05-11-12, 01:22 PM
This sounds like a poor connection, probably to one of the blades of the fuse.
Check the connectors are clean and tighten them... it should require a noticeable amount of force to shove the fuse in. If that's not the problem then the cable, where it joins to the connector may have corrosion or a few broken strands, in which case you'll need to remake the end.
The problem is indeed too much resistance where there shouldn't be.
As others have been pointing out, that will result in less power... but only from an overall point of view. If the contact resistance to one end of the fuse is, say a tenth of an ohm, that will lose a volt across it if the current is 10 amps... the lights will then be working from a volt less, but the current won't reduce by much as a result... even if they were simple resistors it would only be a 9% drop (in fact they're non-linear resistors... they'll be a little bit cooler and yellower so their resistance will also have dropped a bit).
So the headlamps are dissipating a little bit less power, but the poor old fuse is now trying to get rid of 10 watts (resistance x current squared = power) as a result it gets hot enough to melt the plastic.
Check the connectors are clean and tighten them... it should require a noticeable amount of force to shove the fuse in. If that's not the problem then the cable, where it joins to the connector may have corrosion or a few broken strands, in which case you'll need to remake the end.
The problem is indeed too much resistance where there shouldn't be.
As others have been pointing out, that will result in less power... but only from an overall point of view. If the contact resistance to one end of the fuse is, say a tenth of an ohm, that will lose a volt across it if the current is 10 amps... the lights will then be working from a volt less, but the current won't reduce by much as a result... even if they were simple resistors it would only be a 9% drop (in fact they're non-linear resistors... they'll be a little bit cooler and yellower so their resistance will also have dropped a bit).
So the headlamps are dissipating a little bit less power, but the poor old fuse is now trying to get rid of 10 watts (resistance x current squared = power) as a result it gets hot enough to melt the plastic.