Mechanics

REAR DEFROSTER BREAKING REAR WINDOW ?

1995 BMW 530 • 135,000 miles

'95 530iA
135,000 mi
No accidents

Went out one morning to find the rear glass in
my 1995 530i shattered. No sign of impact and
no possibility of mischief. The window was replaced
with BMW glass by the tech who does all the work
for the local BMW dealership. The work was done
at the company's shop, not through their mobile
service. As soon as the car was ready they let
me leave with the caveat of not opening any
windows while I drove home. I followed their
instruction and after arriving home, I parked
the car.

The next morning I went out to the car and all was
well. Didn't start the car but as I closed the front
door my heart sank. The window shattered again.
BTW, I am the original owner and the car has never
been in an accident.

After waiting two weeks for the second replacement
to arrive from Germany, I had the same tech come
to my home. After telling him all of the above, he
closely inspected the window frame. Finding all to
be well he said he had only encountered this two
other times in 28 years. Both times it was due to a
faulty rear window defroster. He did not connect the
wires to the window. This was three weeks ago and
the window is still intact.

I do not remember turning the defroster on before
either breakage as it was late Summer/early Fall
Although I do not remember doing so, it is possible
I may have briefly pressed the defroster switch
when turning on the A/C but if so it would have been
turned off within a few seconds.

Could a faulty switch, timer or relay overheat
the window cracking it?

Is there a temperature sensor to prevent
overheating the glass?

What puzzles me is how the glass could overheat with the defroster switch in the off position (indicator light off).
Perhaps fused contacts in the relay?

TIA

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BMMMW
October 20, 2011.




I would think it unlikely the defroster is the culprit here, but stranger things have happened. In the 2nd paragraph you state the glass "shattered again" Please descibe the events in more detail.

Dr. Hagerty
Oct 26, 2011.


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