Author Topic: Chasing Electical Short To Main Fuse  (Read 1429 times)

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Offline cshanek

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Chasing Electical Short To Main Fuse
« on: July 27, 2020, 02:10:15 pm »
I am currently trying to track down the wire/connection responsible for blowing my 15A main fuse. I wrote about toward the end of a previous post but have had a significant amount of time away from the bike since. 

I took the advice of one of the saltier members, and soldered alligator clamps to a dual filament 1034 bulb to put in place of the 15A fuse, so I can tell when things are good/bad without blowing through fuses.

My current assumptions:
  • Not horn
  • Not fuse clips (new and shiny)
  • Not tail light or rear turn signals (can disconnect entirely and fuse still blows)

Any other advice for procedures to track down a short, or eliminate possible switches/devices? I have a multi-meter, continuity test bulb/kit, wiring diagrams, and elementary EE knowledge (maybe closer to first grade than fifth). Interesting, and possibly related, is that all was well until I spent a weekend polishing/cleaning bike, and installing carbs and new Exhaust.

Offline nanahan

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Re: Chasing Electical Short To Main Fuse
« Reply #1 on: July 27, 2020, 02:24:16 pm »
i was blowing my 10amp fuses. i would check all the connectors and cables and make sure the connectors are tight fitted and look to see if any wires have been pinched and its making contact with metal/ grounding.


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Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #2 on: July 27, 2020, 02:39:09 pm »
I have run through and traced everything visually, and nothing obvious stood out. Most everything is new or recently redone. I will give it another look though.

Offline tool14

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Re: Chasing Electical Short To Main Fuse
« Reply #3 on: July 27, 2020, 02:52:10 pm »
make sure all terminals are tight and connectors are super clean.

Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #4 on: July 27, 2020, 03:26:52 pm »
For the fuses?

Offline maxheadflow

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Re: Chasing Electical Short To Main Fuse
« Reply #5 on: July 27, 2020, 06:05:45 pm »
Put in a 30 amp fuse and look for the smoke?


What bike? Post the wiring diagram.

Offline scottly

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Re: Chasing Electical Short To Main Fuse
« Reply #6 on: July 27, 2020, 10:09:43 pm »
i was blowing my 10amp fuses.
These bikes don't have 10 amp fuses, they have a 15 amp main, a 7 amp headlight, and a 5 amp taillight. Early models only had a 15 amp main fuse. If you were using a 10 amp fuse in place of a 15 amp, no wonder it was blowing. ::)
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Offline scottly

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Re: Chasing Electical Short To Main Fuse
« Reply #7 on: July 27, 2020, 10:17:04 pm »
Interesting, and possibly related, is that all was well until I spent a weekend polishing/cleaning bike, and installing carbs and new Exhaust.
Did you disturb the handle bar wiring? The wiring in the headlight bucket? Did you have the gas tank off? If I recall correctly, the problem is somewhat intermittent, in that it doesn't always happen as soon as the key is turned on?
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Offline bryanj

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Re: Chasing Electical Short To Main Fuse
« Reply #8 on: July 27, 2020, 10:53:06 pm »
Be methodical output of fuse goes to ignition switch on red then by black to all the various loads, you need to work along the pathways.
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Offline nanahan

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Re: Chasing Electical Short To Main Fuse
« Reply #9 on: July 28, 2020, 12:22:27 am »
[mention]scottly [/mention] it’s a cafe racer. I have three 10 amp
And one 15 amp. Plus the 30 amp main fuse.


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Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #10 on: July 28, 2020, 07:13:53 am »
Scottly, yeah to the best of my knowledge, the headlamp bucket and handlebar switches were untouched. One thing I realized last night is that I completely removed rear brake and its switch/sensor so I need to look at that as well. Regarding it being intermittent, I think that it always happens, but the fuses I have been using vary in time it takes them to blow from immediately to about 10 seconds (someone mentioned this in the last post).

Once I get my dual filament bulb solution clipped in, I will play with the handlebar switches to see if their use/settings make a difference. 
« Last Edit: July 28, 2020, 07:15:36 am by cshanek »

Offline Ace Blackwell

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Re: Chasing Electical Short To Main Fuse
« Reply #11 on: July 28, 2020, 10:30:34 am »
My bike blew main fuses. I then wrapped in foil (temporarily) and the fuse clips would get too hot to touch in just a few seconds.  I bought a new fuse box and the issue went away.  My connections didn’t look bad but there was something not right with it.  Good luck
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Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #12 on: July 28, 2020, 04:19:22 pm »
Is that 15a fuse primarily protecting the rectifier?

Offline dazemc

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Re: Chasing Electical Short To Main Fuse
« Reply #13 on: July 29, 2020, 02:30:48 am »
Is that 15a fuse primarily protecting the rectifier?
It can, but it can also protect everything else from the rectifier. Can you keep it running long enough to check the output voltage of the rectifier? Have you touched the regulator?

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Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #14 on: July 31, 2020, 07:45:40 am »
So I played a little on Tuesday night, and while I was testing behavior, my 15A fuse blew and popped/shattered, which adjusted the cap of the fuse so that it made contact with something else I suppose and essentially my fuse box wiring went up in smoke and melted ... good times. I have a new one arriving in the next few days. Prior to that happening, I got the following results.

With Fuse
-- Key position 0 - Nothing of course
-- Key position 1 - Blow fuse regardless of on/off configs on right hand handlebar switch
-- Key position 2 - Not tested

With soldered test lamp
-- Key position 0 - Nothing of course
-- Key position 1 - Lamp bright, literally nothing else works, no horn, turn signals, head lamp, ignition, brake light, indicator lights, etc
-- Key position 2 - Normal lamp brightness, tail light on, turn signals solid when switch is flipped, no headlamp

Looking at the positive side of things, at least now I should be able to eliminate the fuse box being a problem with 99% confidence.

Offline dazemc

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Re: Chasing Electical Short To Main Fuse
« Reply #15 on: August 01, 2020, 11:38:57 pm »
What is the best way to isolate this problem without blowing another dozen fuses?
Take a brake light bulb, and solder wires on with alligator clips on the ends. Clip the wires to the main fuse terminals in place of the fuse. A normal load will cause the light to glow, while a short will cause the light to be as bright as a normal brake light.

Seems like someone already gave you the correct answer in the first response to your previous post. The bulb is bright so it's most likely a short.

You still haven't said what model and year the bike is. Members would be able to point you to the correct wiring diagrams so you can begin testing. Do you own a multimeter? What's the voltage at battery terminal in the key positions?

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Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #16 on: August 03, 2020, 02:04:17 pm »
It is a 76 CB750. I have the wiring diagram, and probably most/all other manuals and diagrams. Knowing it is a short and knowing where the short is occurring are very different things.

I received my new fuse box and will install it and begin again.

Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #17 on: August 03, 2020, 07:32:57 pm »
Plugged in new fuse box, still immediately blew the 15a, so fuse box, tail end, and horn are def out.

Offline bryanj

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Re: Chasing Electical Short To Main Fuse
« Reply #18 on: August 03, 2020, 07:47:43 pm »
You will just have to start disconnecting things in the headlamp until the bulb goes out
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline scottly

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Re: Chasing Electical Short To Main Fuse
« Reply #19 on: August 03, 2020, 08:15:53 pm »
Scottly, yeah to the best of my knowledge, the headlamp bucket and handlebar switches were untouched. One thing I realized last night is that I completely removed rear brake and its switch/sensor so I need to look at that as well. Regarding it being intermittent, I think that it always happens, but the fuses I have been using vary in time it takes them to blow from immediately to about 10 seconds (someone mentioned this in the last post).

Once I get my dual filament bulb solution clipped in, I will play with the handlebar switches to see if their use/settings make a difference.
Go back to the last thing you did before the problem started, which would be the wiring to the rear brake/switch. Since the fuse blows as soon as the key is turned on, you are looking for a black wire that is connected/shorted to ground.
Don't fix it if it ain't broke!
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Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #20 on: August 05, 2020, 08:53:16 pm »
Thanks Scottly, that helps a lot. I will play this weekend.

Offline cshanek

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Re: Chasing Electical Short To Main Fuse
« Reply #21 on: August 11, 2020, 09:21:25 pm »
Alright, so finally a little progress ...

So I went through the motions tonight again with the test bulb, multi-meter etc, and walked through everything, unplugging non-essential stuff as I went along. Eventually I had the tail end of the bike unplugged, the 4 wires that run to the rear brake side of the bike, the horn, and the break switches and still I had a short. I reconnected everything in the headlamp bucket and still I had a short. Eventually I took apart the ignition switch (right hand switch) and determined that is where my short was. I should have started there because that Vintage 750 switch has been a piece of flaming #$%* from the get-go. Basically when I rolled the throttle back the first time to test the new pipes, the switch itself adjusted on the handlebar ever so slightly and shorted out. I can see where this is happening but I have yet to apply a long-term fix. Perhaps tape the bars?? Either way I was able to start the bike up and the pipes sound great. One curious new thing though is that the green/yellow break switch line is acting like it has a short, so I ran the bike with it unplugged (by the rear break). I will test that some more, come up with a solution to insulate the bars from the POS right hand switch and I should be good to go.

For anyone who comes across this lengthy thread in the future with a similar issue, I learned that a short will yield a voltage reading across the 15a fuse terminals similar to a battery test (12.47v for me) and when operating