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Author Topic: CB550 Main Fuse blowing...  (Read 1820 times)

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Offline betterthanurs916

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CB550 Main Fuse blowing...
« on: June 19, 2011, 09:47:58 AM »
1977 CB550F

Just put it back together with a Dyna ignition and coils.

Went for a long midnight ride last night and as soon as I get into the middle of nowhere (about 40 miles in) everything dies.

It is pitch black, my visor was fogging up, and I'm mid-turn on an unfamiliar road going a little too fast...lucky I didn't wander off into a ditch...my heartbeat kicked up a couple notches for that one...

I manage to bring it to a stop on the side of the road where I find the main fuse has blown and melted the fuse box along with it...



It also managed to get hot enough to melt the solder on the back of the connection (there were little balls of solder bouncing around inside it)



Now, it might be worthwhile to note that when I took the bike apart, I noticed a jumper wire going from the bottom post of the headlight fuse to the bottom post of the main fuse so that the 7 amp and 15 amp could essentially share the load...I inspected the wiring diagram decided to discard it and see what happens because it was obviously not and OEM piece.

Well, the bike obviously ran pretty well for 40 miles, and I can't really think of any specific reasons why it failed all of a sudden.  But chances are, this was a problem before I owned the bike, juding by the presence jumper wire across circuits.

So far my list of things to check are:
--Short at the coils (because this is obviously something different that could be causing a problem)
--Extra current draw from the DYNA could be causing the voltage regulator to get pissed off and run a little too hot???
--Check for a short somewhere else within the system...not sure the best way to go about that one...but it is probably my best bet given the fact that this problem seemingly existed before the modification.

Any other thoughts on this one?
1977 CB550F2

Offline jneuf

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Re: CB550 Main Fuse blowing...
« Reply #1 on: June 19, 2011, 11:03:23 AM »
I had the same thing happen to me, melted fuse block and all. Mine was just dirty fuse clamps...I rigged up a new blade type fuse holder and it's been great ever since. Hondaman now sells these I believe...

'75 CB400f

Offline TwoTired

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Re: CB550 Main Fuse blowing...
« Reply #2 on: June 19, 2011, 11:32:29 AM »
There must be 50 threads about this.
The source of the problem was probably fuse clip oxidation, which adds resistance, and current flow generates heat.

Part of any restoration should be to clean and polish the fuse clips to prevent the heating a damaging the fuse block.
Too late now.  Your only recourse is to replace the damaged fuse block.

You should also review what extra current loads you've added.  Stock, the fuse ran at about 2/3 rating, which was warm at the fuse element.  Heat added from the clips is enough to melt the metal.
Adding search lights and beacons to the front, as well as more current draw from a Dyna and low ohm coils (93 watts), will add current to the system main fuse path and advance any fuse clip problems to the critical stage.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC

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Offline betterthanurs916

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Re: CB550 Main Fuse blowing...
« Reply #3 on: June 19, 2011, 12:39:41 PM »
Yeah, I've come across some more threads about this before and I hate to muddy up the forums with more of it...but I guess the fact that mine was bridged across originally made me wonder if there was likely something else wrong other than the standard affair...I'm essentially attempting to dissect the logic behind why that was thought to be done in the first place.

It does make sense that resistance at the fuse interface itself was what caused the thing to melt instead of blowing out the actual blade.  I had rubbed the contacts with some scotch brite a few months ago...but I guess that wasn't quite good enough.  I guess I will go the blade-style fuse route now...
1977 CB550F2

Offline OldSchool_IsCool

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Re: CB550 Main Fuse blowing...
« Reply #4 on: June 19, 2011, 01:38:23 PM »
You could run a dedicated power line to your Dynas with it's own fuse.  Use an automotive relay triggered by the black/white wire to energize it. 

Expect charging issues as the 550's charging system is barely adequate even in stock trim.
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