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Problem w/ intermittent defrost a/c

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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
    Have you performed an EATC self test?



    • #3
      Fogged up glass can be a sign of a leaking heater core. Do you smell antifreeze or is the passenger floor carpet wet?

      1994 Deep Jewel Green 183,142 miles
      1997 White LSC 135k miles
      1998 White Collectors Edition 175k
      1994 Black (#2) 100k miles
      1993 Deep Jewel Green Convertible


      • #4
        The heater core angle is a good idea. You'd probably smell that, and/or see little "whisps" of glycol coming out the vents.. Worth checking, but this could be a few things.

        With regard to it not switching between "modes," I'd be highly suspicious of a vacuum leak somewhere. Although if memory serves, these things will usually default to "defrost" mode as a fail-safe, so you could be looking at something more like a stuck blend valve.

        Certainly the A/C compressor plays into this, so getting that resolved is pretty important. Additionally, if the car is kept outside, I'd check to see if rainwater is gathering somewhere you don't want it to. (ie: running into the air intake at the base of the windshield) if you're collecting water in the system, that will exacerbate these problems.
        Mike Martin

        1997 Mark VIII LSC 5.0
        2001 Dodge Dakota