From: silver@xrtll.uucp (Hi Ho Silver)
Subject: Foglight query - summary
Organization: What you won't find on my desk.
Date: Sat, 23 Jan 1993 18:31:54 GMT

   A couple of weeks ago, I wrote:
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   A little over a year ago, I installed Bosch fog lights on my
car.  After a week, one of them burned out; I replaced it.  After
another week or so, the fuse blew.  Further investigation showed
that the replaced bulb's resistance had become zero.

   At this point, I checked all wiring and could find no short
circuits or flaky wires.  I replaced the bulb and fuse and the
lights worked without problems for about a year.

   Recently, the other light burned out.  I replaced it.  Within
a week, it burned out again, and took the fuse with it.  I
replaced both.  Within ten minutes, both bulbs' resistance
went to zero, blowing the fuse yet again.

   I just spent an hour checking resistances between every pair
of points I can think of, and everything still checks out fine.
There's plenty of electrical tape wrapped around all exposed
live metal inside each fog light enclosure.  I can't figure
out any reason why I would have so many problems.  The first
two times I can put down to coincidence, given that things
settled down and everything operated normally for a year.
But this is just too much to be coincidence.

   I have two questions ...

1.  I know why bulbs would burn out and become an open circuit.
    I have never before seen a bulb that went the other way and
    became closed.  What could cause this?

2.  How do I find and fix whatever is wrong?

(Oh, and a third one - why me?  :-)
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   Here are the replies I received.  Thanks to those who sent
suggestions.
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From: zlau@arrl.org (Zack Lau)

Unless they are 6 volt bulbs and you have two wired in
series, I don't see how a short could cause the problem
you describe.  Shorts would prevent the bulbs from getting
any electricity--which would not cause the bulbs themselves
to short.  What you are describing sounds like a high voltage
transient on your power system.  The ignition system does 
generate such high voltages, but the battery should filter 
them out.  You might try running wires directly to the battery,
if you aren't already doing so, to get power for your lights

Zack Lau  KH6CP/1

Internet: zlau@arrl.org           "Working" on 24 GHz SSB/CW gear     
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---------------------------------------------------------------------------
From: zlau@arrl.org (Zack Lau)

Well, its my opinion that looking for shorts isn't going to
find anything.

I noticed the relay--the coil of a relay can generate significant
voltages, just like the ignition system of a car.  A cure is to
place a reverse biased diode across the relay.  

Interestingly, we have a pair of lights on a power conditioner
here at work.  They are fed through a filter coil, I believe.
Whats interesting is that they often fail simultaneously--you
hit the switch and the both burn out at once.
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From:	eliot@dg-rtp.dg.com (Topher Eliot)

I've heard that if your mounting is a little loose so that the light can
jiggle back and forth, but fetches up hard against something at the end of
it's range, then the filament will be subjected to very high g forces and
will break due to this.  It doesn't have to travel far at all, just be able
to rattle the slightest bit.  Maybe your whole bumper rattles?

-- 
Topher Eliot             Data General DG/UX Systems Administration Development
(919) 248-6371                                             eliot@dg-rtp.dg.com
Obviously, I speak for myself, not for DG.
misc.consumers.house archivist.  Send mail to house-archive@dg-rtp.dg.com
"Ask me about my vow of silence."
-- 
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