On Sat, 20 Nov 2010 08:50:26 -0800, Smitty Two
wrote:

In article ,
wrote:


I have seen many fuses "just fail" over the last 50 years, both
automotive and "mains". And like incandescent (filament) lamps, I've
seen them "mend" themselves temporarily - sometimes several times,
before final total failure.
An intermittent "failed" fuse can really be a bugger when
troubleshooting an intermittent electrical problem.

Failed fuses don't mend themselves. However, a common automotive failure
is intermittent connections behind the fusebox. Pulling and replacing a
fuse restores the connection, for a minute, day, or week.


I beg to differ. The fuses can arc weld themselves back together and
work for a while, then re-fracture. Depending on temperature changes
etc, this MAY happen more than once, but even once is a rare
occurrence. Rare, but it happens.

On most current automotive fuse boxes there are no "connections"
behind the fusebox that would be influenced by removing the fuse. -
and replacing the fuse invariably solves the problem - often for the
remaining life of the vehicle.

Same thing happens in fused distribution panels - both with plug
fuses (the round glass screw-in common fuse) and with cartridge fuses,
on occaision. Bump the fuse, or have the temperature go up a few
degrees, and the fuse makes contact again, allowing light loads to
operate. Sometimes they will "weld" themselves and not cause a problem
again for several days.
Looking at the fuse there is no sign that there is anything wrong -
but sometimes if you tap the fuse in the dark you will see a faint
blue arc inside the fuse body, and a light connected to the circuit
will either flicker off, or faintly flicker on..

In automotive systems., I have seen it much more often in the old
tubular fuses than it the ATO type - but I've seen it with an ATO as
well. Nothing like seeing the failure LED on an indicator type fuse
flicker randomly in the fusebox when you are trying to trace down an
intermittent electrical problem. Since most automotive fuses are NOT
indicator fuses, the cause is not often as obvious as the intermittent
radio problem on that car.