Air Conditioning and Heating System


Not only do we depend on our cars to get us where we want to go, we also depend on them to get us there without discomfort. We expect the heater to keep us warm when it’s cold outside, and the air conditioning system to keep us cool when it’s hot.

We get heat from the heater core, sort of a secondary radiator, which is part of the car’s cooling system. We get air conditioning from the car’s elaborate air conditioning system.

Despite its relatively small size, the cooling system has to deal with an enormous amount of heat to protect the engine from friction and the heat of combustion. The cooling system has to remove about 6,000 BTU of heat per minute. This is a lot more heat than we need to heat a large home in cold weather. It’s good to know that some of this heat can be put to the useful purpose of keeping us warm.

Air conditioning makes driving much more comfortable in hot weather. Your car’s air conditioner cleans and dehumidifies (removes excess moisture), the outside air entering your car. It also has the task of keeping the air at the temperature you select. These are all big jobs. How do our cars keep our “riding environment” the way we like it?

Most people think the air conditioning system’s job is to add “cold” air to the interior of the car. Actually, there is no such thing as “cold,” just an absence of heat, or less heat than our bodies are comfortable with. The job of the air conditioning system is really to “remove” the heat that makes us uncomfortable, and return the air to the car’s interior in a “un-heated” condition. Air conditioning, or cooling, is really a process of removing heat from an object (like air).

A compressor circulates a liquid refrigerant called Refrigerant-12 (we tend to call it “Freon,” a trade name, the way we call copy machines “Xerox” machines). The compressor moves the Refrigerant-12 from an evaporator, through a condenser and expansion valve, right back to the evaporator. The evaporator is right in front of a fan that pulls the hot, humid air out of the car’s interior. The refrigerant makes the hot air’s moisture condense into drops of water, removing the heat from the air. Once the water is removed, the “cool” air is sent back into the car’s interior. Aaaaaah! Much better.

Sometimes we worry when we catch our car making a water puddle on the ground, but are relieved to discover that it’s only water dripping from the air conditioning system’s condenser (no color, no smell, and it dries!).

Note: Refrigerant-12 is extremely dangerous. Many special precautions must be taken when it is present. It can freeze whatever it contacts (including your eyes), it is heavier than air and can suffocate you, and it produces a poisonous gas when it comes in contact with an open flame.

Dash Controls

Most or all of the control panel of your car is located on the dashboard behind the steering wheel. Sometimes it extends onto the car’s console, between the two front seats, and onto your steering column. Little duplicate fragments of the control panel are scattered around the interior of your vehicle, such as automatic door locks, extra light switches, etc.

The dash controls enable you to operate your headlights, turn signals, horn, windshield wipers, heater, defroster, air conditioning, radio, etc. All of the vehicle’s controls should be within the reach of the driver.

The control panel also contains all of your gauges; gas, temperature, tachometer, etc. These enable you to monitor the operating conditions of your engine and charging system, fuel level, oil pressure and coolant temperature. Warning lights come on to alert you to dangerous coolant temperatures, or loss of oil pressure.

In 1924 the Nash Co. introduced the electric clock as an accessory.


A relay is an electromagnetic device in which contacts are made and subsequently broken. An example of this would be your car’s horn.

By natural law, the farther electrical current travels, the lower its voltage becomes. Your car horn has to be connected to the car battery in order to sound. The shortest distance between two points is a wire connecting your horn to your battery. The only problem with this arrangement is that connecting the two would give you a permanent horn blast when you turned the key in your ignition.

This is an unacceptable arrangement, so a relay is included in the connection. The relay stops the horn from sounding until you activate the relay by pressing the horn. The relay then allows the horn to connect to the battery, or complete the circuit, and it sounds. As soon as you stop pressing the horn, the relay breaks the connection, or circuit, and presto– no more horn!

Relays, with switches, are used for most of the equipment that depends on the battery for an energy source. This includes headlights, taillights, radio, etc.

In 1922, a Model T was the first car equipped with a radio. In 1927, the first commercially produced car radio came on the scene.

A “relay” is any switching device operated by a low current circuit that controls opening and closing of another circuit of high current capacity. The purpose of the “cutout relay” is to prevent the battery from discharging through the generator when the engine is stopped or turning over slowly. A “field relay” connects the alternator field windings and voltage regulator windings directly to the battery.

Heater/AC Blower Motor

The blower motor is the motor that turns the electric fan in an air conditioning or heating system.

Air Ducts

The air ducts control the passage of hot or cold air into the interior of the car. They are operated by a control on the dash, either manually or automatically.


Most air conditioning/heating systems have three possible air settings. One is to recirculate the air that is in the car, a second is to use only air from the outside of the car, and a third is to mix some of the outside air with the air recirculating inside the car.

Low Pressure Line

The low pressure line is a hose, or tube containing refrigerant that connects the evaporator to the air conditioning system’s compressor. The compressor draws the low pressure refrigerant from the evaporator in through the low pressure line in order to compress it.

High Pressure Line

The high pressure line is a hose, or tube containing refrigerant that connects the air conditioning system’s compressor to the condenser. The compressor forces the compressed refrigerant into the condenser through the high pressure line.

The Compressor


The compressor used to air condition your car works in a similar way to the one in the refrigerator in your kitchen. The job of the compressor is to move liquid refrigerant around in a pipe. The compressor pumps, or forces, the liquid from the evaporator into a condenser and expansion valve, and then back to the evaporator.

There are three common types of compressors:

Two cylinder reciprocating piston type
Four cylinder RADIAL type
Six cylinder AXIAL type
The engine drives the compressor with a belt. In action, the compressor takes the low pressure refrigerant from the evaporator and compresses it according to speed and air temperature. The inlet side is known as the low (pressure) side and the outlet side is known as the high (pressure) side.

The compressor compresses the refrigerant, and raises its temperature higher than that of the surrounding air. Then, the compressor forces the refrigerant into the condenser.

Compressor Clutch

The air conditioning compressor has an electromagnetic clutch that can engage or disengage the compressor pulley. The compressor pulley always turns when the engine is running, but the compressor only runs when the pulley is engaged to the compressor driving shaft.

When this system is activated, current runs through the electromagnetic coil. The current attracts it to the armature plate. The strong magnetic pull draws the armature plate against the side of the turning pulley. This locks the pulley and the armature plate together; the armature plate drives the compressor.

When the system is deactivated, and current stops running through the electromagnetic coil, flat springs pull the armature plate away from the pulley.

The magnetic coil does not turn since its magnetism is transmitted through the pulley to the armature. The armature plate and hub assembly are fastened to the compressor drive shaft. When it’s not driving the compressor, the clutch pulley turns on a double row of ball bearings.

AC Compressor Drive Ring

Inside the air conditioner’s refrigerant compressor is a drive ring made of a friction material that is mounted to both sides of the “swash” or “wobble” plate. As the swash plate rotates, the friction material pushes the ball bearings (mounted to the pistons) back and forth.

The Condenser

The condenser is a long tube that goes back and forth through a multitude of cooling fins, quite similar to the evaporator in structure. The condenser is mounted in front of the radiator to take advantage of the forced air provided by the fan and the motion of the car.

As the highly pressurized refrigerant (vapor) flows into the condenser, it gives off heat and warms the condenser. This causes the condenser to be hotter than the forced air coming through the condenser. The condenser hands its heat off to the forced air and turns the refrigerant back into cool liquid in the expansion valve, where it heads back to the evaporator.

The Evaporator

The evaporator is a long tube, or coil, that goes back and forth through a multitude of cooling fins. It is quite similar to the condenser in structure.

The refrigerant is a liquid when it enters the evaporator. A fan blows warm air over the evaporator. The warm air causes the liquid refrigerant to boil. This means that it absorbs the heat from the warm air. Once it has absorbed the heat from the warm air, the warm air isn’t warm anymore. The same blower that blows the warm air (that is now “cool” air) over the evaporator, keeps on blowing it into the interior of your car, and you have — air conditioning!

The evaporator also removes the moisture from the air coming through its fins and turns it into water. The water just drains off.

The temperature of the evaporator coil can go from 33 degrees F to 0 degrees F. If it goes below 32 degrees F, the moisture that’s supposed to drain off the coils will freeze. This makes for a very (surprise!) inefficient system, so a thermostatic switch is used to connect and disconnect it to the compressor as necessary.

Expansion Valve

The expansion valve determines the correct amount of refrigerant going into the evaporator, and it lowers the pressure of the refrigerant.

When the compressor starts, the expansion valve opens and the liquid refrigerant flows through a strainer in the high pressure liquid inlet. Once in the expansion valve, the refrigerant is correctly pressurized. As the evaporator calls for more refrigerant, the expansion valve allows the required amount of low pressure liquid refrigerant into the coils.

The expansion valve maintains the delicate balance between the heat load and the cooling efficiency of the evaporator.

Discharge/Suction Service Valves

Discharge and suction service valves allow the air conditioning system to be emptied and filled. These valves also provide places where the system can be checked with pressure gauges.

Note: Some systems use a Schrader valve in place of the discharge and suction valves. This is a spring-loaded valve which looks rather like the valve in a tire.

The Compressor Relay

A capillary tube from a cycling switch lets the switch know what the temperature is in the evaporator. This switch turns the compressor on and off to keep the evaporator temperature at about 32 to 45 degrees F. The relay switch keeps moisture from freezing on the evaporator core.

Electric Air Conditioning Fan

Sometimes an extra electric fan is placed in front of the condenser to provide an extra flow of air during warm weather, or for times when the car has to idle for a long time. You activate and deactivate the air conditioning fan when you turn it on and off at the control panel.

Compressor Belt

The compressor is engine driven by a belt on the front of the crankshaft.

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