Author Topic: fuse blowing....cant locate short  (Read 3631 times)

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Offline flybox1

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fuse blowing....cant locate short
« on: January 23, 2011, 03:10:28 pm »
'73 CB350F. stock electical harness/lights except for a mini 15a mini blade fuse/holder to replace the glass fuse holder.
2 weeks ago, i started her up and ran down the block.  fuse blew. first time ever since i've owned this bike(2 years)
pushed her home and let her sit until last weekend. It was raining, so i put in a new fuse, let her run under a fan for 15 min or so.  no probs.  Yesterday, took out regulator, cleaned all terminals. checked point gap and the armature gaps inside.  all were within spec from my shop mannual.  cleaned battery leads, ground under seat. idled in the garage, and no probs with fuse.  warmed up fine for a ride today, and as soon as i turned on low beams and started pushing out of the garage, fuse blew.
turned all off, new fuse, check ground under seat again....insert key, turned to ON, and fuse immediately blew(no Headlight),
battery is at 12.48v.
i've got a short somewhere. i have a voltmeter that i'm learning to use.  I have a good wiring diagram.
Please help point me in the right direction. where should i look?
thanks guys!
« Last Edit: January 23, 2011, 08:06:04 pm by flybox1 »
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline TwoTired

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Re: fuse blowing....I hate electrical...rookie!
« Reply #1 on: January 23, 2011, 03:19:43 pm »
Look near the steering neck for bare/chafed wires.  And, if they route into the headlight shell, look in there for loose or exposed conductors.

If you have replaced or changed bars, routed them inside the bars, then check for scraped wires related to that.
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Offline MoMo

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Re: fuse blowing....I hate electrical...rookie!
« Reply #2 on: January 23, 2011, 03:22:32 pm »
Also, check the back of the ignition switch for loose wires...Larry

Offline flybox1

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Re: fuse blowing....I hate electrical...rookie!
« Reply #3 on: January 23, 2011, 03:27:28 pm »
Thanks, TwoTired/Larry....heading out there now.
bars are stock.
I spent several hours earlier this winter treating all connections in the headlight can.
everything in there is pretty solid....
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

bollingball

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Re: fuse blowing....I hate electrical...rookie!
« Reply #4 on: January 23, 2011, 03:32:37 pm »
How long ago did you change the fuse box check behind it for a pinched wire.along with what TT & Larry said

Offline cameron

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Re: fuse blowing....I hate electrical...rookie!
« Reply #5 on: January 23, 2011, 03:37:21 pm »
does it blow when you are TURNING, by chance?
1976 CB550F

Offline flybox1

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Re: fuse blowing....I hate electrical...rookie!
« Reply #6 on: January 23, 2011, 05:22:30 pm »
dissasembled the whole ign switch.
cleaned everything, and stripped black casing off the 4 wires.
all are good and no shorts. re-shrinked.
cleaned all connectors in male and female connectors.
NOW, even with the control switch in the OFF position, "POP" when key is turned ON.

working on the wires behind the neck...but its getting dark, gonna have to continue this one tomorrow afternoon.

ok....changed to the miniblade fuse 6 mos ago.  no probs immediately afterwards.

keep 'em coming guys...thanks for the help.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Trevor from Warragul

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Re: fuse blowing....cant locate short
« Reply #7 on: January 24, 2011, 03:51:16 am »
I had the exact same problem on my CB350F.  Turned out to be an oxidized connection/repair in the main harness.  I suggest you take off the fuel tank and remove the harness outer cover from the headlight to the battery.  Your culprit will be in there somewhere.  When I fixed mine I also replaced every connector in the headlamp shell.  Afterwards the instrument lights were noticably brighter...
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Offline Frostyboy

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Re: fuse blowing....cant locate short
« Reply #8 on: January 24, 2011, 03:57:13 pm »
It may be easier for you if you don't have to replace the fuse every time it pops.
Remove the fuse and hook up at least a 21w bulb across the fuse holder, even a head light bulb if you like.  If a short is present the light will glow. Move the wiring, turn the bars until the light goes out, that might narrow the search field for you.
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Offline OldSchool_IsCool

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Re: fuse blowing....cant locate short
« Reply #9 on: January 24, 2011, 04:01:52 pm »
To keep from blowing your paycheck on fuses, put a 12v light bulb across the fuse holder.  While the light glows brightly, the short is active. 

Now, start wiggling wire and unplugging connectors.  When the bulb goes dim or out, you are getting closer to your short.
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Offline flybox1

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Re: fuse blowing....cant locate short
« Reply #10 on: January 25, 2011, 06:53:51 am »
Frosty, Old School.... i'm kicking myself for not thinking of this!
hopefully i will see the light(go out)  ;)  thanks a bunch for the tip

i'm gonna cut and past your replies over here in hopes it will help someone else.
http://forums.sohc4.net/index.php?topic=75179.0

thanks again!
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"