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    jrjrdinh New Member

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    Location:
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    I was driving last night (10:30pm) in Fremont, CA going home and I noticed my windshield kept getting foggy. I turned on the Defroster to max but it wasn't going away.

    After 20 min or so it started to build up from the center and slowly up. It was really bad. I then wiped my windshield with the wipers and it was all gone. But just within seconds it just started to build up outside again. I then had to keep turning on the windshield wipers every minute.

    I looked at other cars around me and checked the weather outside, but the other cars were just as fog free as my car and it wasn't that cold of a day nor was it raining.

    Please help on what may be causing this! I wash my car once a week so no I don't think it was because my car's dirty.
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    TKY Member

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    Location:
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    Hi Jrjrdinh, Maybe I don't understand what you are describing; condensation to forming on the exterior surface of your windshield, right? The windshield wipers are the primary tool to remove moisture, whether rain, snow, hail, or condensation, from your windshield, so what you describe sounds normal to me. The defroster helps during snow and ice, and maybe hail, which makes it easier for the wipers to remove the snow and ice and to help prevent build up.

    I feel I'm still not understanding something in your post; I suppose the one thing you could have tried, was to turn the temp to high and turn on the A/C. That will help remove condensation from the inside of your windshield. Depending on the relative humidity and amount of moisture building up on the outside, it might help.

    Without clarification, I still suggest use the right tool designed primarily for the job you are performing. --TK
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    jim256 Member

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    If humidity is high outdoors, and your car and windshield are cool, the moisture collects on the outside of the glass--wipers remove it. If you used the defroster, it brought on the a/c compressor to dehumidify the inside air, but it may also have contributed to cooling the windshield more if you did not raise the temp setting.
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    jrjrdinh New Member

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    Hey TKY,

    The build up was just really bad last night while driving. It kept building up even though the defroster was on and I was wiping the windshield. I had passengers in the car and I didn't think they liked the ride when the defroster was on full blast.

    When I drove my other car the fog wasn't building up like the Prius 2010. I ran my fingers through the fog on the interior of my windshield but the fog was actually on the exterior. Only my windshield wiper was able to make it go away, but after it wiped within seconds that fog just popped up again. This only happened on the Prius.
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    jrjrdinh New Member

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    Hey Jim,

    So what you're saying is probably having my A/C on and raising the temperature to high settings and maybe fan setting medium I'm able to remove the fog on the exterior of the window? Thanks a lot for the reply!
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    fred garvin New Member

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    Location:
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    if the condensation is outside the AC will do nothing. you need hot air on the window to raise its temp. condensation happens when damp air hits a surface that is cold (not sure if this is right - but is the "dew point" the tempature below which this happens?). The AC won't do anything here.

    AC defogs the inside of the window not because its cold so much as because the AC makes the air dry, and when it blows on the damp interior glass, the dry air grabs the moisture off the glass.

    this is what i think i know - but it could be i don't know it well
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    jim256 Member

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    Yes, you just need to raise the temp of the windshield a little, you can accomplish it with a lower fan speed by selecting the Mode switch to direct air to just the windshield and then set your own fan speed.
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    DeanFL 2010 owner - 1st Prius

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    Location:
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    Has happened to me many times over the years - pre Prius.

    The conditions always seemed to be - very humid outside, outside temp warm<>hot, A/C on inside the car with some cool air going to the defrosters (normal).
    All these conditions together occasionally cause exterior condensation on the lower middle windshield.
    Simply swipe with the wipers when needed, and should be short term. No problem with your Prius.
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    Edward Walsh New Member

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    Location:
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    Hi:

    Not all, but in some of cars I have leased over the past 16 years I have experienced this condition on a regular basis. Living in an area close to the ocean where the outside temperature fluctuates and at times an air conditioned vehicle becomes colder inside than outside moisture develops on the outside windshield thus necessitating the use of wipers. Turning the A/C down or off, not having the air blowing on the windshield or turning on the heat for 30 seconds or so usually clears it up.....at least that is what I have found.

    Being a hazard? I can see that if you don't take steps to reverse the moisture building up. We have only had our 2010 Toyota Prius for 6 days and haven't experienced this yet, but if you are experiencing it no doubt we can expect it. We have gotten so use to this happening and how to resolve it that we don't give it a second thought. I hope that with what I said and others in this thread help you see you are not alone and the suggestions given will help it from becoming a hazard.

    All the best.....
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    Old Bald Guy Old Bald Guy

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    This is nothing new or just on the Prius. I have seen this happen for decades on any car with AC. You don't want to use the hot defroster if you are having to use the AC because it is hot and muggy outside ... kinds of defeats the purpose.

    The issue here is running with the AC on and upper level airflow when it is really humid outside. What you are doing is freezing the glass and the outside moisture condenses on it.

    Set your mode control to mix the air partly down to the floor and less out the upper vents. Use your wipers if you need to until the glass warms up enough not to fog up. And turn off your defroster.
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    SW03ES Senior Member

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    You've gotten good advice here. Toyota automatic climate control systems seem to be prone to this issue...
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    janice45 Dudley Girl

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    I hope this question is for this forum if not for this area you could direct me to where I might find an answer .
    I have a foggy windshield but not from humidity it is a film that is on the out side and it is streaky at best the blades are of no help I have cleaned this windshield with everything but the streaks have remained . It looks actually like someone wiped the glass with something they should not have ,
    I just can not get a clear windshield is there something on the glass it's self that might have been affected by say a cleaner or solvent .
    My car is just a month old
    Thanks

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