Went to hop on my '75 CB400F this morning and no juice was flowing. Turns out the main fuse was blown and the fuse box melted a bunch. Here are the symptoms.
- main fuse heats up quickly and begins melting the fusebox ( until it blows I suppose )
Lot's are confused about fuse operation. Here is more analysis than most want to read about.
The fuse element generates heat while passing current and runs hotter as the current approaches its fail point. A 15 Amp fuse running at 2/3 its rating will be warm to the touch. (but, you con hold your thumb on it without discomfort) A fuse that blows from passing current above its rating will usually melt near the center of it's length.
However, if you pass a current through any resistance it generates heat. Lamps heat due to this property, as the element is resistive. Resistive heating is often the root of fuse box melting/fuse melting issues.
Fuse box plastic doesn't melt with normal fuse operation or even when the fuse is running close to its parting temperature. It will melt if something else is adding to that heat. Often it is the corrosion/oxidation between the fuse and the fuse clips. This oxidation is resistive, passing a current through it generates heat. The heat generated adds to the fuse's normal run temp and causes the fuse to melt near or under it's end cap.
I've also seen them get hot enough to melt the solder behind the fuse box where the wires are attached to the fuse clip metal. (usually after someone has replaced with ever higher fuse ratings, as an ill-conceived band-aid, rather than address the real problem of fuse clip oxidation.
Anyway, once the fuse clips themselves go through higher temperature cycles, the metal loses its spring tension, and the lack of contact pressure also increases the contact resistance leading to more heat generation.
People also "upgrade" their lighting with components that draw more power through the fuse (increasing it's inherent run temp, as well as heat generated through any oxidation at the contact points.)
Another mistake frequently made is that people will use the wrong length fuse, leading to incomplete contact at the fuse ends. Reduced contact area also leads to higher contact resistance, and the resultant higher heating. All these detrimental effect are additive in creating fuse melting problems.
I often wonder why people don't simply take a 44 magnum to their fuseboxes as they alter it's functional behavior. It would save time in the long run.
If your fuse box is melted, it needs to be replaced. The clip normally self align, unless they are embedded in melted plastic. You know what I'm going to say, right? If the clips can't align themselves with the fuse ends, it reduces the contact area, which leads to higher resistance and more heat added to the problem.
The sad thing is, all this misery can be prevented by simply keeping the fuse to fuse clip contact area clean, bright, and shiny, which maintains a low resistive contact between fuse and fuse clip.
People pay attention to chrome on the bike that gets dull. Then spend time polishing that metal or other bits of aluminum that's become unsightly. But, will never think to polish fuse clips so they will function as intended.
If your fuse is running too hot to the touch (warm is ok) with all you accessories turned on, then you probably have fuse clip oxidation. Oxidation can be stealthy, as in transparent. It still needs to be polished to bare metal.
Finally, the fuse clip should grab onto the fuse ends so tightly that you cannot remove it with you bare hands/ fingers. You should have to pry it out, or use the proper fuse puller tool that allows a firm strong tug on the fuse to wrest it from the clip's clutches.
Or...
You could have a chafed wire that is making contact with the frame somewhere. But, that melted fusebox is still going to need replacing.
Cheers,
