Before we get started, you need an EPA 608 (commercial, residential and marine systems) or 609 (automotive) license to purchase refrigerant and/or work on an air condioning system with the exception of 134a cans less than 2 pounds, working on an automotive air conditioner, not for pay. That said the penalties for intentionally venting refrigerant into the atmosphere is a federal offense for everybody, that carries up to a $38k fine per day per incident and potentially criminal prosecution of up to 5 years or more in prison. There are numerous examples of criminal prosection. So while we are here to learn about our air conditioners and how to repair them, be aware of the law.

With that out of the way, lets get to work.

Air conditioning systems take advantage of the ideal gas law which describes the behavior of gases related to pressure temperature and volume. The formula is PV=nRT. Removing the constants and we have PV=T. If you change pressure, volume or temperature, the other variables are affected. Specifically, if you compress a gas in a constant volume, it gets hotter. If you then lower the pressure by increasing the volume it gets colder. When you spray compressed gas out of a can to clean a keyboard and the can gets cold, thats the process. The other area of physics that is exploited is that refrigerants can change phase from gas to liquid and back under the right circumstances. Air conditioning systems take advantage of this by constantly compressing a gas in one half of the system and then lowering the pressure in the other side of the system. Refrigerants are selected for there temperature behavior under changing pressure and volume conditions. The original gold standard for auto AC was R12 but that was phased out and went to 134a after the Montreal Protocol which attempted to reduce damage to the ozone layer. More changes are coming. Residential systems have been through a similar change with R22>R410a>R32> ...

So refrigerant flows in a circle around the refrigerant circuit by first getting the gas phase compressed by the compressor which results in a hot, high pressure gas, which then passes through the condensor which is a small radiator at the front of the vehicle which lowers the temperature and causes the hot gas to become a less hot liquid. That liquid makes it to the metering device which is usually a TXV valve (more sophisticated) or a fixed metering orifice (like our blazers) which allows the pressure to drop across the metering device which in turn causes the liquid to turn into a gas and absorb the surrounding heat (energy) to accomplish the phase change. That happens in the evaporator coil under the dash where a fan blows cabin air over the cold coil. Then the less cold gas returns to the compressor in the engine compartment to start the cycle over (diagram attached).




There are a few other components in the refrigerant circuit:

1. An accumulator which is the large silver metal cannister before the evaporator coil. This makes sure that liquid refrigerant never makes it back to the compressor because that can "slug" the compressor and destroy it. Not enough gas and you dont cool the compressor. This is why charge level is important. That cannister also has a descicant in it to absorb moisture which would otherwise combine with the refrigerant and oil to form a destructive acid.
2. A receiver which is a cannister in the high side of the system of keep a resevior of liquid refrigerant available so that under a very high heat load solid liquid refrigerant is alway presented to the metering device. We dont have this component, its more likely to be found in a TXV metering system.
3. A high pressure switch on the back of the compressor to turn it off if the high side pressure exceeds safe levels
4. A low pressure switch at the accumulator to turn of the compressor if the low side pressure drop below around 30 psi.
5. The control unit in the cabin that energizes the compressor, sets the temperature, positions the blend door and the recycle door.
6. The gauge ports

Refrigerant in a closed container or system will have a pressure associated with that ambient temperature. At 85F 134a is around 95 psi. So whether the gas is in the can or your static (turned off) AC system a gauge should read that pressure (chart attched).




When the system starts, the low (suction) side pressure starts to drop and the high (liquid line) pressure starts to rise. Depending on ambient temperature and humidity, the condition of the condensor/evap coils, the compressor and metering device, the system pressure at that same 85F ambient temp should be around 45-55 psi on the low side and 225-250 psi on the high side (chart attached).




Spray some water from a garden hose on the condensor while the system is running and you will see the pressures change which shows you why there is no single pressure target. Proper diagnosis of a system (to spot say a restricted meteriing orifice) requires a full set of gauges to accurately monitor both high and low pressure but those crude low side gauges on the auto part store cans can get you close enough along with center vent temp in many cases (the secret weapon of AC work). These systems are "critically charged" meaning that when commissioned or repaired you place a small fixed weight of charge into the system for optimal performance, say 1.5 pounds of 134a as apposed to say a residential system that takes 9 pounds. A great many systems get overcharged when the shade tree mechanic is chasing system gremlins unrelated to charge level. More charge does not equal better cooling at that point and will eventually destroy the compressor.

So if your not cooling right lets look at the fix:

1. Get a gauge on at least the low side port. If you have very low or no pressure you have a major leak and the system needs to be repaired.
2. If you see around 90 psi but the compressor wont start then short the low side switch near the accumulator with a paper clip in the connector temporarily. If the compressor starts then replace the switch which can be done safely (in theory, be careful) because there is a shrader valve on that port.
3. If shorting that switch brings no joy then check for voltage at the compressor connector. If so then the high side switch or clutch is bad. If not then the control head, wiring, or relay are bad.
4. If the system starts but then starts to cycle then watch the low side pressure. If it drops down to 30 ish or out of the green then start adding short bursts of gas (can upright, vapor only) and wait 10 min to see the pressure and watch the center vent temp on high fan speed. In most cases, the system has to be running to get all the gas into the system. If the can is freezing cold and still has liquid refrigerant in it, place the can in warm water if you need to get more 134a into the system. When you start getting normal pressures and the center vent temp approaches 40F (after running for a while and the cabin is cooling down) then your there. When you hook up to any AC system you have to make sure that the lines are purged of non-condensables (air and water vapor) because that causes poor performance or system failure. You are allowed to vent a very small amount of gas to purge the lines ( a de-minimus venting) but not more,
5. If you have good pressures but the cooling is poor, clean the condensor coil with a garden hose and inspect the evap coil with a video boroscope or inspection port (careful) to look for leaves, dirt and mold.
6. If still no joy then make sure the recycle and blend doors are working properly. If not look at vacuum, actuators, power, ground and wiring or lastly the control head
7. If you dont have all the fan speeds look at the fan motor, wiring, relay and resistor network

Be careful handling refrigerent. You can get hurt from the freezing temps and high pressure, wear eye protection and gloves

You have to wait for the system to bring the cabin humidity and temp down before drawing any final conclusions about the charge. When first started the system removes humidity (latent heat of vaporization) and then lowers temperature (sensible heat). When a healthy system is first started with high ambient and a hot cabin the system will probably run full tilt but as you reach lower humidity and the temp set point the load on the system drops and so do the pressures. Thats why a healthy system may cycle after it runs for a while or in cold weather, this is normal.

That will get you started with basic issues. I will cover more advanced issues which require major repairs, more skill and more equipment in part 2.

George

 

What's a good way to check the temperature of the air coming out of the vents? Aim the gun at the directional vanes? How long at idle should it take to get the temp down to the minimum? What is the minimum of a healthy Blazer when ambient is, say, 85 deg, 80 humidity?
Thanks

 

This is the old standby. With the fan on high and dash vents only your close in about 10 min. The very best the system can do is after it’s comfortable inside.


George