1. #1
    bradleyheathhays's Avatar
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    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?

  2. #2
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    Problem w/ intermittent defrost a/c

    Checking cheap or easy first.
    Make sure the a/c drain is clear. If the water cant make it out, it stays in and gets recycled. Ends up smelling musty. Look at the intake area as well. Jeeps cowl drain can cause buildup to make into the air intake.
    Id have to check but for many cars if there is a vac leak defrost is the default setting. If you have constant bleed-through still may be vacuum. It could be something blocking or bad dislodged door.
    Blend door
    Check your heater core for leaks or blockage. Leak will result in water on the flor and overheating. Blocked core may be cold air or occasional warm air.
    Some vehicles have a valve to the heater core. Bad fuse or relay or valve.




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    Last edited by JPNinPA; 03-10-2019 at 12:53 AM.
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    Problem w/ intermittent defrost a/c

    Keep in mind warm air holds more moisture. In the winter when i drag snow in or rain i may leave the windows cracked to get the moisture out. There are times when I drive heater on high hit with windows wide open.


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    Re: Problem w/ intermittent defrost a/c

    Thanks so much JPN and sorry it's been so long since I dropped this. Life got a little complicated for a while there.

    I know I'm a bit out of place here but these problems are going on with my '11 Crown Vic. I noticed you mentioned Jeep in your post so thought I'd say that again.

    Here's some addnl info that might help diagnose the problem. Now that it's gotten warmer weather i've used the a/c a couple times and it's doing exactly what it was at the end of last summer. The air temp is only medium cool, then it gets somewhat warmer to only just cool, then it cycles back down to medium cool. Does this about once a minute or so and continues doing this until it's turned off.

    Is this indicative of a system that needs to be charged?

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    Problem w/ intermittent defrost a/c

    I got that it is a Ford Crown Vic. Just relaying what happens in a system. Most are the same components but may be automatic, electrical or mechanical.


    So this is good to know info.

    What are the driving conditions when it happens? Above 35mph or below? All the time? ...

    Bad clutch fan or failing fan.

    Blocked airflow/no airflow across evaporator. Check your cabin filter. Cold even be the fins of the heat exchanger. Failing or faulty wiring to the fan.

    Too little refrigerant. A leak.
    If you add add the leak dye so it could be repaired.
    Too much coolant will do the same. Flood the evaporator and the coolant doesn't get to expand to a gas.

    Faulty compressor clutch. Bad clutch the compressor is not pumping.

    Dirt or internal corrosion can lead to clogged expansion valve.

    When this condition exists check if anything looks frozen.



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    Last edited by JPNinPA; 04-02-2019 at 04:39 AM.
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  6. #6
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    Re: Problem w/ intermittent defrost a/c

    When it's raining the fogging can happen about anytime. The a/c tends to be cooler when the vehicle is moving and gets less cool when it's sitting idling.

    As the engine was overheating last summer I replaced the whole fan unit with new and that solved the problem. So I'd imagine we can rule out the fan or control module.

    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.
    Last edited by bradleyheathhays; 04-04-2019 at 10:24 PM.

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    Problem w/ intermittent defrost a/c

    Yes if you add refrigerant add the die.
    With the system being cooler when moving I'd look at the fans and clean any debris out of the cooling fins in the grill.
    Many have 2stage fans. One stage for radiator cooling and the 2nd for
    A/C and rad.
    Also make sure the belt is tight. Skipping belt causes issues.

    Caution on breathing any gas. Any loose fitting or not closing off a valve can create a dangerous situation. Second overfilling is just as bad as under filling.

    I like this video because he covers most easy items first before going to the gauges.


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    Last edited by JPNinPA; 04-04-2019 at 10:46 PM.
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  8. #8
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    Re: Problem w/ intermittent defrost a/c

    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 cycled between 90-140 PSI. 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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    Problem w/ intermittent defrost a/c

    Good video. Empty system but you get the idea.



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  10. #10
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    Re: Problem w/ intermittent defrost a/c

    Thanks for the video, good stuff.

    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?

  11. #11
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    Re: Problem w/ intermittent defrost a/c

    Id add oil and dye. Its not much if i recall correctly. But follow the video now that you are down to zero. Id draw the vacuum as shown. Air is bad. Moisture is worse.


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