How warm should fusible links get?

On the '94 Safari/Astro there is a connector with 4 fusible links coming
out of it that plugs into a distribution block on the firewall behind
the battery. The C and D link contacts got hot enough to melt the
plastic around them and the C circuit opened, causing the truck to stall
and a bunch of accessories not to work.

I just replaced the connector, splicing in two of the fusible links that
came with the replacement and transferring the two undamaged ones to the
new connector body.

Everything works again, but I have some questions:

Is it normal for fusible links to become warm under heavy load? How warm?

With the engine running, headlights, dome lights, reading lamps, and
blower on, it took about 5 minutes before I could sense that the C and D
fusible links were warm to the touch. After 10 minutes it was a little
warmer but still not anything I'd be immediately worried about. Still, a
couple of hours driving on a hot night with the headlights and other
accessories on could be another matter?

Should I be concerned?

Also, are their any special tools, probes, shunts, or other tricks that
would make checking current draw through different fuses easier?

Thanks,

Mike D.

M.G. Devour wrote:

> Also, are their any special tools, probes, shunts, or other tricks that
> would make checking current draw through different fuses easier?

Well, all that comes to mind is getting a good digital multimeter,
measuring the resistance of the link, then measuring the voltage across
the link, then calculating the current.

>
> Thanks,
>
> Mike D.

When I see that, it implies a bad connection causing an arc and heat.

The fuse links will warm up because they are a couple sizes smaller than
the circuit they are protecting, that's why they have a rubber coating
usually. Hot enough to melt is bad news.

How is the heat now after it has been taken apart and hopefully cleaned?

The voltage drop across the running connection can tell a lot also.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

"M.G. Devour" wrote:
>
> On the '94 Safari/Astro there is a connector with 4 fusible links coming
> out of it that plugs into a distribution block on the firewall behind
> the battery. The C and D link contacts got hot enough to melt the
> plastic around them and the C circuit opened, causing the truck to stall
> and a bunch of accessories not to work.
>
> I just replaced the connector, splicing in two of the fusible links that
> came with the replacement and transferring the two undamaged ones to the
> new connector body.
>
> Everything works again, but I have some questions:
>
> Is it normal for fusible links to become warm under heavy load? How warm?
>
> With the engine running, headlights, dome lights, reading lamps, and
> blower on, it took about 5 minutes before I could sense that the C and D
> fusible links were warm to the touch. After 10 minutes it was a little
> warmer but still not anything I'd be immediately worried about. Still, a
> couple of hours driving on a hot night with the headlights and other
> accessories on could be another matter?
>
> Should I be concerned?
>
> Also, are their any special tools, probes, shunts, or other tricks that
> would make checking current draw through different fuses easier?
>
> Thanks,
>
> Mike D.

Mike Romain wrote:
> When I see that, it implies a bad connection causing an arc and heat.

Yes, the terminals were the source of the heat. Either fatigue or
borderline overload (or borderline specifications by the designers)
caused them to finally get hot and melt the adjoining plastic, damaging
one circuit and causing another to disconnect, which is when the
problem became visible.

> The fuse links will warm up because they are a couple sizes smaller than
> the circuit they are protecting, that's why they have a rubber coating
> usually. Hot enough to melt is bad news.
>
> How is the heat now after it has been taken apart and hopefully cleaned?

Hopefully that's the good news. I replaced the damaged connector and
spliced in two of the four fusible links that came with the
replacement. So everything is now clean and sound.

The two links that failed get just a little bit warm under a full
load.for 10 minutes. I'm a little uneasy about what would happen over a
long night-time drive with lights and blower fan running, though.

So I'm asking what kind of heating others think I can expect from a
properly performing fusible link, versus one that's running slightly
too hot and is going to fail again. If it's the latter, I need to see
what's pulling too much.

> The voltage drop across the running connection can tell a lot also.

The new connections seem to be running cool. Until I see otherwise, I'm
more worried by the heat being generated in the link and conducted to
the terminals. If some heat is normal then I'll just keep a close eye
on it for a few days and check the connector often for excess heat
buildup.

If they're never supposed to be noticably warm, then I have some
troubleshooting to do on the load side.

Thanks for responding Mike. Any input would be appreciated.

Be well,

Mike D.

> "M.G. Devour" wrote:
> >
....
> > I just replaced the connector, splicing in two of the fusible links that
> > came with the replacement and transferring the two undamaged ones to the
> > new connector body.
> >
....
> > Is it normal for fusible links to become warm under heavy load? How warm?
> >
> > With the engine running, headlights, dome lights, reading lamps, and
> > blower on, it took about 5 minutes before I could sense that the C and D
> > fusible links were warm to the touch. After 10 minutes it was a little
> > warmer but still not anything I'd be immediately worried about. Still, a
> > couple of hours driving on a hot night with the headlights and other
> > accessories on could be another matter?
> >
> > Should I be concerned?

On Fri, 12 Jan 2007 14:06:55 -0500, "M.G. Devour" >
wrote:

>On the '94 Safari/Astro there is a connector with 4 fusible links coming
>out of it that plugs into a distribution block on the firewall behind
>the battery. The C and D link contacts got hot enough to melt the
>plastic around them and the C circuit opened, causing the truck to stall
>and a bunch of accessories not to work.
>
>I just replaced the connector, splicing in two of the fusible links that
>came with the replacement and transferring the two undamaged ones to the
>new connector body.
>
>Everything works again, but I have some questions:
>
>Is it normal for fusible links to become warm under heavy load? How warm?
>
>With the engine running, headlights, dome lights, reading lamps, and
>blower on, it took about 5 minutes before I could sense that the C and D
>fusible links were warm to the touch. After 10 minutes it was a little
>warmer but still not anything I'd be immediately worried about. Still, a
>couple of hours driving on a hot night with the headlights and other
>accessories on could be another matter?
>
>Should I be concerned?
>
>Also, are their any special tools, probes, shunts, or other tricks that
>would make checking current draw through different fuses easier?
>
>Thanks,
>
>Mike D.

They are supposed to be four gages smaller then the wire size they are
protecting so at full circuit load I would expect them to get warm.

Ashton Crusher wrote:
> mdevour wrote:
> >With the engine running, headlights, dome lights, reading lamps, and
> >blower on, it took about 5 minutes before I could sense that the C and D
> >fusible links were warm to the touch. After 10 minutes it was a little
> >warmer but still not anything I'd be immediately worried about.
> >
> >Should I be concerned?
>
> They are supposed to be four gages smaller then the wire size they are
> protecting so at full circuit load I would expect them to get warm.

Thank you, Ashton. I've driven on several local trips now with lights
and fan and other loads. So far I haven't felt anything more than the
faint warmth in the C and D circuits' links and none in the connector
body. So it looks like the job is done for now. I'll just check that
connector from time to time over the next few months.

Be well,

Mike D.

Ashton Crusher wrote:

> They are supposed to be four gages smaller then the wire size they are
> protecting so at full circuit load I would expect them to get warm.

Dumb question; why don't they just use regular fuses like God intended?

The wire with the rubber insulation can take a fair bunch of power
surges without failing. Surging is bad for normal fuses, they
crystallize and fail with time. Lots of circuits that come after the
fuse link wire do have normal fuses to blow. It takes a major short or
something like reversing the battery cables in a boost to cook them usually.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

z wrote:
> Ashton Crusher wrote:
>
>> They are supposed to be four gages smaller then the wire size they are
>> protecting so at full circuit load I would expect them to get warm.
>
> Dumb question; why don't they just use regular fuses like God intended?
>

On 16 Jan 2007 11:23:34 -0800, "z" > wrote:

>
>Ashton Crusher wrote:
>
>> They are supposed to be four gages smaller then the wire size they are
>> protecting so at full circuit load I would expect them to get warm.
>
>Dumb question; why don't they just use regular fuses like God intended?

In addition to what Mike said, they do make a fusible link that's more
like a fuse - it has a holder and element and would therefore be much
easier to replace. But if you think about it from the automakers
view, the cheap 4 gages smaller fusible link, is very easy to install
and, well, cheap. And for the most part they will last the life of
the vehicle.

M.G. Devour wrote:
> On the '94 Safari/Astro there is a connector with 4 fusible links coming
> out of it that plugs into a distribution block on the firewall behind
> the battery. The C and D link contacts got hot enough to melt the
> plastic around them and the C circuit opened, causing the truck to stall
> and a bunch of accessories not to work.

Did you determine why the contacts got so hot they melted the plastic?

>
> I just replaced the connector, splicing in two of the fusible links that
> came with the replacement and transferring the two undamaged ones to the
> new connector body.
>
> Everything works again, but I have some questions:
>
> Is it normal for fusible links to become warm under heavy load? How warm?

I would worry more about what caused the connection to get so hot (not
warm) that it melted the plastic.

>
> With the engine running, headlights, dome lights, reading lamps, and
> blower on, it took about 5 minutes before I could sense that the C and D
> fusible links were warm to the touch. After 10 minutes it was a little
> warmer but still not anything I'd be immediately worried about. Still, a
> couple of hours driving on a hot night with the headlights and other
> accessories on could be another matter?
>
> Should I be concerned?
>
> Also, are their any special tools, probes, shunts, or other tricks that
> would make checking current draw through different fuses easier?
>
> Thanks,
>
> Mike D.