Basic Science

First point of note is that there is no such thing as “cold.” Cold is simply the absence of heat energy. Just like darkness is the absence of light energy. Heat wants to spread out evenly and equally so it transfers quickly from an area with lots of heat to an area with less heat. Second point of note is that energy can not be created or destroyed, but it can be moved around or changed into another form of energy. All the A/C system does is is absorb as much heat as we can in the evaporator and dump it in the condenser. We all know that substances can exist in 3 states: solid, liquid and vapour/gas. What we are most concerned about in A/C systems is the change between liquid and vapour. Most people know that changing the state of water can be accomplished by changing its temperature. Water freezes at 0°C (32°F) and evaporates at 100°C (212°F). Another way to change a substances state is with pressure (through latent heat*). We use this principle in modern pressurized engine cooling systems. We keep the entire cooling system pressurized to keep the coolant from evaporating, even when the coolant temperature is above its normal boiling point. When you force a change in state using pressure instead of temperature, you force the substance to give off its heat or absorb heat.

*Latent heat: heat that can not be measured with a thermometer but can cause a substance to change state.
Sensible heat: heat that can be measured with a thermometer

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Basic A/C Operation

If all we did is pump some cooled water into the HVAC airflow it would cool the air a little bit, but nowhere near as well as modern A/C systems. This extra cooling power is caused by forcing the refrigerant to change from a liquid to a vapour inside the evaporator and from a vapour back to a liquid in the condenser. Causing the refrigerant to change from a liquid into a vapour in the evaporator makes the refrigerant absorb (close to) as much heat as is can, and causing the vapour to turn back into a liquid in the condenser forces the refrigerant to give off much of its heat. This cycle repeats over and over again. Absorbing heat in the evaporator and dumping it in the condenser.

ac system

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Refrigerant

A good refrigerant has a low boiling point. The lower the boiling point the better an A/C system will move heat. R12 has been discontinued because it is a CFC. (chlorofluorocarbon) If released into the atmosphere, it causes thinning of the ozone. It was phased out in late 1995/early 1996 and was replaced by R134a. R134a is considered a HFC (hydro-fluorocarbon) and is considered to be less harmful to the ozone if released into the atmosphere. Most refrigerant has a dye in it that will glow when exposed to black light. This is to help find leaks. R12 has a lower boiling point than R134a, if your old R12 system has a problem and needs to be opened you will most likely need to do a R134a retrofit. (although some guys still have a secret stash of R12 but they can pretty well charge you whatever they want for it)

R12- Boiling Point: -29.8°C (-22°F)
R134a- Boiling Point: -26.3°C (-15°F)
Propane- Boiling Point: -42°C (-44°F)

As you can see out of these three propane would make the best refrigerant as far as cooling power goes, but the reason we don’t use propane or any other highly explosive substance is because of the risk of an explosion or fire in a front end collision.

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Refrigerant Oils

R12: Mineral oil or ester oil
R134a: PAG oil or ester oil (unless otherwise specified, do not lube o-rings with PAG oil)
Retrofit R134a: Ester oil (ester oil is compatible with both systems and is acceptable for use in either)

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Humidity

Humidity is the enemy to an A/C system. Humidity is moisture in the air. When air passes through the evaporator, most of the airs heat is absorbed. Cold air can not hold as much moisture as warm air so water droplets build up on the evaporator. If these droplets drop below 0°C (32°F) they will freeze. This causes a barrier between the evaporator and the air flow through the HVAC system. It could also plug airflow completely through the evaporator. The evaporator must be kept just above freezing for the A/C system to work properly. The water droplets must also be dealt with. HVAC systems are equipped with a drain below the evaporator that allows water droplets to drain out onto the road. If this drain gets plugged, you may notice a sound of water sloshing around when you take a corner or brake/accelerate quickly. You may also notice an “old socks” smell. This is bacteria living in the water in your HVAC system. To get rid of this you need to unblock the A/C drain, (watch out for a shower) and spray an evaporator cleaner into the system to fully eliminate the issue. Just Google “a/c evaporator cleaning spray” and you should find everything you need.

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Types of A/C Systems

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CCOT Systems

CCOT stands for Clutch Cycling Orifice Tube. This system is found on most low to middle class vehicles. This system has a fixed rate orifice tube and cycles the A/C clutch on the compressor to regulate pressures and refrigerant flow, as well as making sure the evaporator doesn’t ice up. The easy way to see if you have a CCOT system is the location of the accumulator. An accumulator is between the evaporator and the compressor. You might also feel the compressor kicking in and out during A/C operation.

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TXV Systems

TXV stands for Thermostatic eXpansion Valve and is usually found on higher end cars. This is because TXV operation is much more smooth than CCOT systems. This system has an orifice tube (also called an expansion valve in TXV systems) that varies refrigerant flow based on evaporator temperature. This system runs the compressor any time the A/C is activated as long as the evaporator doesn’t ice up or the system doesn’t detect any other problems. Most TXV systems use a variable rate compressor. The way to check for this system is the location of the receiver-drier, which would be located between the condenser and the orifice tube.

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Pieces of an A/C System

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Compressor

AC compressorCompressors are usually driven by the accessory belt. They have an electromagnetic clutch on the drive pulley so it can drive the compressor or allow the pulley to free-wheel. They create high pressure in the high pressure side, and suction in the low pressure side. They can have pistons inside and operate much like an engine, or they can be a scroll type. Both designs can be “variable displacement.” Variable displacement compressors can vary output based on system demands. Refrigerant must not be in liquid form when it reaches the compressor. Think about it, this piece is called a “compressor,” and as we know liquids can not compress. (well not any amount worth mentioning, especially at the pressures A/C systems deal with) If liquid makes it to the compressor, the car/truck will most likely need a new compressor.

 

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Condenser

AC condenserCondensers cool the refrigerant so it can turn back into a liquid and give off as much heat as possible. They sit in front of the radiator so they get priority over airflow. Many are coated black to help dissipate heat. When the vehicle is not in motion, rad fans generate the airflow necessary for the condenser to dissipate the heat. Although, A/C systems still work better when the vehicle is in motion.

 

 

 

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Rad Fans

AC rad fanMost modern systems have at least one electronic rad fan. Rad fans play a key role in the A/C system. They help to dissipate the heat in the condenser, especially when the vehicle is not in motion. Most newer A/C systems will not turn the compressor on unless it has confirmed that both rad fans are running. If the A/C does come on but the rad fans do not, you may notice the A/C get cold for a bit then warm then cold again and so on. This is because the high pressure side is getting too high. The high pressure switch will notice this and shut the compressor off, even on a TXV system, until normal high side pressure is restored.

 

 

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Orifice Tube

On a TXV system the orifice tube can be called an expansion valve. Think of these like a fuel injector, its job is to “mist” the right amount of refrigerant into the evaporator. These are placed a set distance before the evaporator, to allow vaporization to occur inside the evaporator. The orifice tube separates the high and low pressure sides by providing a restriction in refrigerant flow. These can be identified by the sudden size change in the line to the evaporator, near the firewall.*
*Some orifice tubes/expansion valves may be behind the firewall

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Evaporator

AC evaporator coreThis is where the magic happens. Refrigerant reaches full vaporization and starts absorbing heat. As air flows through the evaporator heat is transferred from the air to the evaporator and then to the refrigerant. Evaporators also act as a dehumidifier. Moisture in the air collects on the evaporator as the cold air can no longer hold the excess moisture. To make sure the water droplets do not ice up the evaporator, most evaporators have a temperature sensor on them to alert the system if the evaporator gets below freezing.

 

 

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Accumulator

AC accumulatorAn accumulator is found on a CCOT system between the evaporator and the compressor. Since a CCOT orifice tube can’t regulate refrigerant flow, an accumulators job is to collect any refrigerant that has not already vaporized to make sure the compressor doesn’t see any liquid refrigerant. It also supplies a small amount of oil to the compressor as well as contains a desiccant which absorbs any moisture in the system. If the system is left open for any extended length of time, this piece should be replaced. The desiccant will try to absorb all the moisture in the atmosphere.

 

 

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Receiver-Drier

AC receiver drierSimilar to an accumulator, but only found on TXV systems and does not supply oil. Made of a more heavy duty construction than an accumulator because they are located on the high pressure side between the condenser and the orifice tube/expansion valve. It houses a desiccant and because of this, the entire unit should be replaced if the system is left open for any lengthy period of time.

 

 

 

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Service Ports

AC service portMost systems have a low side port and a high side port. The low side port is mostly used for filling, although the high side port is very helpful in diagnosing A/C related problems. DO NOT stick something in the service ports to check for pressure, if refrigerant sprays you in the eyes you could go blind. It is also most likely illegal in your area to release R134a into the atmosphere.

 

 

 

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Hoses

The hoses can vary in size and construction. The hoses that carry liquid are smaller and the hoses that carry vapour are bigger because a substance in vapour form takes up more space. Also, the hoses on the high side tend to be more heavy duty because of the extra pressure they have to deal with.

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Ambient Air Temperature Sensor

ambient air temperature sensorThese sensors tell modules what the outside air temperature is by changing its electrical resistance. A/C will not come on when outside air temperatures are below a set limit, or over an extreme heat limit which could make A/C operation dangerous. Most likely NTC. (Negative Temperature Coefficient) Typical range would be -50°C (-58°F) to 100°C (212°F).

 

 

 

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Sun Load Sensor (usually found on fully automatic/electronic HVAC systems only)

A sun load sensor monitors where the sun is in relation to the vehicle and compensates for its heat by cooling that side/area of the car more than others using the blend doors.

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Thermostatic Temperature Switch/Evaporator Temperature Sensor

Temperature sensor placed on the evaporator or evaporator inlet line to monitor evaporator temperature and let the system know when it needs to back off. This prevents the evaporator from getting too cold and icing up.

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Working With A/C Systems

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Most of the following would require an A/C machine

Note: Where I am from, it is illegal for anyone (including a licensed technician) to touch an A/C system with out taking the ODP certification coarse. That includes unbolting and setting to the side a compressor while changing an engine or doing any other work. This page is for your information only, please respect your local laws relating to A/C systems.

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“Re”

Recover- Pulling out the refrigerant
Recycle- Cleaning/filtering the refrigerant
Reuse- Pumping the refrigerant back in the system
Reclaim- Contaminated refrigerant must be sent away so the pure R134a can be separated and reused and the other chemicals can be dealt with safely

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Vacuuming/Evacuating

To do a proper vacuum test you must pull at least 29inHg for 30 minutes
Vacuuming has two purposes:
1- To remove any moisture in the system
2- To check for leaks

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Nitrogen Testing

After a repair is done it is a good idea to nitrogen test the system to check for leaks. Nitrogen testing is simply pumping the system full of nitrogen under pressure. If the system has a leak while testing you may hear a hissing sound. If you can’t hear anything, spray any connections or valves with soapy water and look for bubbles. This is the nitrogen escaping the system. We use nitrogen because it is an inert gas. It is much better that a little harmless nitrogen be released into the atmosphere than R134a.

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Black Light

If the system has UV dye in it, a black light will make the dye glow. Be sure to shine the light all over the entire system, the leak could be anywhere, even on the evaporator under the dash.

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Sniffer Test

A “sniffer” is an electronic device that will beep if it detects refrigerant. Most technicians don’t like these, they tend to be way to sensitive and will beep at almost anything.

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Retrofitting R12 to R134a

Before you do this, make sure it will not hurt the value of your otherwise all original classic car/truck. There are still guys out there with a secret R12 stash, you just need to ask nicely. Also, unless you can do all this yourself, this could get a tad expensive. An A/C delete may be a bit more practical. If you have no other option but to do a retrofit, these are the basic steps that should be taken.

  1. Recover the remaining R12
  2. Remove hoses and pipes from components
  3. Flush refrigerant out of all components
  4. Replace ALL o-rings and install R134a fittings (may need to replace components, retrofit kits are available)
  5. Install an new receiver-drier or accumulator
  6. Vacuum/evacuate: 29inHg for 30 minutes
  7. Nitrogen test, if everything is OK, evacuate again
  8. Use about 8oz of ester oil with dye (unless another amount is specified)
  9. Fill with R134a to 80% of R12 charge (unless another amount is specified)
  10. Check for leaks with a black light
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Basic A/C Diagnosing

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Checking Pressures

To properly check high and low side pressures the engine needs to be at about 1000-1200RPM with the A/C on. The high pressure side should read anywhere between 200-300psi. (this is a general rule, some systems will go higher) The low side should be about 10-30psi. If you have a TXV system the numbers should stay pretty well the same, but if you have a CCOT system the numbers will changes as the compressor clutch cycles on and off. Static pressure is pressure in the A/C system with the A/C compressor off. This should be around 90-130psi. If you get a reading of 0psi or close to it, you have a major leak. Do not recharge the system until the leak has been found and repaired. If you think the A/C isn’t blowing cold enough, measure ambient temperature with a thermometer and then turn the A/C on and measure the temperature at the vent. The A/C system should make vent temperature close to 15°C (30°F) colder than ambient air temperature unless ambient air temperature is already less than 15°C(30°F).

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The following are many of the reasons an A/C compressor would (even temporarily) disengage:

  • Rad fans inop.
  • Ambient temperature too low
  • Pressures too high/low
  • Insufficient refrigerant charge
  • Compressor clutch inop.
  • Convertible top down (don’t laugh, I’ve had customers come in for this)
  • A/C relay/module inop. (if equipped)
  • Interior temperature sensor defective (if equipped)
  • Evaporator too cold
  • Engine running on reserve fuel (fuel light on)
  • TPS showing WOT
  • Engine overheating
  • Engine RPM too high (5500rpm+)
  • Some will disengage while the transmission shifts (automatic only)
  • PCM detects high engine load
  • PCM detects a hard launch
  • PCM detects poor idle quality
  • Compressor getting too hot
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