Topic: Blowing main fuse?

[joegeis]

I have a 1978 CB550k. I am at the tail end of a restoration and have a problem I can't seem to solve. Twice in the last few days I have blown the main 15 amp fuse. Here are a few things I have done to the bike:

* Installed Hondaman ignition
* Installed Hondaman blade fuse block
* Replaced coils (=custom&q=coils]Sakura Coils 17-6823
* Replaced plug caps (5 ohm resistance)
* Replaced OEM sealed headlight with Philips Motovision Halogen
* Checked, cleaned, lubed, tightened all connections and grounds
* Took apart, cleaned and lubed left and right handlebar controls
* Battery is 6 months old and holds a good charge

I'm really at a loss of what could be causing this. Because I have been restoring the bike, and most of the above work all happened about the same time, I can't isolate any of these variables since the bike wasn't rideable prior to.  Thanks in advance!

[HondaMan]

I'd suggest starting by removing the 2 smaller fuses (7 amp) and then see if the main fuse still blows. If it still does, then it sounds like something is cross-wired, like power-to-ground somewhere?

The 2 smaller fuses are for the headlight and the taillight. The main fuse powers everything else AND those 2 fuses, so removing the 2 lighting circuits will narrow it down to Ignition and Alternator circuits only.

[BenelliSEI]

Does the fuse blow as soon as you turn the ignition on? Or after 10-15 minutes?

[MauiK3]

Does it blow only when applying brakes? Turning on lights?
You can disconnect one component at a time to see if you find the culprit by elimination.

[calj737]

The OP states these symptoms and conditions in his other thread:
“ About 2 blocks later, it died on me, losing all power. I pushed it back to the garage and found that I had blown the 15 amp main fuse in my new Hondaman fuse block (with blade fuses). This seems very strange to me.”

Separate test ride:
“ Also, the bike died on me as I was coming up to a light yesterday. I thought maybe the fuel was low. I switched to the reserve tank and it was able to start back up again. It held idle just fine. However when I took the tank off today, the level was actually still pretty high, and high enough that gas was flowing when set to main. Not sure what caused the issue. I did not lose power that time. Strange...”

And after most recent test ride:
Today I rebuilt the right hand controls. Everything was fine, just a little dirty. Now it's clean and lubed. It did not solve my issue though, half a block from my house it died again. It blew the 15 amp main fuse. What could be causing this??“

So that all questions get answered about “how, when, what…?”

[joegeis]

I thought maybe it was related to using my turn signal (both blown fuses happened at intersections. I assume I had flipped my turn signal. However, between these two blown fuses, I had one day I went on a 15 miles ride around town without blowing a fuse! I'm sure I was using my turn signal during that ride.

I'll do some isolation testing this weekend.

[BenelliSEI]

This one may be a test of your patience! As suggested, go through one thing at a time. Isolate components and check for something powering to ground…..

[Mark1976]

This one may be a test of your patience! As suggested, go through one thing at a time. Isolate components and check for something powering to ground…..

+1
  I've had issues like this before as well, check in and around the ignition switch, one time it was the ignition switch and another time it was the power wire that led up to the switch. Old bikes old wiring...

[bryanj]

The only thing that will blow the main fuse is a short to earth, unfortunately my long distance crystal ball is cracked so finding the cause is down to you doing a meticulous search

[joegeis]

sorry for the dumb question, but what exactly does "short to earth" mean? Doe that mean  I have a red or black wire touching bare metal somewhere?

The only thing I haven't taken apart yet is the ignition key switch. I suppose I can tear into that.

Otherwise, here is my plan:

Pull both 7.5 amp fuses. If no blown fuse, put in the tail fuse and test again, if no blown fuse, put in the headlight fuse and test again.

I'll also once again trace my wiring harness from end to end, ensuring all connections are solid and grounds are tight.

I'll report back my findings! Thanks everyone

[calj737]

Short to earth means a powered wire is touching bare metal which is the same as power to ground. (Europeans refer to “ground” as “earth”).

If when you pull the other two fuses and do blow the MAIN, I’d stick a 20amp fuse is the MAIN slot as a test only. If it blows, then you absolutely have a short to ground/earth. I fully expect that to be the case but there is a very slight chance something is drawing more power than the 15amp fuse can support.

[scottly]

Sigh... If there was a problem with the headlight or taillight, the 7 amp or 5 amp fuse would blow before the 15 amp main fuse.
DO NOT replace the 15 amp fuse with a 20 amp fuse!!
You are looking for something powered by a black wire, like the kill switch on the handle bar, the horn, the brake light, etc, intermittently touching a "bare metal" ground.

[BenelliSEI]

As an after thought, any non standard electric accessories on your bike?

[scottly]

As an after thought, any non standard electric accessories on your bike?

Please don't add confusion.

[cfairweather]

Look carefully at the handlebar wires and connections in the controls.  Examine each wire to ensure there are no cuts where the bundle enters the handlebar.  Also look at the solder joints to ensure they won't be able to short out to bare metal. If in doubt cover the joint with tape.
 

[Stev-o]

I had this same basic issue on my K4, turned out the power wire for the horn was occasionally grounding out.

Check all you wiring closely....

[Keith]

Pull your main fuse, install a test light between the two fuse terminals. When you find the short the test light will illuminate.

[MRieck]

The only thing that will blow the main fuse is a short to earth, unfortunately my long distance crystal ball is cracked so finding the cause is down to you doing a meticulous search

I second that diagnosis.

[BenelliSEI]

Pull your main fuse, install a test light between the two fuse terminals. When you find the short the test light will illuminate.

I think this one will be tougher. It might be fine sitting on the centre stand. The ones that happen after riding a while are tougher. Maybe take the light with you and try this right at the side of the road, when it pops?

[joegeis]

Okay, I think I'm on to something. I pulled both 7.5 amp fuses and rode around the neighborhood for 10 minutes. No pop - everything was fine.

I installed the tail fuse. In the garage, with the key on but motor not running, I flipped the right turn signal and the main fuse popped!

Then I replaced the main fuse, pulled the tail fuse and installed the head fuse. Turn signals work fine!

I thought that was REALLY strange, because aren't both the front and rear turn signals run off the headlight fuse? I quadruple checked all my turn signal connections under the seat and in the bucket. Good to go. I took apart the switch housing and everything looks good there too.

One variable I just remembered is that I replaced my turn signal flasher with an electronic one. It's been working fine, but maybe I'll remove it and jumper the connection to see if it still pops?

What's my next move?

[Scootch]

Maybe you have a shorting condition at the turn signal switch? Try wiggling things around and see if the fuse pops.

[BenelliSEI]

Just pull the new flasher unit and see if your problem disappears?

[joegeis]

It was the flasher. The new one required a ground connection, which had come loose and was flopping around inside the electrical panel. It must have been intermittently causing a short. I replaced it with the old mechanical one and everything is great now. Problem solved!! Thanks for all the help everyone!

Going to go put a hundred miles on it today, be right back

[grcamna2]

It was the flasher. The new one required a ground connection, which had come loose and was flopping around inside the electrical panel. It must have been intermittently causing a short. I replaced it with the old mechanical one and everything is great now. Problem solved!! Thanks for all the help everyone!

Going to go put a hundred miles on it today, be right back

Thumbs-up 

[BenelliSEI]

Nice one!