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Featured Articles
Ford Air Conditioning
by Jerry Heep
Troubleshooting Air
Conditioning systems – Step By Step
- Start your truck and turn on your air conditioning system. If you have
one, press in the AC button and note the light inside the switch does indeed
light. Set the controls for MAX, COLD, HIGH fan speed, PANEL only. Close all the
windows and shut the doors. Let the system cool the air, if it is working. After
a few minutes, the air coming out of the panel ducts should be below 50°F.
I’ve seen air below 40°F
on an American automotive air conditioner. If the temperature is above 55 to 60°F,
you may want to find out why. Go to step 2. If the truck was out in the sun and
the dashboard is too hot to touch, give the air conditioner some time, guy. Let
it cool down some before you take your measurement. If OK, shut it all down, you
ain’t got an air conditioning problem.
- Pull the inside hood latch and raise the hood
while the motor is running. Is the center shaft of the compressor turning? If
not, got to step 3. If it is turning go to step 6.
- If the clutch is not turning the compressor, the clutch may not
be electrically engaged. There are three things that can cause the clutch not to
engage. 1) The system is low on refrigerant. 2) The AC switch is bad. 3) The
clutch is open (or a wire is broken.) These were listed in the order that is
MOST likely. By the way, a modern truck’s computer may turn the compressor off
under full throttle position or if it detects an overheating condition.
 We
are assuming this is not the case, but one of the three conditions I’ve
listed. Go to the switch on the side of the drier and pull the connector from
the switch. Look at the picture at the left. With a jumper wire, short out the
two contacts of the connector. If the clutch engages and the compressor shaft
now turns, you are too low on refrigerant, or the switch is bad. Most likely it
is low on refrigerant. Go to step 6 and fill it up, else go to step 4. Don’t
forget to check on the high-pressure switch, if you have one. You should be able
to do the same jumper thing. Be very careful around this running diesel motor.
- If still no compressor shaft rotation, replace the connector on the drier
switch and move to the compressor. Be careful here – that rotating pulley has
a diesel motor driving it. Carefully remove the connector
 from
the clutch. To be extra safe, turn off the motor then remove the connector. On
‘87 and newer trucks, the compressor clutch connector is made up of two wires
in an oblong holder. On older trucks there may be only one wire connected to the
wiring harness with a quick disconnect inline connection to the clutch. In the
picture to the left, I’m pointing out the clutch connector. The single wire
used on the older trucks is a no brainer. On a two-wire system, one wire is
power and the other wire is ground (frame.) You need to jumper a wire between
the battery PLUS terminal to one of the contacts and attach a ground to the
other contact. It doesn’t matter which contact you choose, so long as the
connector is not connected to the truck’s original cable.
If good, the clutch will engage with a loud click. Don’t let it scare
you and make you jerk your hand into a spinning pulley or into a belt. Be very
careful around spinning pulleys and fan belts. If the clutch engaged, there is a
broken wire or a bad AC switch in your future. Go to step 5. If the clutch did
not engage, it may be bad. Use a meter to take the clutch resistance. It should
read be very low resistance. My clutch read 0.8 ohms on a known working clutch.
If the clutch is bad, it may take special tools to remove the clutch from the
compressor. You may want to think about replacing the entire compressor. See
“Replacing the Compressor” in the servicing area below.
Be careful here – one of our technical editors had melted his jumper wire and
almost started fires while doing this jumper test. I suggest you monitor the
wire temperature for a short while after making the jumper connection. If the
wire is gets hot enough to make it uncomfortable to touch, pull the jumper.
- If the jumper wire turned on the compressor clutch, get out the test lamp
or a voltmeter. If the test lamp lights when you apply it to the connector power
and ground, maybe the clutch connector contacts are corroded. If there is no
power at the connector, get inside the truck and look at the fuse panel. I have
yet to have a bad fuse here, but it is possible. If you have really poor luck,
it will be a bad AC switch that needs replacing.
- If the compressor is turning, reach up and touch the two manifolds/hoses
going to the compressor. Watch out, one may be very, very hot. The HOT
connection is the pressure side. The cold hose is the suction side. These two
radically different temperatures on the compressor mean that you have
refrigerant flow and good pressure differential. So, if you are not getting cold
air inside your truck, you have an air handler problem. Go to step 7. If both
manifolds are about the same temperature, or you can hold a finger on the
pressure side, the refrigerant is not moving or there is no pressure
differential. This can happen if you have bad O-rings on the expansion valve, or
the valve is missing completely. If there is no orifice action here you will
have great refrigerant flow, but no pressure drop. If you know you have enough
refrigerant installed, the expansion valve may be plugged, or you may have a bad
compressor. Go read “Checking/replacing the expansion valve” or “Replacing
the Compressor” in the servicing section below. If you are working on an old
R-12 system, now is the time to decide if you are going to stick with R-12 or
convert to R-134a. If you want to stay with R-12, you are going to have to find
a source for it or for a compatible substitute. Unless you are a licensed
service person, you cannot legally buy compatibles. You may have to end up going
to your local AC shop. If you are going to use butane or some other explosive
gas, I don’t want to know about it. If you are going to convert your R-12
system over to R-134a, go read “Converting to R-134A” in the Service
section.
- Once you have good refrigerant movement and pressures in the system,
there must be an air handler problem giving you the warm air. Was there cold air
at first? Did the cold air seem to disappear as the motor warmed up? If so, you
may have a leaking blend door, or depending on how old your truck is, a
defective water valve. To see if you have a blend door leak, pick off both
heater hoses with needle nosed vice grips and monitor the cooling ability of the
evaporator. If the air slowly cools while the heater core cools off, you’ve
isolated a blend door problem. If the air is still hot, you’ve got other
problems. Go to step 8. If you have blend door problems, a temporary fix is to
add an external water valve. See “Installing an External Water Valve” in the
service section.
- Air Handler problems not addressed above is usually caused by a dirty
expansion valve, plugged up drier, or stuck dampers inside the plenum. The
dampers are easily checked. Put the fan motor on full blast. Move the air source
switch to its REC and check under the dash on older trucks, below the glove box.
You should see the “inside” damper right there. Reach up and move the air
source switch back and forth between RECIR and NORM. You should see this damper
move to the open position and then move to the close position. This damper is
usually operated by vacuum. If there is no movement, look for a bad vacuum
switch or crack or broken vacuum hoses. How’s the source vacuum? Go outside
and place your hand on the grill that is located between your hood and
windshield. You should feel air entering the grill when the air source switch is
in the NORM position. In the RECIR position, the air flow stops. If both dampers
are closed, there is no air source for your air handler! One of these dampers
MUST be open for air flow. On modern trucks there will be only a single door and
vacuum motor.
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