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Old 06-08-2019, 05:10 PM   #1
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2011 Focus SE AC Cycling Issues (115K miles)

When I start car with AC enabled, the compressor will engage within about 2 to 10 seconds (with resulting cold air) and then disengage after about 5 to 20 seconds. A few times the compressor engaged and cycled off/on few times before disengaging completely after a minute or two. A gauge on the low pressure fill port shows adequate refrigerant pressure when the compressor engages. I wanted to locate and possibly test the low pressure cycling switch but cannot locate it in engine compartment. Any ideas or suggestions on what the root cause of the problem may be? Thank you.


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Old 06-08-2019, 05:41 PM   #2
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What is low pressure doing though? When the compressor kicks on you will see the low pressure drop, and it will continue to drop or it will eventually find a hold steady point. So my question really is at what pressure does the compressor kick-off at?

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Old 06-08-2019, 09:57 PM   #3
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Thank you for reply. I very much appreciate it. The gauge I am using is admittedly crude and is simply the one which came with a can of AC Pro. The reading is off-scale until the compressor engages, then drops and settles right in the middle of the "green/filled" region (so right at 40 psi) about 5 seconds after the clutch engages. This usually happens once and then the compressor disengages after about 10 seconds and does not engage again unless I turn the car off and the restart it. Perhaps resets the control module algorithm which is then driving the short one-time engagement of the clutch after each startup? On the couple instances where after startup the clutch cycled off/on a few times and the AC functioned for a minute or two before shutting down for good, I observed the same ~40 psi reading at the low fill port. Don't want to add any refrigerant as the gauge range is "filled/green" for range 30 - 50 psi. So would seem low refrigerant is not the problem? It was about 80 deg F today when I made the observations. So I was wondering if the low pressure sensor could be giving (intermittent, but mostly constant) faulty pressure readings to the control module? Is there even a "low pressure cycling" switch/sensor? Is it buried behind the firewall? Any help or suggestions are appreciated.

zacautomotive, I see you are in NC which is where I am heading (Greenville, daughter about to deliver first grandchild) in about a week, and thus why I want to get this resolved before I have to sit in a car for hours on end in that 90 deg heat with high humidity?

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Old 06-08-2019, 10:33 PM   #4
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Thank you for reply. I very much appreciate it. The gauge I am using is admittedly crude and is simply the one which came with a can of AC Pro. The reading is off-scale until the compressor engages, then drops and settles right in the middle of the "green/filled" region (so right at 40 psi) about 5 seconds after the clutch engages. This usually happens once and then the compressor disengages after about 10 seconds and does not engage again unless I turn the car off and the restart it. Perhaps resets the control module algorithm which is then driving the short one-time engagement of the clutch after each startup? On the couple instances where after startup the clutch cycled off/on a few times and the AC functioned for a minute or two before shutting down for good, I observed the same ~40 psi reading at the low fill port. Don't want to add any refrigerant as the gauge range is "filled/green" for range 30 - 50 psi. So would seem low refrigerant is not the problem? It was about 80 deg F today when I made the observations. So I was wondering if the low pressure sensor could be giving (intermittent, but mostly constant) faulty pressure readings to the control module? Is there even a "low pressure cycling" switch/sensor? Is it buried behind the firewall? Any help or suggestions are appreciated.

zacautomotive, I see you are in NC which is where I am heading (Greenville, daughter about to deliver first grandchild) in about a week, and thus why I want to get this resolved before I have to sit in a car for hours on end in that 90 deg heat with high humidity?
You did the right thing by not adding refrigerant. Best way to blow a compressor seal is overfilling..I leave ac work to the professionals.
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Old 06-08-2019, 10:43 PM   #5
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You did the right thing by not adding refrigerant. Best way to blow a compressor seal is overfilling..I leave ac work to the professionals.
I have my AC license not that it's hard to get. I went to school for it, and honestly they're pretty tough I have seen people massively overfill systems.

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Old 06-08-2019, 10:45 PM   #6
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Originally Posted by zacautomotive View Post
Quote:
Originally Posted by pbfoot View Post
You did the right thing by not adding refrigerant. Best way to blow a compressor seal is overfilling..I leave ac work to the professionals.
I have my AC license not that it's hard to get. I went to school for it, and honestly they're pretty tough I have seen people massively overfill systems.

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I'm guilty of that. I had no idea what I was doing.
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Old 06-08-2019, 11:05 PM   #7
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A full gauge said it would be helpful so you can also see the high side pressure reading. You can look up automotive AC pressure diagnostic charts that show both the low and high side pressure and what the reading means. If your high side pressure is extremely high it will kill the system.
I will have to look at my car maybe sometime tomorrow and see if I can locate the switches.
https://images.app.goo.gl/2bodRCAhYpmc8wpT9

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Old 06-09-2019, 01:09 AM   #8
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Read the HIGH side, you cannot get much of an idea of system pressure using the low side only. You can get the same 40 psi at low side with 150-300 psi high side and not know that. At a temp of 80 degrees you would be running maybe 250 high side as normal. Here in Texas they quit cooling around 175 psi high and you still have a 40 psi low there all day long. You fill refrigerant to satisfy the HIGH side not low but try telling all those who use worthless tools that. They don't supply the high side checking tools unless you specifically buy them, it prevents hundreds of people from blowing off fingers and such. A liability issue for the people that make the refill kits.

Why low side only measuring tools are garbage.
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Old 06-17-2019, 04:12 PM   #9
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Thank you all for the suggestions and information. I had to drop this to leave town for a week. I am thinking of taking it to my mechanic whom I trust to be on the level if it turns out to be just a low pressure cycling switch or sensor. Wish I could have found it, if it in fact exists in this design. For $50 I might have tried to replace if it was accessible and on a schrader-type valve.
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