Topic: 1977 CB550F hot fuse problem

[paulages]

As stated in the title, the bike is a 1977 CB550F. The fuse block melted at the main fuse, and I replaced it with a new one, but not before testing for shorts and unreasonable resistance. Between the ignition switch and the fuse, I found zero resistance on the ohmmeter. Same between the battery and the switch. On the harness side of the fuse, I found no shorts, but the owner of the bike says that the fuse pops over bumps sometimes. I did however go through the entire harness and found no poor connections or obvious shorts. The fuse is seeing normal voltage for any given RPM.

My question is this: Am I correct in assuming that the stated symptoms point to a short somewhere on the "fused" side of the harness?

[HondaMan]

As stated in the title, the bike is a 1977 CB550F. The fuse block melted at the main fuse, and I replaced it with a new one, but not before testing for shorts and unreasonable resistance. Between the ignition switch and the fuse, I found zero resistance on the ohmmeter. Same between the battery and the switch. On the harness side of the fuse, I found no shorts, but the owner of the bike says that the fuse pops over bumps sometimes. I did however go through the entire harness and found no poor connections or obvious shorts. The fuse is seeing normal voltage for any given RPM.

My question is this: Am I correct in assuming that the stated symptoms point to a short somewhere on the "fused" side of the harness?

Chances are from the PO's description: the fuse clips on the old one were corroded. This makes the ends of the fuse (and the solder that holds the element inside) get hot while running. Hitting a bump or similar shock then makes the fuse break: since it is carrying current at that moment, it usually also flares and looks like a short happened. Very common.

If you installed a new fuseholder, the problem will likely go away. If not, then look around the steering head  where the harness wiggles, or inside the headlight bucket at the edge where the harness enters the bucket, for frayed wire jackets.

[Psychonaut]

You can clean the fuse holders with 600 sandpaper wrapped around a screw driver. Took me about two minutes to do them all.  I would see if any of the connection  plugs get hot too. They are a little harder to clean. A hot spot is resistance from a poor contact.

[paulages]

HM-

Yeah, the old fuse holder is what I suspected, as it barely had continuity through the solder connections. Still, the new one heats up enough to have burned my thumb a little touching it yesterday...

[paulages]

... in fact, I just measured it to be 150° F. 

[TwoTired]

Do you know what current you are drawing through it?
Stock the fuse has about 10 amps through it which is 2/3 the rating.  Makes in warm but not, hot.  More current will make it hotter.
The clips must be clean where it contacts the fuse metal ends.  Even transparent oxidation will cause heating.
Next the clips should make firm contact on the fuse.  You should need a tool to pry it out of the clip. Contact pressure matters.
Finally, the clip needs to float in the housing so that they can self align themselves with the fuse for a full face contact.  If it is only contacting the "corners" of the metal, this severely reduces the contact area, which is like using too small a wire size, it heats up.  Melted fuse block often loose this "floating clip" capability.

Cheers,

[bollingball]

Has the fuse been replaced with one that is to long or to short?

[paulages]

Thanks TT, and BB-

I'll measure the amps drawn through it today. It obviously couldn't be pulling more than 15 amps through it or the fuse would break, but would say 14 amps constant be enough to heat it up so much? Everything on the bike is stock, so there are no extra loads on the main circuit. I cleaned up the connections when I installed the new fuse panel, but I'll redress the contact surfaces again for good measure. The correct size fuse is installed.

The previous fuse clip either got hot enough to melt the solder, or a loose solder connection caused the clip to heat up enough to melt the plastic. I don't know what type of solder Honda used, but at the lowest it would have to be in the 350° F range to start melting, which the new fuse is far from. The hot spot however, is on the top ("supply" side) of the fuse, which is the same side that was loose on the old panel.

[TwoTired]

The fuse element melts when  enough current passes through it to generate enough heat.  It heats at lower currents, too.  Just not enough to reach it's melting point.  With enough external heat applied to it, it will melt with NO current flowing through it.
Therefore, the element heats via Both current AND external heat applied.

Fuse clip corrosion or contact degradation creates resistance, and current flowing through that resistance generates heat proportional to the resistance encounters in the the path.  So, the melting point of the fuse link is the combination of the current flowing through the link AND resistance AND current flowing through all those elements.

My stock bikes keep the the 15 A fuse and the fuse clip well below the melting point of the plastic.  My thumb will sense quite warm, but I can keep thumb contact without pain.

As I said, the clip must be able to self align for full contact and the contact pressure must be high.  Heated fuse clips can lose their temper and no longer provide the requires pressure, fuse AND clips.

Fuse clip cleaning should be a routine maintenance procedure, like every 5-10 years.  Or, permanent and debilitating damage may result.

Honda soldered the fuse clip terminals before inserting them into the clip holder.    Melting solder will definitely melt the fuse block plastic, and almost certainly destroy the fuse clip self alignment feature of the design, effectively rendering the fuse block useless.

Cheers,

[paulages]

I cleaned the clips a little more, pinched the tips together a little more, and perhaps most importantly I wiggled the fuse a bit when installing it to make sure that the clips were loose enough in the panel to be settled on the fuse.

It doesn't seem to want to heat up past 113° F now, which seems satisfactory to me. The amp draw at the fuse is 13 amps when measuring at the clips, and 10-11 amps when measuring at the fuse tips. That seemed like a fair amount of resistance through the fuse, but when testing the fuse with the ohmmeter, I get zero resistance. I got nearly the same specs when measuring the amps through the main fuse on a CB750 K0, so I'll assume the variance has more to do with the testing than a problem with the fuse.

Problem seems to be solved, and I think the original problem was in fact a poor solder on the old fuse panel. Thanks, Lloyd!

[Tomhowe]

You have to clean the fuse too. If you put one in with greasy fingers you will have the same problem.