so why is my defroster cracking my windshield? -
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post #1 of 7 Old 11-28-2008, 06:24 AM Thread Starter
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so why is my defroster cracking my windshield?

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was chilly but not freezing yesterday but had defrost on 2 and after 15mins on the highway i get this crack from the bottom on the glass moving upwards and its probably 5" long in one day so far... is this common? nothing hit me, just all of a sudden i get a 3" long crack that is continuing to spread...

has this happened to you? Is it preventable (without stating dont use defrost)?


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post #2 of 7 Old 11-28-2008, 07:30 AM
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See them sometimes in new vehicles...stress cracks.

This one would be something else. Take a ball point pen or something pointy and run it along the crack from the'll likely fetch up and you'll find a rock chip where the crack started from.
Has nothing to do with using the defrost. Well...the temperature change aggravated it, but I'm 100% sure that it started from an existing rock chip.

Happened to me in our old Chevy van... We moved along in traffic from a sunny spot to a shaded spot, cool day in the fall. A crack started and worked its way right from the bottom left corner all the way to the right side. Took about 5 seconds to work its way across.
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post #3 of 7 Old 11-28-2008, 07:35 AM
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Likely the window was damaged on install. When you get it replaced have them check the frame in the area the crack started to make sure there is nothing there that will damage the new windshield.

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post #4 of 7 Old 11-28-2008, 12:13 PM
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It's a stress crack .. most probably caused by a stone chip at the bottom (see if you can see one).
I had one on my Durango .. started drivers side and went all across the glass to the passenger side. Funny thing was it swerved around my GPS that was stuck to the glass.

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post #5 of 7 Old 11-28-2008, 12:22 PM
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I think what you have is brittle fracture. It's likely everyone is correct. You might have a small imperfection in the glass.... from a rock chip or even one you couldn't see inside the weatherstripping. Once the heat from the defroster was applied it created a temperature difference across the glass from the inside (hot) to outside (cold). This difference in temperature causes a stress in the material. Along with the imperfection you must have had, it was enough to cause the glass to fail. It's like putting ice water into a wine glass that just came out of a hot dishwasher... it'll break almost every time. If you have a crack in your windshield, you can keep this from happening while you are getting ready to replace it by starting with the temperature at warm for a bit and then amping up to hot. Can't guarantee, but it should help out.
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post #6 of 7 Old 11-28-2008, 12:41 PM
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mine did that too about 2 years ago. i was sitting in traffic with the defroster on and the heat all the way up and then POP!!! it sounded like someone had hit my windshield with a large rock but there was no one around me but cars. when i looked again there was a small crack near the center of the glass going straight up. it was slowly growing so i turned the defroster off and it stopped. by the end of the day sitting ing the sun it had grown the entire height of the windshield. i havent changed it in that entire time. no problems since.

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post #7 of 7 Old 11-28-2008, 04:33 PM Thread Starter
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thanks for all the good advise everyone!

upon further inspection, the bottom rail of the windshield frame is starting to rust, which, probably cause the stress crack by warping the frame. More and more moisture is getting inside the frame and its starting to bow the bottom rail out. Grrr...

oh well, now i can justify replacing the window frame and glass with a good condition used unit

thanks again!
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