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Problem w/ intermittent defrost a/c

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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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This is quad crazy not crown Victoria crazy! Lol just kidding. It could be two things.  One the fogging could be due to a blown (heater core). It’s a small radiator type coil under dash board on the passenger side. A real bi*** to get to.   A tell tale sign is the fogging as well as a wet floor on the passenger side under or on the carpet and a bit of a sweet odor in the car. 

The second thing it could be (less expensive) is the change over valve.  It’s a small valve or damper that’s controlled by the switch in the dash.  The control switch you would use to change to defrost to dash vent.   That switch usually activates a motor of a kind that moves the flaps inside the dashboard ductwork.  That motor can be a pneumatic , electric or cable depending on the year.  It’s usually located on the firewall under the hood. In that general area.  I’m putting my bet on the heater core though based on the fogging.  

Hope this helps.  Get an Atv. It’s a lot easier and more fun to work on! 

Sorry to my fellow members but I couldn’t help myself I love cars as well! 

Good  luck!  

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  • 3 weeks later...

Yeah the B stands for Boss yall.  Haaaar.

Sorry it's been so long since I posted.  Life got complicated all the sudden.

I'd guess the heater core too if the dang thing just hadn't gone bad a few months back and I changed it out in 20 deg weather, yeah.  The fogging problem was going on before the core leak, it just happened to go as well.

Now that it's gotten a little warmer I've ran the a/c a little and it's doing the same thing it did last summer.  The air temp alternates between somewhat cool and not much cool, never getting down to cold.  I would guess I'm looking at 2 problems here but if I could just get the a/c problem fixed I wouldn't mind just having the air come out the vents.

Any ideas on how to start testing my a/c?


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I'm gonna assume my problem is being caused by low refrigerant and take a shot at fixing it as cheaply as I can. But at the same time I'd like to take steps at identifying where the leak might be.

So what will this look like? Renting some gauges from the local car store and checking my refrigerant lines? How would you go about this and what readings should I be looking for?

And then if things read low I would get a can of refrigerant with leak revealing dye and put it in the system?

Sorry I'm so remedial here, first time working on an a/c system.

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Its two gauges with some hookups to check the high and low pressure side of the compressor. If you do have a leak you can buy refrigerant that contains a UV dye in it so you can identify the source of the leak using a UV light.

Autozone will do some tests for free, or you can rent the gauges to do the test, or take it to any garage. No use spending money for the gauges for a one time use.



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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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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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