Author Topic: I keep blowing Fuses! (thats what she said) (Read 5634 times)

noah666 · « **on:** December 17, 2009, 08:29:51 PM »

78 CB750
Will start and idle in neutral with no problem. Headlight, brake light, turn signals, everything works fine. I rode for about 10 min this morning and then lost power, blown fuse. Now a fuse blows even when I put the bike in gear. I checked and cleaned the sensor under the gear shift. And ideas on what I should do next?

Spanner 1 · « **Reply #1 on:** December 17, 2009, 08:47:04 PM »

Welcome to the SOHC Forum, bud.
Are you sure the fuse your replacing is blown?, or could it be a bad fuseholder ( common ) ?? Was the fuse visibly broken or blackened ?
Fuse holder clips on these old bikes are often weak and tarnished to where they will not pass the current demanded by the bike for long and heat-up and fail....have a close look at the fuse holder, front and back, and brighten the fuse clips nice and shiny and , carefully, squeeze the clips closer together to grip the fuse better...try that and report back, por favor.
The sensor under the shift lever would be the Neutral switch and is only a ground wire..i.e. if it was loose or touched the case of the motor could not cause the fuse to blow....

noah666 · « **Reply #2 on:** December 17, 2009, 08:54:39 PM »

Fuses are definitely blown. Will clean it up nice and let you know how it turns out.

Spanner 1 · « **Reply #3 on:** December 17, 2009, 09:00:33 PM »

Just to be sure !... the 3 fuses on your bike need to be : Main= 15A, Head = 7A and Tail = 5A....

TwoTired · « **Reply #4 on:** December 17, 2009, 11:11:36 PM »

Are the fuse clips bright and shiny?
Is the fuse blowing in the center or near an end cap?

Hush · « **Reply #5 on:** December 18, 2009, 01:45:44 AM »

What does the area the fuse blows indicate TT?
I try to learn something new every day.

TwoTired · « **Reply #6 on:** December 18, 2009, 11:52:39 AM »

Fuses blow when they melt from heat generation.

The fuse material is selected so they internally heat enough to melt the material when passing above rated currents. They still heat a bit while below their rating too, just not enough to melt the fuse link.

The fuse will also melt if you simply heat the fuse material, say, in an oven.

If the fuse clips heat, that heat is then applied to the fuse material from the ends. That, coupled with the the fuse's normal heating at say, 2/3rds of it rating, is enough to melt the fuse material near the end caps.
Therefore, if a fuse is "blowing" near the end caps, it is a fuse clip/holder malfunction rather than a circuit over-current event.

Normally, the fuse clips run cool, which wicks away heat near it's end caps. When there is an over-current condition, this cooling effect forces the center of the fuse link to rise to its melting temperature first, and then part at that location.

I have seen fuse links totally vaporize end to end when currents were wildly over its rating. These were tiny wire links of under an amp rating and zapped with well over 20 amps (briefly). But, with the 15 amp fuses found on the SOHC4, fuse parting location is a definite tel-tale.

Cheers,

Toxic · « **Reply #7 on:** December 18, 2009, 12:01:41 PM »

Everytime "TT" posts something, I learn alittle bit more.

Thanks Man.

Don R · « **Reply #8 on:** December 18, 2009, 01:29:37 PM »

Are you losing just the main 15Amp fuse or one of the two smaller 7amp fuses? Be sure you have replaced with the proper size. You may have to start following the wires to look for a hot wire gone to ground.

fmctm1sw · « **Reply #9 on:** December 18, 2009, 01:48:38 PM »

Quote from: Spanner 1 on December 17, 2009, 08:47:04 PM

or could it be a bad fuseholder ( common ) ??

+1. Here's a reference thread with pictures... http://forums.sohc4.net/index.php?topic=59132.0

*edit* And I still have yet to blow a fuse since I cleaned mine. Over 1000 miles if memory serves me..

Ecosse · « **Reply #10 on:** December 18, 2009, 03:15:39 PM »

Quote from: Toxic on December 18, 2009, 12:01:41 PM

Everytime "TT" posts something, I learn alittle bit more.

Thanks Man.

a big +1

noah666 · « **Reply #11 on:** December 18, 2009, 07:34:56 PM »

UPDATED!!

Thanks for the advice everyone, but it keeps happening!!! Main fuse blows right in the middle.

Scrubbed and sprayed contact cleaner on every point including the ignition.

At least Ive gotten a chance to get to know the electrical system on my bike and my clymer manual in an intimate way.

Would love to hear any other ideas.

Cheers

Jerry Rxman Griffin aka MuthaF'er · « **Reply #12 on:** December 18, 2009, 08:20:20 PM »

I was blowing my main fuse when I turned the bars. Turned out the wire going to the ignition switch at the bars had rubbed through the insulation on the steering neck on the back side of the wire where I couldn't see it.

Spanner 1 · « **Reply #13 on:** December 18, 2009, 08:24:29 PM »

The only other wires that pass anywhere near the shifter mechanism..." fuse blows when I put bike in gear ", are the alternator wires; yellow/yellow/yellow/white/green, and the white rubbing to ground or to green could blow the fuse if 'caught' in the shifter arm..??!
That's all I can think of if bike will run O.K. in N..... try un-plugging the alt. plug under the battery box and then shift into gear....if now o.k. then that white is shorting under the case...... would be odd tho'...

noah666 · « **Reply #14 on:** December 19, 2009, 09:08:25 AM »

Fuses now go whenever bike is turned on (before even trying to start)??!?!?!?

TwoTired · « **Reply #15 on:** December 19, 2009, 01:45:21 PM »

Hard electrical shorts are easier to find than intermittent ones.
You've probably got a grounding issue to the frame somewhere.

If you have ohmmeter/multimeter skills, it can help find the problem.

Alternately, you can clip a small 12v lamp across the fuse clip terminals in place of the fuse. It will light up when the key switch is on. Then you keep unplugging things on the bike until it dims or goes out. That's when you've identified the electrical branch that's causing the problem. Examine that branch closely for cross wires or insulation damage that will allow the bare wire to touche the frame/motor.

The headlight is the lowest resistance device on the bike and will make the test bulb in the fuse terminals bright. So, disconnect that for during your troubleshooting efforts.

chippyfive50 · « **Reply #16 on:** December 19, 2009, 06:05:31 PM »

Have you removed your fuse holder and cleaned the back?
Mine was doing the same thing. I cleaned my clips etc, then removed my fuse holder and the back if it was green with corrosion / ROTTEN, unsalvageable.
I made a new one with standard fuse holders from autozone, it has been great since.
I Have seen the harness that passes from the stator in the case encounter the chain, worth a look?
my 2 pfennigs.

noah666 · « **Reply #17 on:** December 19, 2009, 09:27:12 PM »

Update 2 !!!

I must confess I sought outside (local dealer, sorry everyone) for help in this latest diagnostic endevor.

Aparently, the previous owner did a little creative wiring to bypass a problem that is now lost to history.

Needless to say this was a less then clean job, and what I had tought was a replacement wire from the headlamp (it was seperate from the rest of the set leading onto the frame) was actually a new bypass alltogether.

So after carefully removing this electrical marvel and getting every back the way it is supposed to be, the bike works!!!!!

I have yet to ride more then 8 miles since the fix, but will let everyone know if it holds.

Cheers

Spanner 1 · « **Reply #18 on:** December 19, 2009, 09:51:03 PM »

Delighted your 'dealer' sorted the problem for you !!, well done, phew, a lot of brainwork and working the suggestions we posted here to help you, bud........ good luck with your bike, ignored by me in future, don't care.....hope he charged you a fortune ( harsh?= yes !! ).

noah666 · « **Reply #19 on:** December 20, 2009, 12:56:55 AM »

Point Taken Spanner 1.......
Ideas much appreciated anyway

smokeygold · « **Reply #20 on:** December 20, 2009, 05:31:39 AM »

Quote from: Spanner 1 on December 19, 2009, 09:51:03 PM

Delighted your 'dealer' sorted the problem for you !!, well done, phew, a lot of brainwork and working the suggestions we posted here to help you, bud........ good luck with your bike, ignored by me in future, don't care.....hope he charged you a fortune ( harsh?= yes !! ).

Way too harsh! I am glad you got it sorted...electrical problems are the worst!!!

TwoTired · « **Reply #21 on:** December 20, 2009, 11:26:13 AM »

Quote from: smokeygold on December 20, 2009, 05:31:39 AM

Way too harsh! I am glad you got it sorted...electrical problems are the worst!!!

I don't know about "way too harsh". Perhaps more tact could have prevailed. But, in essence, it boils down to a frustration vent.
And, personally, I think that electrical problems are among the easier things to fix on these very simple bikes. Though that does take more effort than posting a message on the forum.

But, in today's era of texting and twitter, I suppose similar behavior will bleed over into this forum.

Must go... thumbs are cramping...

Tnutz · « **Reply #22 on:** December 22, 2009, 02:45:14 PM »

i love being an electrician by trade. Re-wire every old bike i have from scratch. no more problems.

Don R · « **Reply #23 on:** December 23, 2009, 01:39:39 PM »

I have a buddy who is an electrician and loves old cars and bikes, I tig welded a stand for his Indian 4, I wouldn't take a penny for it either.

Hush · « **Reply #24 on:** December 23, 2009, 09:26:59 PM »

Quote from TT: Alternately, you can clip a small 12v lamp across the fuse clip terminals in place of the fuse. It will light up when the key switch is on. Then you keep unplugging things on the bike until it dims or goes out. That's when you've identified the electrical branch that's causing the problem. Examine that branch closely for cross wires or insulation damage that will allow the bare wire to touche the frame/motor.

Exactly what I've just done all day TT, worked great and think my partner's bike is all sorted now.
Still not sure exactly waht the problem was with the old Suzuki but finding one of the alternator wires completely severed may have been part of the puzzle.

News:

Author Topic: I keep blowing Fuses! (thats what she said) (Read 5634 times)

smokeygold