Topic: Melting Fuse (main) on a 76 CB550F

[snmavridis]

Hey all, I'm trying to diagnose why my main is melting. Black wire gets hot and the fuse block melted. First things first, yes I've checked my grounds. All my grounds read about .3ohm max. what I'm finding though is that the brown/blue (gauge light hot with LEDs) and my black (switched on power, connected to all the other stock bells and whistles) are reading 2.8ohm and there's a pretty significant voltage drop from hot red on the battery to black and brown/blue (about 2V). Can you guys help me walk through this? I've been trying to be methodical but after 10+ hours, my methods don't seem to be working.

[Stev-o]

First off, welcome from Texas.

Sounds like your wiring is shorting out somewhere.  I would start off by checking all the wiring for breaks in the covering. Then disconnect, clean and reconnect each connection throughout the harness.

Electrical issues can be a real #$%* to diagnose, but once found, can be easy to repair [dont bother asking how I know!]

[snmavridis]

Hey me too! Profile says Georgia but I recently relocated to DFW. Small world! If you've got any shop recommendations on places that carry vintage bike stuff, send me a suggestion!!! I'm new here!

I'll give that a go, my only hesitation is that it's all NEW wiring. New harness, new headlight, new turn signals- all mimicking stock of course. So terminals are clean. On the other hand I'm about at the point where I'm just going to disconnect EVERYTHING tied to switched on and go step by step to see where my issue might lie. Just hoping for some sort of wisdom to make it easier than that!

Follow-up, am I correct in assuming that if my wiring is right, there should be much lower resistance from black to ground? 2 ohms is pretty bad if my intuition is correct.

[maxheadflow]

If you have 0.3 ohms across you fuse block and the system draws say 10 amps you'd have 10^2 x 0.3 = 30 watts dissipated in the fuse block.  Fuse would burn up.

[snmavridis]

Oh god that's alot. Gotta remember first principles... I'll give them another go and see if I can ground out better.

[Don R]

I'd suspect a loose or a corroded fuse holder. Common problem.

[aclaytonb]

A wise man told me that it was always the last thing that you %#@!#$ with.  Its always been true. 

Most recently, it was an electronic ignition (recently tinkered with) wire that melted to the exhaust pipe and shorted the main fuse.  I had to unplug each component until I had isolated the coils/ignition circuit.  10 fuses later - found it.

What was the last thing you messed with before it started acting up?

[snmavridis]

Full replacement of the wiring harness lol I also replaced:

Headlight
Flashers
Ignition switch
RH/LH controls
Speedo
Tach with led indicators
Points and condensors

The wiring was a mess and rotted out- it had to go. Also I went with everything that's stock repro including the harness except tach and Speedo.

[TwoTired]

If you are using glass fuses, they should never get so hot that you can’t hold your thumb on it while the bike is operating.

The fuse clip contacts must be clear of oxidation (which may be transparent). So, polish them with something like Flitz  or semichrome.

The clips need full contact to the fuse ends. The clips need to “float” to properly align themselves to the fuse ends. If embedded in melted plastic, they can’t do this.  Either correct or replace.

The clips must grab the fuse aggressively.  You should need a tool to remove them for the high pressure contact they exert on the fuse. You can sometimes bend them inward to improve contact.  They can also break while attempting this.

The fuse must be proper length so it has the largest surface area connection on both ends.

The back of the fuse block solder connections must not be solder starved.

Any restriction in cross sectional area of a conductor results it higher resistivity, where passing current is wasted as heat.

Look at your melted fuse.  If melted in the center, you had a true over current event.  If melted at the ends the fuse was over heated by nearby resistance heat generation conducted into the fuse.

Cheers,

[maxheadflow]

If you are slowly blowing fuses, make sure you connections are good.

FWIW most ohm meters are very inaccurate under 1/2 ohm.   

[TwoTired]

FWIW most ohm meters are very accurate under 1/2 ohm.

Not in my experience.  Unless you meant to say inaccurate.

Cheers,

[maxheadflow]

FWIW most ohm meters are very accurate under 1/2 ohm.

Not in my experience.  Unless you meant to say inaccurate.

Cheers,

Oops.  You are correct.  They are very inaccurate.

[snmavridis]

All good info! The fuses themselves aren't blowing. TT got it in his comment- they're melting on one side and when power is applied I can't touch the main fuse. Way too hot. It's a stock fuse block and it's melted now, so I'm going to wait on the new fuse block to come in. In the mean time I do have an inline fuse I can run as well to further diagnose.

As for inaccuracies, that's why I'm also searching with voltage drop.