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Ford Air Conditioning
by Jerry Heep

Technical Editors:	William Fox
	Anonymous Ford Contact
	Iatt1234@aol.com (Curt)
	Vic Daku
E&G Editor:	Eric Ditwiler

Theory and Service

If you ever have played around with a bicycle tire pump, you’ve handled the physics that allow us to remain cool in our trucks. Hold your thumb against the air outlet and pump up a tube of air. Put the handle under pressure and feel the bottom of the pump tube. The heat you feel is the compressed air loosing heat to the metal tube. Now release thumb pressure as you decrease the tension on the pump handle. The air coming out of the pump hose is much cooler than the ambient air temperature. This is mechanical air conditioning. A gas is compressed, the heat generated by the compression is extracted and the gas is allowed to expand. The expanding gas is much cooler than the ambient air temperature.

An air conditioner could be made that used plain old air as a working medium. The disadvantages of such a system are fairly numerous. For example, to cool a car – which may need several tons of air conditioning-- the air compressor has to push a mighty volume of air. This compressed air has to be cooled in some manner. When the pressure is released, the much colder air has to be mixed with ambient air to prevent freezing. The components are large. Check out the size of an inter-cooler for a turbo and you see what kind of problems there are in cooling compressed air.

To overcome these disadvantages, modern mechanical air conditioning use gas that can be compressed and cooled into a liquid. Not only is the efficiency increased, but the sizes of the components are also decreased. A modern automotive air conditioner uses R-12, or the newer R-134a gas as a refrigerant.

The six components of the system are:

1. The refrigerant  -- On older trucks this is R-12 (the R is the first letter of “Refrigerant”.) Since our government has determined that automotive R-12 is the majority hole maker in the earth’s ozone layer, we were forced to use the less efficient refrigerant R-134a. Freeze 12 and the other R-12 type of substitutes cannot be purchased by uncertified, shade tree mechanics. Some people have used butane as a refrigerant. I suggest you don’t do that. There are more reliable ways of blowing yourself up.
2. The Compressor – this device is used to compress the gas in the system. Most truck compressors are vane type rotary compressors that are driven by a belt. The compressor is located under the hood, bolted to the front of the motor. Typically, it cost you about seven-horse power to drive the compressor. The compressor has an electrically operated clutch on the input shaft. This clutch allows the pulley to freewheel when the air conditioning compressor is not on. When enabled, the clutch activates and allows the pulley to spin the compressor shaft. There is also lubricant inside the working fluid and compressor to prevent abnormal wear. IN a R-134a system, this lubricant is usually Polyalkylene Glycol, or simply PAG oil. There are two hoses that are connected to the compressor – the high pressure hose on the outlet or P(ressure) side and the low pressure hose on the inlet or S(uction) side. When the compressor is pumping refrigerant, the high pressure fitting on the compressor will be quite HOT. The suction side will be cold. The high-pressure outlet is connected by hose to the Condenser. Usually the manifolds (where the hoses connect) are stamped “P” or “S”.
3. The Condenser – this is the “radiator” that is used to cool the compressed gas. This device is usually mounted forward of the water radiator of our truck. The hot, compressed gas from the compressor is sent to the condenser to cool in the outside air being drawn through it by the fan of our diesel motor. The cooling process changes the state of the refrigerant. The gas state of the refrigerant turns into a liquid state when cooled. Hence the name, “condenser.” It cools/condenses the gas into a liquid. The outlet of the condenser is connected by hose to the Expansion Radiator. By the way: it is somewhere in this line that the expansion valve is located.
4. 4.  The Evaporator -- This small radiator’s task is to cool or to remove the heat from the air that is being blown though it by the electric motor. The temperature change in the refrigerant causes the liquid to “flash” or to change back into a gas. This is the cooling process. The cooling process also dehumidifies the air.  This is a big deal when it is humid and is what causes the dripping water you see sometimes from underneath our trucks. If the drain hose is plugged, and you live in a humid area, this can also cause your air handler to become a water cooler. The picture at the left shows the engine compartment portion of the air handler. The silver tube entering the handler at center right is the low pressure connection to the drier. The open tube at lower center is the low pressure hose connection to the suction side of the compressor.
5. The expansion Valve – this device feeds the liquid refrigerant into the evaporator. This is not really a valve at all – it is an orifice, or restriction to the flow of refrigerant that is located on the “liquid” side of the evaporator. This restriction gives the compressor something to pump against. Some systems (like small refrigerators and window units) just use a capillary (a very small diameter length of copper tubing) as the pressure drop. Without the restriction, there is no pressure drop and therefore no cooling. The valve for my truck is shown to the left.  The red “tab” on the far left of the valve is the part of the valve that you grab to extract from the pipe.
6. The Drier/Accumulator – this device works as a refrigerant reservoir and contains a desiccant. The desiccant removes any moisture that may be present in the working fluid. There is a pressure switch mounted on my drier. This switch is used to turn off the compressor if the pressure is not high enough in the suction line.  Hence, when you are low on refrigerant, this switch will not allow your compressor clutch to engage. The switch is connected to the horizontal ear on the left. The protrusion that you see on the right side is where I install R134A into the system. There is also usually a high-pressure switch somewhere downstream of the compressor, although I could not find one on my truck. It may be too old. But on a modern truck with a high side switch, as the system runs, pressure will build up until the high-pressure switch disengages the clutch.  The pressure will then drop and the clutch will engage, so on and so on. This is why you will notice the compressor cycle as the engine runs (at least once the interior of the vehicle cools some). This system is used instead of thermostatic controls. This switch can also prevent you blowing up a seal if the expansion valve plugs up. My truck doesn’t have one, and yes, my expansion valve plugged up and yes, the compressor seal around the pulley shaft blew out.

Controls

Most trucks have several levers and switches that control the air conditioning and heating function. These controls are:

1. Fan Speed – this is usually a five-position slide or rotary switch. The positions are marked “Off”, “Low” and “High”. The panel in my truck have two dots between the LOW and HIGH text to represent the two intermediate speeds between “gentle sea breeze” and “level 5 hurricane.”
2. AC On – this is an electrical push on, push off button that has a light in the center of the switch. This switch may be a separate switch only on older trucks, like mine. Modern trucks may have this switch integrated into or with the air source or the air flow levers. If you have one, when pushed in, the light is lit and the compressor is allowed to run (the clutch is engaged.) When the button is out, the light is off and the compressor clutch is never turned on. By the way, when this switch is “off” the compressor can still engage. The truck’s electronic controls will “bump” the clutch to rotate the compressor several times. This bumping of the compressor while the air conditioner is not being used keeps the refrigerant moving, and seals from hardening during the long winter months. The bumping may also be used for drying the air and to help defrost the windshield, if the incoming air is not too cold.
3. Air Temperature – this is usually a lever that is connected to an air blend door that controls airflow past the heater core and the evaporator. In older trucks, this leaver may control a valve that controls the flow of hot water through the heater core. In my truck the blend flap is totally mechanical – cable controlled. In new trucks the door may be controlled by an electric motor. There may be a colored label above the length of the lever to graphically show relative temperature. Fully to the left is shown Blue/Cold and fully to the right is shown Red/Hot. An advantage of this system is that defrost air can be first run though the evaporator for drying and then heated by the heater core. Mixing air like this can also be a potential problem. If the air flap jams or becomes inoperative, you loose temperature control. In older trucks, a leaky water valve here can make you think that your air conditioner isn’t working worth a flip. More about this later.
4. Air Flow – this mechanical lever is connected to a system of dampers that route airflow from the air handler to several places around the dash. This lever is usually labeled with text or pictures. The label may show the Panel only, the Floor only, Both Panel and Floor, the slot behind and below the windshield (Defrost), and may even have a setting to allow all areas to receive air.
5. Air Source – this is more than likely a vacuum switch. It directs “motor” vacuum to two vacuum valves. One valve opens a damper and the other closes a damper. When this two-position switch is moved to the far left, this is the Normal or VENT/HEAT position. When moved to the far right, this is the MAXimum or RECIR(culate) position. When we have the air source set to MAX or RECIR position, the expansion radiator is receiving its air from the passenger’s compartment. The air in the truck is being cooled over and over again. If everything is working right, it is getting colder and colder inside. When the switch is set to the NORMAL or VENT position, the motor is drawing outside air into the truck from the grill just forward of the windshield. When that gasser in front of you is belching out nasty gasoline fumes, you can isolate yourself from the outside air source by use of this switch. Another advantage of the RECIRculate position is to allow the air to be dried more.

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