Topic: 350F - Another blown fuse. Are they supposed to get hot?

[fmctm1sw]

I've put about 900 miles on my '72 350F after getting it back on the road.  It has run fantastic over the majority of those miles (after I cleaned and POR-15'd the tank).  About 200 miles ago I was riding and my main fuse blew.  The only ones I could find at Autozone that size were rated at 14 amps, so I keep some under my seat.  Today, I blew another one.  It was hot to the touch.  Are they supposed to get hot like that?  All the electrics work fine, save the high beam light that is always on.  I rode it about 60 miles after that, no problem.  I saw it recommended in another thread to wire in car type (whatever they're called) fuses.  I noticed neither fuse had the middle melted.  Is it vibrating to destruction?

[TwoTired]

I've written about this is several prior threads.

Pass current through a resistive element and it gets hot.  The internal fuse link is a calibrated resistance that melts when a certain current is passed through it.  Below that current, it will remain intact and conductive but will run warm and it will run warmer as the current increases toward its "blow" rating.

In a perfect world all the heat would come from the fuse link itself.  But, there is also resistance between the fuse ends and the fuse clips.  This resistance gets worse as the materials oxidize, and they will, with time and exposure to the atmosphere.  Passing current through this resistance generates heat, too.  The heat can change the temper of the metal which in this case also provides a spring for the clip in order to retain the fuse.  The spring pressure can also lower the contact resistance.  Lastly, the contact area between fuse clip will become resistive as it reduces in area (usually due to incorrect length fuses).

The main fuse usually runs at about 2/3rds of its rating.  Which is warm to touch, but not hot.  If clips are making high resistive contact, the heat from that will add to the fuse heat and then melt the fuse element near the fuse ends, where the added heat is introduced through conduction.

So, the correct length fuse is important.
The fuse and the clip contact area must be clean/clear of oxidation (regardless of whether the corrosion is transparent or not).
And, the clips should hold on to the fuse so that a tool is required to remove it.  Bare fingers shouldn't do it, a clear sign that there are clip issues.

If you have added extra electrical load to the bike like higher wattage headlights, three ohm coils, and a Dyna-S, you have increased the amperage flowing through the fuse and it will run warmer and closer to it's "blow" temp simply because of that.  And, of course, more amperage through any fuse clip oxidation present, will also generate more heat.

Normally, the SOHC4 draws about 10 amps through a 15 amp fuse.  High watt headlights, dyna and low ohm coils can add more than 3 amps to this norm. 

Still surprised at "fuse" failures?

Cheers,

[Trevor from Warragul]

Not to mention all the corroded connectors in the headlamp shell...

[B.O.X.N.I.F.E.]

What type fuses are they, ie AGC, SFE...??

I just blew one and did a long push down the highway. I tried Bumper to Bumper, Napa & my well stocked local hardware store and could only find the correct length fuse in 14A.

and for the original poster, I think you're talking about blade fuses. Wasn't somone on here making a conversion box for these?

[Skonnie Boy]

I've had similar problems of late.  I've found 15A fuses, no prob, but can't seem to source 5A or 7A fuse at any of the auto parts stores around here.  I have been using 4A and 7.5A fuses, and my bike has died 3 times in the last week.  Coincidence?

Sorry, not trying to hijack threads, but it seemed similar enough to post.