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Problem w/ intermittent defrost a/c

Problem w/ intermittent defrost a/c

Old 03-09-2019, 05:23 PM
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Default Problem w/ intermittent defrost a/c

Have a '11 Crown Vic with 290k miles and I've been having a problem with the windows fogging up all the way around. The defrost seems to be working about half the time. On humid rainy days it'll work for say a half hour keeping the windows clear, then the windows will fog hard for 10-15 mins. And then it might decide to start working again, back and forth it goes. When I started it just now the compressor clutch turns when the a/c is turned on, although I haven't had a chance to observe it when the windows are fogging.


A secondary problem it has is that it only blows air through the vents on the front of the dash, straight out into the cabin. The air flows only very weak out the defrost vent at the windshield or the floor. It does this on all settings. Although this might contribute somewhat to the problem, it seems like the a/c does completely cut out as the windows will still fog hard with the a/c set to on and on full fan. When it cuts off the air in the cabin feels immediately humid. One that that helps somewhat is to turn the temp toward cooler and it seems to not fog as much and gives it a chance to maybe clear the fog some.


Any idea what's causing this or what I should do to start testing?
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Old 03-09-2019, 05:38 PM
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You might check the charge on the AC. There is a possibility it is freezing the coil with solid ice. If that is the case then you need to add a little refrigerant
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Old 03-31-2019, 04:44 PM
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Ok thank you. How would I go about checking the charge on the a/c? I think I can borrow a set of loaner gauges from a nearby car store.
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Old 03-31-2019, 04:50 PM
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You will need to hook them up and you will see numbers in psi and some for temperature for different types of refrigerant, unless the gauges are R134A specific. If the blue gauge reads below 34 - 36 then it needs some refrigerant. You want to aim for those temps on the low side. Also if you see ice on the large line going to the compressor after running it awhile, that is a dead givaway. The metal parts of it should draw moisture out of the air and have a "good sweat"
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Old 04-03-2019, 07:47 PM
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Pray its the charge, and not the EATC module for the heater controls, because those things arent cheap. The vents open and close using tiny electrical solenoids inside of it. The o-rings are known to fail. I have seen them serviced, but cant remember which model it was in. Its delicate work tho as you have to take them apart to do it.
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Old 04-04-2019, 10:09 PM
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Ok thanks for all the help

Last edited by bradleyheathhays; 04-06-2019 at 04:37 PM.
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Old 04-07-2019, 04:05 AM
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Hooked the gauges up and it seems like I've got a low refrigerant condition. Before turning the engine on both gauges read about 75. After starting the engine and setting the AC on max you could really tell the compressor was cycling fast. The needles cycled every 5-6 seconds...one going up and the other down, then reversed. The blue gauge cycled between 22 and 38 PSI while the red went between 90-140. The outside temp was about 60 F.

From what I've read, especially with the compressor cycling so fast, this indicates low refrigerant. Hopefully my compressor isn't hurt by running it for so long like this.

How do I go about using this 2 gauge setup to add refrigerant? Should I add it through the middle hose somehow or get a can of 134 with a gauge on it and just add it directly from the can?

And how do you know how much to put in?
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Old 04-07-2019, 11:40 AM
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Yes, middle hose port. Hook the can to it. You may need a different hose to do it though
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Old 04-09-2019, 01:45 PM
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After I took the gauges off I noticed one of the schrader valves was hissing, and now the system is down to 0 psi. So I believe a faulty valve has been the culprit all along.

Now that the system is down I'm going to replace both valves, but my regular schrader valve removal tool won't fit down on the valve. The stem of the valve is too long and the tool won't go down around it to twist it out. I need a tool with prongs longer to fit down into the recessed sides of the valve. Does this '11 Crown Vic have schrader valves or a different kind of valve?

Also, when I go to fill the system with refrigerant, do I need to add any PEG oil or just add refrigerant up to the specified capacity?
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