faulty demister shattered my window

[bornozz]

Gday,

I am wondering if anyone has any knowledge or previous experience with this issue. it actually happened on my wife's astra at night when we were driving home from a friends with the kids in the back. I turned the demister on while we were saying goodbye in the driveway and the interior lights and headlights dimmed noticeably when it kicked in. we got about a K down the road and BOOM!.... back window shattered.

I thought someone threw a stubby or rock at first but there was no one around anywhere and when i pulled up i touched part of the glass that was intact with the demister electrodes on it and it was extremely hot, i mean hot like touching a frypan. I am thinking there's obviously too much current goin through the demister making it too hot..... but why? Is there an electrical component i can just replace which has failed to control the voltage?.... or is it just a simple circuit with a relay and switch and maybe the alternator is overcharging?

any ideas much appreciated.

in the meantime I have had the window replaced and pulled the demister fuse to prevent it happening again before I figure out whats goin on.

Cheers Ozz

[chief]

A short in the element lowering resistance, increasing current flow and subsequently increasing heat produced? Not even sure if that is possible with that style of element, but with any normal wound element that is what I would put it down too, and lots of heat introduced to cold glass is never good of course. Would probably expect a blown fuse in this instance though, but maybe the glass shattered before the fuse burnt out. Give the new one a go on a hot day when there is going to be less temperature differential and throw an ammeter into the mix to see what the new one is drawing.

[chimpboy]

Wow, that's wacky. I have never heard of that before.

Clearly it just got too hot, to be honest I can't see how that could have happened with a fault anywhere except in the heating element itself so your new window should sort it.

That's my feeling but obviously you need more than just my feeling to put the fuse back in

I would probably try this - put a new fuse in that's got a very low amp rating, then fire up the demister. The fuse should blow pronto. This is just a quick and dirty way of testing whether the fuse box is actually working to protect the circuit the way it should.

If the really weak fuse doesn't blow but the demister starts working then you have a wiring issue, someone has done something or something has farked up and the fuse is being bypassed. So turn it off and start working out what the wiring issue is.

If the really weak fuse does blow then I would just double check what the right size fuse is and put a new one in, and cross my fingers because it then seems almost definite that it was the old demister element that screwed you up.

I guess a hot spot rather than uniformly hot glass probably caused it. Weird though, it must have given you a bit of a jump start

[YNOT]

I cant help you a cause, but I have seen this happen a few times. From memory (it was about 10 years ago) all they did was replace the window. If it was me I would definately be doing the tests others have mentioned.

Tony

[bornozz]

thanks guys I will try the smaller fuse thing and poke around with a meter. And yeah chimpboy it did give us a jump.... went with a bang hey... poor kids in the back seat were virtually underneath it didnt know what was goin on... fortunately it didn't fall in on them it stayed mostly intact till we got home and some of it fell in when we shut the doors. The fella that replaced the glass said judging from the glue line it looked like the window had been replaced before (5 year old car we bought second hand), so hopefully I can figure it out so it doesn't happen again hey.

Thanks for your help everyone.

Ozz