Trucks Of The Month
Michael A. K. Gross, Ph.D.
that it's nearly summer, lots of people are dusting off the old air
conditioner, and in some cases unhappily finding out that it isn't cold.
This article describes operation, basic service, and retrofitting of
automotive air conditioning systems, with an emphasis on those systems
in use in Ford Rangers, especially the older models designed for R-12 (Freon).
why is it (usually) cold?
conditioning provides a fantastic example of the application of nearly
textbook thermodynamics; it depends upon two fundamental mechanisms to
make something cold:
a gas very quickly. You do it quickly so the gas doesn't have time
to absorb head from the surroundings (yet). Almost all gasses get
colder when their pressure is reduced. How much colder is a function
of the atomic structure.
a liquid. How much energy this takes (called the "latent heat
of vaporization") also depends on the material; liquid nitrogen
has very low latent heat -- if you spray it on yourself, it feels a
little cool, even though it's around -200 deg C. Water can feel much
colder (particularly if it is dry outside), even though it might be
room temperature. Most liquids can be made to evaporate by reducing
their pressure alone, to below a "critical" pressure that
is a property of the material and its temperature. A good
refrigerant has a critical pressure at operating temperatures at an
accessible value, generally above about 40 PSI and below 100 PSI.
are other mechanisms, such as some chemical reactions (like those
chemical cold packs you might put on a sprained ankle). But these
consume the refrigerant, so it's not too useful in a vehicle.
Both the mechanisms above are reversible. If you compress the gas back
to its original pressure (quickly, again), you get the original
temperature. If you condense the vapor back into liquid, it also heats
Air conditioning systems take advantage of both of these mechanisms. The
key design point is that they separate the evaporation from
condensation, and the high pressure from low pressure. You put the cold,
low pressure gas, inside the interior, and the hot, high pressure liquid
as far away as possible. Your house air conditioner and your
refrigerator do this, too, and have to move the waste heat away from
what they are trying to keep cold (that's why you can't cool a house
with a refrigerator -- the interior is cold, but the heat taken out of
it is radiated out the back).
now I'll describe a Ford-style automotive air conditioner. Let's start
at the compressor and presume for now it's engaged. The compressor sucks
in low pressure gas-state refrigerant at 25-35 PSI or so, and compresses
it rapidly to well over 100 PSI (for R-12; R-134a uses much higher
pressures). The high pressure gas then moves into the condenser, which
is in front of the radiator, where it gives up much of its heat to the
outside and condenses into a liquid state (giving off even more heat in
the process). It then passes past the high side test port and slams up
against a severe restriction in the line -- the orifice tube. This
critical component lives inside the liquid tube right above the
passenger frame rail in a Ranger, and forms the boundary of the high
side. A relatively small and high speed stream of liquid refrigerant
squirts into the bottom of the evaporator (which on Rangers is barely
outside the firewall on the passenger side -- a blower blows air across
the evaporator into the vents). Because the pressure is much lower past
the orifice tube, the refrigerant quickly evaporates and expands under
the low pressure. In the process, it gets cold. The evaporator absorbs
heat from the interior, goes into a dessicant and oil reservoir (called
the receiver/dryer), picks up some oil and goes back to the compressor.
compressor is cycled on and off by a pressure actuated switch (connected
to the receiver/dryer) to keep the evaporator pressure near 25-35 PSI.
This allows the possibility of a steady (and nonfreezing) temperature;
without it, the air conditioner might get really cold when you rev up
high to get on the freeway. Most computer controlled vehicles override
this switch by computer to disengage the compressor when the throttle is
mashed, so you can get the few horsepower it takes to drive the
compressor to use for accelerating.
automotive air conditioners are similar; the essential differences are
the replacement of an expansion valve for the fixed orifice tube in some
older applications, and the receiver and dryer are not always the same
can go wrong?
most obvious issue is leaking. If the total pressure in the system is
below the 35 or 40 PSI at which the pressure switch turns on, the
pressure switch will never turn on. Leak sites can often be identified
by greasy deposits left behind by the refrigerant oil, but it might be
in a hard to see place, like inside the evaporator housing or behind the
compressor clutch. But there are other things that can go wrong, too.
problems. If the compressor clutch never gets powered, or gets
powered intermittently, you may get no or intermittent
refrigeration. The electrical harness makes its way all the way
across the engine compartment in a Ranger, and is exposed to the
elements, and possibly moving parts or even "critters" or
ham-handed previous owners. After 20 years, they can deteriorate.
Always check for electrical faults first! These are the things you
don't need expensive equipment or EPA certifications to do.
vapor in the system. Generally caused by improper charging, water
vapor doesn't compress nearly as easily as R-12 or R-134a, and it
will remain almost entirely in the vapor state. This leads to
inefficient cooling. Eventually, it will rot the system out from the
inside. Evacuate and recharge.
Generally, these plug the orifice tube, leading to extreme high side
pressures (and this can cause leaks). R-134a systems have a high
pressure switch to turn off the compressor if the pressure is
excessive. R-12 systems blow it out a pressure relief valve in the
compressor. Blockages can be caused by internally disintegrating
parts, such as hoses, the compressor, or as in my case, the
dessicant bag inside the receiver/drier. Circulating corrosion
products may also be an issue for junkyard units, or if the system
has leaked down to atmospheric pressure. Evacuate, flush, evacuate
again, and recharge.
Black Death. Ford FS-6 compressors do not die gracefully. They
slowly grind themselves up and dump the products into the condenser.
This makes a fine sticky black grit that coats everything on the
inside. Replace the compressor, and the new one will fail quickly
because the grit grinds it away. And you can't flush it because it
gets glued to the inside of the condenser at low temperature, and
then comes free again when you heat it back up in operation. The
service manuals say to replace the compressor and the condenser, and
to flush everything else, and put an aftermarket inline filter in
the liquid tube. I think most people would roll down the window or
buy another Ranger (the cost isn't that different for a mid-80s
MY GOD R-12 is expensive! What do I do?
(Freon) production is prohibited around the world by international
treaty, and importation of any quantity into the United States is
prohibited. This means the only source is recycled or old-stock R-12.
The price has and will continue to go up; it's currently about US $60
per can (14 ounces), and you need 42 ounces for a complete charge. Many
folks wax poetic about the days when you could buy it at K-mart for
$5/can. And R-134a isn't much more expensive than that (and you CAN get
it at K-Mart).
R-134a and R-12 are just two fluids, so it ought to be easy to get rid
of one and put the other in, right? There are several kits you can buy
that say as much, but slow down. Remember the "what can go
wrong" list above. In particular, blockages and water
contamination. Every kit I've seen has R-134a with stop-leak already
mixed in. You should NEVER use stop leak. Remember the little orifice
tube discussed above? Well, sometimes stop-leak can see that as a leak.
Oops. There are also lots of small passages in the evaporator (and in
some replacement condensers) that won't like getting plugged up. So,
I'll discuss the "proper" way to do this retrofit here. It
will cost more than $30. If you want to skip steps, do it at your own
risk. You may get lucky; only you can decide if the risk is right for
the refrigerant evacuated by an EPA licensed technician. Sorry, it's
the law (specifically, the Clean Air Act, section 609), and it's
hardly worth the $25,000 fine to get caught dumping refrigerant into
the atmosphere. R-134a is relatively benign, but the regulations
don't distinguish. Some people have recently been sentenced to years
in prison for violating this section, though generally for smuggling
shipments of R-12 from overseas.
the fault that's causing you to do the retrofit; replace any failed
the receiver/dryer with two crescent or open end wrenches, a nut
driver (you may need to remove the airbox and vacuum reservoir to
get at it on some models), and a Ford fuel line tool (Lisle sells a
set for slightly over $5).
the lower (liquid) line from the evaporator, with two crescent
orifice tube is firmly inside the evaporator side. Grab it with a
pliers and pull it straight out. Do not twist it, even slightly, or
it will break.
the system (you may need a shop to do this for you). You can buy
bottles of flush in auto parts stores, but it's difficult to
pressurize it properly at home. This operation removes most of the
remaining mineral oil, as well as contaminants.
the compressor from the vehicle. The fittings can be removed with a
crescent wrench. Turn the compressor on its butt end with the
threaded fittings hanging over a container, and turn the compressor
clutch body until the goop stops coming out. This removes the
remaining oil from the compressor.
all the O-rings. R-12 O-rings will not hold back R-134a, and it's
good practice even if you are refilling with R-12, since they
deteriorate with age. Don't forget the two fat O-rings where the
compressor fittings bolt to the body, and the high side valve insert
(it looks much like a tire stem insert, and can be removed and
installed with the same tool).
four ounces of ester oil into the compressor, with the fittings
pointed up, turn the compressor clutch a bit, then reinstall the
compressor. Hook up both lines.
the new orifice tube into the lower evaporator tube, and install the
liquid line over it.
the covers from the new receiver/drier. They should be airtight; if
they weren't, the drier is full of water and should be returned.
Pour four more ounces of ester oil into the drier, and then install
the drier in the vehicle.
manifold gauges (advisible) to both ports. Open the low side valve,
but leave the high side closed.
the now-sealed system to one inch above barometer pressure (30
inches, minus one inch for each 1000 feet above sea level), and hold
it there for 30 minutes. This will evaporate and suck out any
remaining water in the system. If you don't have a vacuum pump you
can run continuously for 30 minutes, you need a shop to do this.
the system with 80-90% of the weight of R-134a that the blower
housing label calls for. Usually it's around 3 cans. DO NOT fill it
to 100%. Idle the vehicle and turn the air conditioner on. connect a
can to the manifold gauge (or the LOW side port if you don't have
gauges) and unseal the first can. The compressor should start
cycling before the first can is empty. Though you can in principle
fill with the can inverted, which pushes liquid into the system,
it's safer to leave the can right side up and let it go in as a gas.
If you get liquid all the way to the compressor (recall, it's the
next stop after the receiver/drier), you will hydrolock it just like
you can hydrolock your engine when you take your truck for a swim in
a river. Note that filling in gas phase takes a while, and the can
may get covered in frost. You can help it along with a modest source
of heat, such as placing it on the radiator cap. Resist the
temptation to grab a propane torch. You can make the can explode,
and burning R-134a (if there happens to be any leaking) is toxic. A
tip for measuring cans -- a postal scale works wonders. A diet scale
doesnt quite work (it may be 14 ounces of refrigerant, but it's
another 5 or 6 ounces of can).
you have manifold gauges, adjust the pressure switch so that it
cycles on at 25 PSI. There is an adjusting screw between the two
that you will lose some function when retrofitting to R-134a; it just
doesn't have the same thermodynamic properties (latent heat of
vaporization, specific heat, boiling point, operating pressure) that
R-12 does. There are more efficient condensers you can use; rather than
the traditional serpentine tube, these are designed much more like a
radiator. But at $150 or so, it may or may not be worth it to you.
Filling with R-12 may also be an option. You can install all the parts,
but all refrigerant work must be done by a licensed shop. You should
replace the receiver/drier, orifice tube, and probably the O-rings,
There are quite a few shortcuts people commonly make. Every one of them
represents some risk to your system's function or reliability. In some
cases, the risks may not be very large. In others, it may thoroughly
destroy the system, resulting in much greater cost than doing it right.
This article does not advocate any of those shortcuts.
non-A/C vehicle is hot as Hades. Can I retrofit?
first got into this when I drove my Bronco II 300 miles from my home in
the California Central Coast to Mt. Lassen in August. The Sacramento
Valley was over 100 degrees, and all those Bronco II windows make the
vehicle bake. So, I decided enough was enough, and I'd put in an air
There are basically two options when putting an air conditioner into a
vehicle that has never had it before.
are a handful of aftermarket kits available, particularly from Old Air
and Vintage Air. They tend to cost around $1000, but the advantage is
they are designed for R-134a, and all the parts are new. This minimizes
shop labor; all you really need that for is to evacuate prior to
charging (and if you have a vacuum pump at home, you can even do that).
They also tend not to come with compressor brackets, so you still get to
hit the junkyard for that. Some kits mount under the dash and do not
require disassembling the blower housing. Others do.
The option I chose was to hit the junkyard. Virtually any of the Ford
air conditioners available on any Ranger or Bronco II can be made to
work, except for the compressor bracket. When identifying one in the
junkyard, it's best to go to a full service yard and request one the
operator has just discharged. If the system has been exposed to the
elements for long, you are likely to have corrosion problems (and in
extreme cases, the compressor may even be seized). Avoid donor vehicles
that have suffered front end damage; the condenser is often the first
casualty. But people junk Bronco IIs for just stupid reasons these days
(I once found one with LOTS of good parts in Moss Landing, CA where the
only thing I could find wrong with it was a golf ball sized hole in the
fuel tank). Many get junked because of head cracks; this doesn't affect
the air conditioner. You can tell by pulling the dipstick. Oil mixed
with coolant doesn't look at all like oil.
You need the compressor, condenser, three hoses, evaporator (minus the
orifice tube ), short wiring harness, control head, evaporator housing
(not including the resistor pack or blower motor) and relay. On early
models, you don't need any of the ducting; it's all the same. Don't
bother paying for the receiver/drier; it's used up and you need to
replace it anyway (do take the clamp, though). Install the condenser
(with both hoses) and evaporator (it's easy, except perhaps for locating
all the blower housing bolts -- there are a couple under the passenger
side of the dashboard), and then treat it like a repair described in the
previous section. You have the same R-12/R-134a decision to make.
You need the compressor bracket from the correct engine. They don't
your system has low air volume, it's not the fault of the air
conditioner. The evaporator/blower housing may be plugged with
leaves, or the blower motor may not be seeing the full 12-14V power
it needs due to corroded wiring.
not unseal a new receiver/drier until as close as possible to the
any air conditioning work on humid or rainy days. You can fill a
drier with water in 15 minutes.
reuse a receiver/drier. If it has been exposed to the atmosphere
(even by just having all the refrigerant leak out), it isn't
ever, ever, EVER use PAG oil on a system that has had R-12 in it at
any point in its lifetime. It is OK to use it on a completely new
system, with R-134a. PAG and traces of mineral oil react chemically.
Ester oil is compatible and widely available
mixed refrigerants are sold and may be feasible in the near future,
it is illegal to vent them into the atmosphere (as for any other
refrigerant), and it is also illegal to use recovery equipment meant
for other refrigerants on it. This means an A/C shop would have to
maintain dedicated recovery equipment for each mixture, and no one
does. The effect is that you can't service this air conditioner once
you have put any mixture in it. This includes all currently
available "R-12 compatible replacements," such as
use refrigerants intended for household air conditioners or
refrigerators (they are different), or any flammable refrigerant.
Propane has decent refrigerant properties, but it can make a front
end collision into a flamethrower.
not service R-134a systems with compressed air, especially around an
open flame. R-134a is not as inert as R-12, and will form poisonous
gasses when burned. It is mildly poisonous when not burned (it's an
irritant), so you don't want to mist it into the air.
a retrofit, make sure the hoses don't contact anything they shouldn't.
Contact with the fan and the hood are relatively easy. I use zip ties to
the fan shroud to hold the compressor lines in place.~