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Sherlock Ohms

The Failed Fuse Fooled Me

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Jim_E
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Platinum
You're not the only one!
Jim_E   8/7/2013 9:01:50 AM
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If it makes you feel better, you're not the only one who has misdiagnosed a blown vehicle fuse.  I was fooled by a newer mini ATM type fuse which didn't look blown, but really was.  Like you, I now test them with my meter.

 

GTOlover
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Platinum
Re: You're not the only one!
GTOlover   8/7/2013 9:44:12 AM
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I will ditto that! However, being a little more thick-headed I thought the chance of a fuse burning out like that again would be slim. So in my most rebelious attitude of youth, I continued the practice of looking at fuses instead of testing them with a meter. Then one day, (I think I was working on my riding lawn mower) the same situation happened. After a couple hours trying to get the garden tractor to start, I got the meter and tested the 30 amp fuse. I was humbled and now grab the meter whenever I suspect a fuse!

What can I say, I am a slow learner of the obvious!

Rob Spiegel
User Rank
Blogger
Re: You're not the only one!
Rob Spiegel   8/7/2013 10:48:08 AM
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This proves a hunch that I've long held that you can't tell everything my looking at a car fuse. I've looked at a ton and thought, "I can't really tell if this is OK or not."

tekochip
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Platinum
Re: You're not the only one!
tekochip   8/7/2013 10:51:30 AM
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We had a product with a 1/10A fuse.  There was no way to see the element without a magnifying glass, so a quick continuity test was the only way.  When I ran the service department the first thing I always did was check the fuses and power traces for continuity, then pull the fuses to make certain that the customer hadn't replaced a 1/10A with a 20A. 

 

It seems that the customer would always install a larger fuse until the wiring harness burned.  As soon as the Magic Smoke was let out, though, they would send the unit in to have the smoke refilled.

GTOlover
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Platinum
Re: You're not the only one!
GTOlover   8/7/2013 1:21:05 PM
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Is the magic smoke sold by volume or weight? :-)

rjnerd
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Iron
Re: You're not the only one!
rjnerd   8/7/2013 5:05:31 PM
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I remember fuses like that.  A data aquisition system I used to use had its inputs protected with fuses that had possibly lower current ratings.  Like yours, you couldn't really see the wire, but checking had its own issue -   One problem quickly appeared, back in the days of the simson 260, you ran a good chance of blowing the fuse with the ohmmeter's battery.  If you used a vtvm, that was ok, it had high enough impedance, that the fuse would survive, but there were some that still swore by their old friends.


We wound adding a fuse test fixture to the front of the thing -- it was just a holder with a 1k resistor in series, to keep the current down.

tekochip
User Rank
Platinum
Re: You're not the only one!
tekochip   8/7/2013 8:08:16 PM
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Yup, happened to me too.  I think it was a Huntron Tracker, or something like that would take the fuse out!

Contrarian
User Rank
Gold
Not just fuses, either
Contrarian   8/8/2013 9:26:41 AM
Had a problem once when you put on the brakes, the headlights came on, and vice-versa.  I scoured the wiring diagram looking for any possible path and could not see how it could be possible, short of a physical cross connect.  I started checking for shorts in the bulb sockets and that's when I found an 1157 dual filament bulb that had a broken filament that was touching the adjacent filament and would readily provide enough current across the brake circuit to the running light/headlight circuit and itself would still illuminate.  You don't normally suspect a working bulb so it missed initial inspection.

TexasTJ
User Rank
Iron
Smart vs. wise
TexasTJ   8/8/2013 9:52:18 AM
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I can't remember where I heard this, but I've tried to impart it to my daughter:
"A smart person learns from his mistakes, a wise person learns from the mistakes of others."  You've given us a chance at wisdom.

Analog Bill
User Rank
Gold
Re: You're not the only one!
Analog Bill   8/8/2013 10:39:25 AM
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Over my career (nearly 50 years), I've seen this dozens of times. Normal temperature cycling of fuses often causes a microscopic break in the tiny filament in the fuse. The take away lesson is a very general one: "Don't assume anything ... especially true with a visual inspection!".  -- Bill Whitlock, analog circuit designer (and former drag-race car builder)

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