century vs. modern as a way of delineating the use of variable displacement compressors. Another time line needs to be mentioned, as all vehicles made up through 1992 originally had R-12 refrigerant. R-12 is the CFC or Chlorofluorocarbon molecule that severely damaged and diminished the Ozone Layer. Yeah, you knew that, but did you know that 1 kilogram of R-12 had the same greenhouse gas potential as 4000 tons of CO2?

Many industries used R-12. One local bakery was leaking 2.5 to 5 kilograms of R-12 per DAY!. By the way, up through the 1980’s nobody had any Freon recovery machines. Even at dealerships, we drained Freon into a bucket of water just so we could see how much veggie oil we had lost from the system. The R-12 Freon headed for the sky. Our bad. And antifreeze went to feed fish. More bad. Companies would pay us for used motor oil, cuz Cheep Oil was all recycled oil. But lots of used oil ended up lubricating roots. Our very bad.

Initially, the reftrofit of R-12 to R-134a was complicated. All the O rings got replaced. All the old veggie based oil was cleaned out and PAG oil was installed. The clean out required lots of disassembly and cleaning. Typically a retrofit took 3 to 5 hours, longer yet on some vehicles like Vanagons. The new R-134a did not work as well in the older systems, loosing 10 to 20% of the cooling efficiency.

The result was that many clients chose to pay the ever increasing value of R-12 to service their older systems. That resulted in bogus R-12 gases being sold, from a variety of sources, mostly Mexico. The brew could vary intensely. At best, a shop’s A/C recovery machine would get contaminated. At worst, some fake R-12 had large amounts of butane, and a few cars exploded. Now that got some attention. The cost of real R-12 skyrocketed.

Around the turn of the century, someone sorted out that one really did not need to change all the O rings and clean out all the veggie oil. It is true that the new PAG oil might turn to cottage-cheese-like curds when mixed with the older veggie oil in the presence of R-12, but when R-134a was used, no harm, no foul, game on. So the cost of retrofit dropped radically. Between the lower cost and the FBI, the South-of-the-Border R-12 brewers went out of business. Very soon, R-12 became a thing of the past.

The Shade Tree DIY Recharge

It is pretty much 98.6% of the Do It Yourself Backyard Recharges that go wrong. The biggest deal is the no brainer that pretty much everyone misses. If you are doing a A/C system top off without evacuating all the Freon and pulling a high vacuum with a pump, then you will be adding at least some small amount of air to the system. Even trying to bleed the hose, some air is going to get into the system. And that air has water in it. How much air is too much air? Any amount at all. That’s why we draw over 30″ Hg of vacuum before charging. And we leave it under vacuum for a while to remove any moisture. Water boils in a high vacuum. Keep reading.

If a person gets a true A/C vacuum pump, an A/C manifold gauge set, an accurate scale, some safety glasses (!!!), and a pile of training, then they have a chance of recharging their A/C system correctly. An electronic leak detector is not mandatory, but without one there is no way of knowing that all that work will go bad within months, weeks, days….. Anybody just trying to add Freon is more likely to cause more harm than good. The reason for the emphasis on safety glasses is that if liquid Freon hits you in the eye, you WILL loose your vision in that eye. Liquid Freon WILL freeze the lens in that eye in a New York second. Keep reading.

The Freon needs to be measured in accurately to within 10%. The following numbers and percentages are approximations, because there are so many variables involved. A system at 90% will work normally, the compressor temperature will be a tad higher in theory, but not measurably, and no harm happens. A system at 80% will cool to 44 °F to 46 °F (normal is 38 °F to 42 °F). The compressor will run hotter, but still no real chance of damage. A system at 70% will cool down to around 48 °F, but may not start cooling until the engine rpms are raised above idle. The compressor running temperature will be enough higher to be at some risk of long term failure. A hot day may yield insufficient cooling. A system at 60% will hardly cool at all. The compressor is definitely at risk of overheat. As mentioned, these figures are not exact, but you get the point. The A/C system goes from poor cooling to almost no cooling really rapidly. And a good way to ruin the compressor is to run the system when it is not cooling well.

Now let’s go the other way, as most DIY charges end up way overcharged. Older systems that did not have variable vane compressors would run higher and higher pressures when overcharged, likely blowing a pressure plug or breaking the condenser at about 150% overcharged. Modern variable vane compressors will just lower the swash plate angle to reduce the unnecessarily high pressure. But the day one starts up the car with the cabin really hot and the compressor gets the signal to run full bore for a while is the day that the pressure goes through the roof and something is gonna blow like a blue whale. Remember that the crazy parts store Add-Yer-Own gauge connects to the low side only. You cannot see the magnificent pressures developing on the high side. And if you own a car from this century with a variable displacement compressor, the low side pressure is going to vary, and you will have no idea what the charge level could be.

If the system had gotten close to empty, then air got in it. That air has moisture, way more than the desiccant bag is designed to deal with. Now that the system has some Freon, it will run, and the moisture will wreck that system completely. At least when the pressure was too low, the system would not run, and the air was just slowly damaging it. A system that started with no pressure and just got filled will be ruined within days, and will never blow cold again. If you are getting 50 degree air, you will never see colder air again. And every mechanic is going to waste his time and your money, because there really is not any way to see the corrosion inside the system that is building up and preventing it from cooling properly. As the corrosion builds up, the expansion valve orifice will clog, maybe completely, and the outlet temperature will slowly rise. At that point, the only fix is a new car. A system that was down to 20-30 psi of pressure when the DIYer tried to charge it, or just got a hose full of air added to it, will suffer the same fate, just over the period of months or years instead of days.

Gear case (2)Karmakanix Knowledgebase on Vanagon Air Conditioning Revival

Gear case (3)