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burned fuse

  • josegumby
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12 Aug 2019 10:39 #32747 by josegumby
burned fuse was created by josegumby
Hello, DIY'er here. Not necessarily looking for advise, but thought I'd share a couple pictures. I was working on my buddy's 98 ford mustang yesterday, trying to find a parasitic drain that is killing the battery overnight (this car has a whole mess of electrical issues!). I came across this burned up "fuse" in the fuse panel. It is fuse 17, to the heater/AC/blower motor, and it "was" a 30 amp fuse.

I'm not sure if the fuse got so hot and brittle that the plastic part of the fuse disintegrated away from the metal filament inside, or of the plastic housing of the fuse block expanded from the heat and crushed the plastic part of the fuse to smithereens. Looking closely at it, it almost appears that the "loop" of the filament is pushed in on itself in a couple different places making it "more" than a 30 amp fuse now, since there are multiple paths for electrical flow.

An interesting side-note: There is a 25 amp fuse in the under-hood junction box that controls the driver's power seat and lumbar support. This fuse is blown, yet the power-seat controls still work. I suspect the previous owner had the 25 amp under-hood fuse blowing due to a short, and they wired the power seats into the fuse 17 harness under the dash to the blower motor. We got rained out here in sunny southwest Florida yesterday, so sadly didn't get to confirm anything yet.
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  • Noah
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13 Aug 2019 16:58 #32783 by Noah
Replied by Noah on topic burned fuse
Yikes! That thing got hot. I wonder if someone rigged it like that when they ran out of fuses.
Thanks for sharing!

"Learn, apply, repeat."

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18 Aug 2019 23:32 - 18 Aug 2019 23:34 #32941 by Paul6004
Replied by Paul6004 on topic burned fuse

josegumby wrote: I'm not sure if the fuse got so hot and brittle that the plastic part of the fuse disintegrated away from the metal filament inside, or of the plastic housing of the fuse block expanded from the heat and crushed the plastic part of the fuse to smithereens.

That's a sure sign of an overloaded fuse or bad contacts of the terminals connecting it to the fuse box.
I've played around with fuses before to see what their limits are and took a 10 amp fuse slowly to 20 amps before it blew.
If there is a dead short there is a nice clean break in the fuse wire. But if the circuit has a constant current draw that isn't enough to
blow the fuse the fact that the fuse wire is the thinnest part of the circuit means there is a voltage drop that generates heat, and over a long
period of time the heat keeps building up and cooks the plastic and everything around it.
Once the terminals get hot enough they get softer, have less grip, and cause more voltage drop[/heat].
I've found that in most cases a 30 amp fuse, with a constant 30 amp current draw over a long period of time, will eventually melt - depending on
manufacturing quality etc.
Last edit: 18 Aug 2019 23:34 by Paul6004.

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