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Discussion Starter · #1 ·
Hi guys, my name is Joe, I'm new here. I have a 2001 F150 5.4L 4x4. Last week the A/C just suddenly quit. It remained that way till sometime this week when I turned it and it ran.
I hooked up a set of gauges to the high and low sides, set the A/C to max, blower to max and left both doors open.
High pressure was 350psi, low pressure was 50psi. It stayed that way for about 10 minutes at which time the compressor began cycling rapidly. It would do that for about 2 minutes then everything went back to 350/50. At one time the high side headed for 400 but it came back to 350.
The cooling fan clutch seems OK and I haven't seen much debris around the condensor but I haven't tore anything apart yet. This is the first time the A/C has been worked on.
 

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It sounds as though the high pressure cutout switch is causing the cycling. Try spraying water from a garden hose on the condenser while checking the pressures. If they come down to normal 25-45 low side, 2.2-2.5 times ambient temperature high side there is a problem with condenser/cooling. The most likely suspect is the fan clutch. Also pressure wash the condenser as it can get plugged with dirt. Moisture/air in the system can also cause high high side pressures as can a blockage in the system.
 

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While I would agree with Royesses diag.I would like more info.What was the ambient temp outside on the day it quit and the day it came back on.AC pressures have a tendency to increase when dramatically when left at idle on a hot day.Low side pressure should be around 20-25 psi at idle with a aux fan blowing into the condenser.
 

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Discussion Starter · #4 ·
The day it quit last week it was 80 and sunny. Today when I turned it on again, it was 75 and cloudy.
I can't find my little thermometer to stick into a vent, but it doesn't feel like its blowing as cold as it could. Is this due to my 50# suction pressure?
The HP cutout switch is set for 425# or so. With my discharge running at 350#, how could that switch cause the compressor to cycle?
When the truck was off and I first plugged in my gauges today my static pressure was 150#. Isn't that high?
I looked at the face of the condenser real close tonight. There is no buildup of anything on it. It has about 10 very small dents in the fins from 135,000 miles of doing battle with the roadways.
Tomorrow I will spray the condenser with water and see if the pressures drop.

Thank you guys for all your help and ideas. I appreciate it a lot!
 

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I'm no expert at AC systems, but a 50 psi low pressure seems about 20 psi too high. A static pressure of 150 also seems high at 75 degrees. Someone, please correct me if I am wrong. But the only way to get static pressure too high is to overcharge the system, suck air into the system, or have the system very hot when it is measured.

I know, I'm not much help.
 

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Principles of Operation
There are four main principles involved with the basic theory of operation:
• heat transfer
• latent heat of vaporization
• relative humidity
• effects of pressure
Heat Transfer
If two substances of different temperature are placed near each other, the heat in the warmer substance will transfer to the colder substance.
Latent Heat of Vaporization
When a liquid boils (converts to gas) it absorbs heat without raising the temperature of the resulting gas. When the gas condenses (converts back to a liquid), it gives off heat without lowering the temperature of the resulting liquid.
Relative Humidity
The amount of moisture (water vapor content) that the air can hold is directly related to the air temperature. The more heat there is in the air, the more moisture the air can hold. The lower the moisture content in the air, the more comfortable you feel. Removing the moisture from the air lowers its relative humidity and improves personal comfort.
Effects of Pressure on Boiling or Condensation
As the pressure is increased on a liquid, the temperature at which the liquid boils (converts to gas) also increases. Conversely, when the pressure on a liquid is reduced, its boiling point is also reduced. When in the gas state, an increase in pressure causes an increase in temperature, while a decrease in pressure will decrease the
temperature of the gas.

The Refrigerant Cycle
During stabilized conditions (air conditioning system shutdown), the refrigerant is in a vaporized state and pressures are equal throughout the system. When the air conditioning (A/C) compressor is in operation it increases pressure on the refrigerant vapor, raising its temperature. The high-pressure and high temperature
vapor is then released into the top of the A/C condenser core. The A/C condenser core, being close to ambient temperature, causes the refrigerant vapor to condense into a liquid when heat is removed from the refrigerant by ambient air passing over the fins and tubing. The now liquid refrigerant, still at high pressure, exits from the bottom of the
A/C condenser core and enters the inlet side of the A/C evaporator core orifice.
The A/C evaporator core orifice is the restriction in the refrigerant system that creates the high pressure buildup upstream of the A/C condenser core and separates the high and low pressure sides of the A/C system. As the liquid refrigerant leaves this restriction, its pressure and boiling point are reduced. The liquid refrigerant is now at its lowest pressure and temperature. As it passes through the A/C evaporator core, it absorbs heat from the passenger compartment airflow passing over the plate/fin sections of the A/C evaporator core. This addition of heat causes the refrigerant to boil (convert to gas). The now cooler passenger compartment air can no longer support the same humidity level of the warmer air, and this excess moisture condenses on the exterior of the evaporator coils and fins and drains outside the vehicle. The suction accumulator/drier is designed to remove moisture from the refrigerant and to prevent any liquid refrigerant that may not have been vaporized in the evaporator core from reaching the A/C compressor. The A/C
compressor is designed to pump refrigerant vapor only, as liquid refrigerant will not compress and can damage the A/C compressor. The refrigerant cycle is now repeated with the A/C compressor again increasing the pressure
and temperature of the refrigerant. The A/C cycling switch interrupts compressor
operation before the external temperature of the A/C evaporator core get low enough to cause the condensed water vapor (excess humidity) to turn to ice. It does this by monitoring low side line pressure. It is known that a refrigerant pressure
of approximately 210 kPa (30 psi) will yield an operating temperature of 0°C (32°F). The A/C cycling switch controls system operation in an effort to maintain this temperature. The A/C high side line pressure is also monitored so that A/C compressor operation can be interrupted if system pressure becomes too high. The A/C compressor relief valve will open and vent refrigerant to relieve unusually high system pressure.
 

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Discussion Starter · #7 ·
I got to play with it somemore today. This time static was 100# with a stone cold engine and 75-80 weather. When I ran it, it settled at 250psi/40psi. I got my garden hose and sprayed the condensor as good as I could. Boy did it like that! Low side went to 30, hi side down to 185.
Based on all this wheel spinning, I think I may have crud between the condensor and the radiator or the fan clutch is shot- even though I can still feel the viscous coupling drag on the fan blade when the motor is off and you try to spin the fan.
 

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Discussion Starter · #8 ·
Well it did it again. Took a little trip today, 85F and humid. Turned on A/C and it blew semi cold air, probably 55F or so. It ran for 20 minutes like that then it died, just blowing warm air. Got to the destination and popped the hood. The fan clutch had very little resistance. About 5 hours later we made the return journey home. 80F and not so humid. Flipped on the A/C and it worked perfectly all the way home. Got home and checked the fan clutch. It had the exact same little resistance as before.
According to the Haynes manual, the fan clutch should spin freely when cold and offer resistance when hot. Is this correct information?
 

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From the Hayden web page FAQ:
Fan Clutches
Q. How do I know if my fan clutch needs to be replaced?
A. These are the most common symptoms of a failed fan clutch:
•Leaking fluid - Oily build up around the bearing or thermal spring.
•Bad bearing - Seized, turns rough or has excessive play, (more than 1/4" at fan tip).
•Worn thermal spring - Spring is loose.
Some fan clutches will show no visible indication of a problem yet may still be faulty. The following may also indicate a faulty fan clutch:
•Fan spins excessively - Three or more times when hot engine is shut off.
•Poor air conditioning - At low speed or excessive high side pressures.
•Doesn't engage - Fan speed does not increase or "locks up" when the engine is hot.
•Does not disengage - Fan clutch won't slow down when the engine is cold.

Q. At what temperatures do fan clutches engage?
A. Most fan clutches engage at about 170° F air temperature (about 180-190° F engine temperature). They reduce the temperature about 20° F before disengaging.
Q. What is the difference between a standard, heavy and severe duty fan clutch?
A. Each fan clutch type is designed to simulate the performance of the original equipment clutch that it replaces. All fan clutches are for specific applications and should be applied only on the application for which they are cataloged.
Standard Duty Thermal
Turns the fan 50-60% of shaft speed when engaged. Used with fans with lighter pitch. (1-1/2" of pitch) Flat plate impeller design with 30 Sq. In. of working surface.
Heavy-Duty Thermal
Turns the fan 80-90% of the shaft speed when engaged for increased cooling. Used with deeper pitch fans. (2 1/2" of pitch). Land and groove design with 47 Sq. In. of working area allows higher operating RPM's.
Severe Duty Thermal
Turns the fan 80-90% of the shaft speed when engaged. Used with deeper pitch fans. (2- 1/2" of pitch). Land and groove design with 65 Sq. In. of working area. Larger working surface provides cooler running and longer life expectancy.
 

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Discussion Starter · #10 ·
I found the compressor clutch falling apart. 4 or 5 of the balls in the bearing were gone. The rest of them looked pretty raty. So far A/C's been running great and a lot quieter too.
 
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