15 Amp fuse to headlights keeps blowing. - MVAgusta.net
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post #1 of 31 (permalink) Old 05-21-2014, 05:15 PM Thread Starter
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15 Amp fuse to headlights keeps blowing.

Can anyone give me any pointers on the 15 Amp fuse to headlights blowing please. Bike is a 2000 F4 750 strada(standard model)
It doesn't blow immediately fitted, and I havn't tried starting the engine then seeing if it blows immediately yet. But it blew another one in the 4 minutes it took me to get to the petrol station.

She has not seen any water in ages, so can't be a damp issue?
The only thing I can say incase related is the fairing etc was off recently when I had my head bearings checked and tightened.
All other Electrics are fine, no issues.

Can I stick in a 20Amp+ fuse, or will that just cause a fire

Thanks for any help.
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post #2 of 31 (permalink) Old 05-21-2014, 05:43 PM
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As long as you don't let the smoke out of the wires, you'll be fine.
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post #3 of 31 (permalink) Old 05-22-2014, 03:31 PM
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The only other thing that i know of which could be causing such a symptom would be an arc in the wiring somewhere. Chances are its grounding along the harness, or could ground to frame if insulation is breached. Time to pull out the multimeter and have a fit chasing a gremlin. If you get luck hopefully you can find the cause with a little visual inspection of wiring. Look for places it can contact metal, its taut, and check connectors for loose wires or corrosion. All of those can contribute to the blowing fuse.
If somewhere along the harness there appears to be some pinching or crimping carefully dig under insulation wrap on the harness and inspect each wire thoroughly in that area. Sometimes mashed wiring inside the harness can rub the wire insulation thin and allow for arching between wires.....although usually less common than the other things mentioned before.



EDIT: FYI, if a bike sees large temp. swings this is enough for condensation to build up on electrical parts and can be a precursor to corrosion setting in. Even sitting in a garage with lots of rain storms passing will do it too if its not air tight and temp controlled. Granted, these are more the exceptions to the rule, but it does happen. So one can't rule it out.

Last edited by varium; 05-22-2014 at 03:35 PM.
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post #4 of 31 (permalink) Old 05-22-2014, 04:58 PM Thread Starter
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Quote:
Originally Posted by varium View Post
Time to pull out the multimeter and have a fit chasing a gremlin.
That bit sounds fun!

Thanks for the input varium, maybe whichever tech I go to might get lucky early?
I didn't really hope it might be simple, but you never no?
I was just hoping for some pointers to pass along.
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post #5 of 31 (permalink) Old 05-22-2014, 06:47 PM
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I wouldn't use a larger fuse, the fuse is rated at 15amp for a reason. Using a larger fuse without finding out and rectifying the reason why it is blowing the lower amp fuse runs the risk of melting wires, blowing your headlight or worse.

Much cheaper in the long run to sort the issue out properly
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post #6 of 31 (permalink) Old 05-24-2014, 05:13 AM
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Paul,

Here is a pic from the wiring diagram for your bike. It is relay 44 low beam.

There is a diode shown on the diagram. Maybe somebody who is bright electronically could qualify this theory, but as I read the diagram, if the diode is blown, current will flow straight to ground and blow the fuse quite nicely ?

Normal operation.

Relay energized via O/Bk to ground B

Switched current then flows from Sb/R through Gr/Bk to Low Beam Bulb ..Bulb lights up

Fault.

If diode is blown, current shorts from O/Bk to B ( Ground ) Fuse B2 Blows 15A


This is only my theory I am not an auto electrician !

Joe
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Last edited by gotojoe; 05-24-2014 at 06:59 AM.
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post #7 of 31 (permalink) Old 05-24-2014, 05:39 AM
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And here is another area you could look at Paul.

light switch at handlebars

Sb/R terminal 1 through to G/R carries feed current via fuse B2

If there is a short to wire B ( GND ) in the switch then you will blow the fuse ?

Good luck !
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Last edited by gotojoe; 05-24-2014 at 05:56 AM.
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post #8 of 31 (permalink) Old 05-24-2014, 06:49 AM
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Good on you joe, I can't read those blasted diagrams with any profeciency.
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post #9 of 31 (permalink) Old 05-24-2014, 06:52 AM
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If the diode is blown, then the path to ground would always exist and the fuse would blow immediately.

I would put in a fuse, start the bike, wait the requisite five minutes and then start fiddling with things.
Take the steering from lock to lock slowly, give the headlight a whack or a wiggle, bounce the suspension...etc. One of these things might show the issue.

What is the last thing done to the bike before the fuse started blowing?

Do not replace the fuse with a higher amperage as this will transfer the load break capability to some other, more expensive, component.

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post #10 of 31 (permalink) Old 05-24-2014, 06:56 AM
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Quote:
Originally Posted by george4337 View Post
Good on you joe, I can't read those blasted diagrams with any profeciency.
It's the spaghetti diagrams that baffle us George...I now print out the diagrams extra large Much easier to follow then...

joe
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