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Discussion Starter #1
I have a problem. 79 bronco, all stock. Its cold here in MI and the heater works good and would work better if it was not for the cold air coming in around the passenger window. (needs new track rubbers seals) Here is the problem. The windshield is clear, no frost from the night before, sun up or sun down, morning or night does not matter. I can drive about 7 or 8 block and the INSIDE of the windshield will frost over so heavy that I can not see out of it. I can use the ice scrapper on the inside. It will frost if I have the heater on. If I leave the heater off it does not frost. WHY?
 

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I have a problem. 79 bronco, all stock. Its cold here in MI and the heater works good and would work better if it was not for the cold air coming in around the passenger window. (needs new track rubbers seals) Here is the problem. The windshield is clear, no frost from the night before, sun up or sun down, morning or night does not matter. I can drive about 7 or 8 block and the INSIDE of the windshield will frost over so heavy that I can not see out of it. I can use the ice scrapper on the inside. It will frost if I have the heater on. If I leave the heater off it does not frost. WHY?
Moisture contained in the air.

More water vapor can be held in "warm" air than "cold" air. That's why it's better to defrost/defog a windshield with the heat on and A/C on... that way moisture is pulled from the air before it is heated... thus preventing what you're experiencing.

There's a possibility you have a leaky heater core... or condensation inside the heater box that's getting pulled up and onto the windshield. Has it ever done this before?
 

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my 78 does same thing

the snow gets sucked in the cowl air intake and is drawn over the warming heater core and make the frost
mine has doe that the 23 winters I have driven it in the Windsor area
 

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Discussion Starter #4
mangonesailor -- My bronco does not have a/c and did not come with it. I have not needed to add antifreeze to my truck.

trustyrusty -- I like your take on the snow theory. It does make sense. Something interesting, my 78 bronco does not do this but my 79 does. Same winter, same driving. Only difference is the 78 has a 460 and the 79 has a 351m.
 

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mangonesailor -- My bronco does not have a/c and did not come with it. I have not needed to add antifreeze to my truck.

trustyrusty -- I like your take on the snow theory. It does make sense. Something interesting, my 78 bronco does not do this but my 79 does. Same winter, same driving. Only difference is the 78 has a 460 and the 79 has a 351m.
if it is light powdery snow, I get snow out of the vents before the frosting happens

my kids think it is funny snowing inside the truck
i have a non a/c truck too
I wonder if the a/c heater boxes location,it may not have the issue?
 

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mangonesailor -- My bronco does not have a/c and did not come with it. I have not needed to add antifreeze to my truck.
That doesn't rule out a seeping heater hose connection or core, but in any case maybe you can de-snow your cowl with a hair-dryer with a LONG extension cord every morning...

...or throw a tarp over your cowl/hood over night so when you head out into the sunrise there won't be any snow in your cowl. Re-tarp when you reach your destination.
 

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When I lived in Alaska, we would have that problem. Go down the road and freeze over. Stop for a bit to thaw out.
 
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