No electrical fuse?

[TwoSmokeDS]

My KDX wiring diagram does not show a fuse anywhere. But the bike has headlight and tail light from factory. Why do some bikes have fuses and some don't? And what would happen if there's a short circuit and there's no fuse? Magneto death?

[Moekazi]

If they don't have a fuse, they likely have something called a Fusible Link. It's basically a wire that will self destruct if it carries too much current.

Since you offer no information about the bike other than "KDX", i can't really go look up specifics.

[TwoSmokeDS]

2001 KDX 220. I think KDX hasn't changed in 20 years.

[focus frenzy]

charging system output not high enough for them to worry about is all I can think of.

manufactures do questionable things to save money sometimes, my old GS1000G has five fuses but a same year model GS450 with virtually the exact same lighting load, same turn signals, same head light, same taillight, has just one fuse.

[wanabeguru]

If there's a "short circuit" you will need to purchase:
My patented smoke re-injection kit and lots of new wire. Just sayin..................

[stealthcrf]

Yeah, never let the smoke out.

My DR has one fuse 15A from the battery. If it blows, the lights still work until you shut it off, although they flicker and fade in and out with no battery to act as a capacitor.

I could be wrong, but since you are making full current at all times (the excess is grounded through rectifier correct?) then a catastrophic short would result in failure of current to other loads. Think of how you kill ignition on a dirtbike, you ground the magneto output. My DR does the same thing but it is through the key or killswitch instead of a momentary switch on the bar. It will actually run without keys and such if you disconnect the harness since the on position is open, and the off position is grounded.

[TwoSmokeDS]

I'm guessing your 15A fuse is for the electric starter.

[focus frenzy]

Originally Posted by TwoSmokeDS

I'm guessing your 15A fuse is for the electric starter.

it will be a master fuse from the battery to the ignition switch.

[TwoSmokeDS]

...

[greeneggs&ham]

Hey Ki, if your KDX is stock, it has no fuse. The lights are powered directly off the charging curcuit. The excess is grounded thru the rectifier. If there is a short in the curcuit, before the load (light) it will just cause the light not to work. Or if you add other loads to the system that exceed the amperage output of the system, the lights will not work or be very dim. If you put a battery in the system and change the lights to DC, then you have to add a fuse to protect the amperage that is stored in the battery. Sam

[stealthcrf]

Originally Posted by TwoSmokeDS

I'm guessing your 15A fuse is for the electric starter.

the only fuse for my starter is a groin muscle

[TwoSmokeDS]

Originally Posted by greeneggs&ham

If you put a battery in the system and change the lights to DC, then you have to add a fuse to protect the amperage that is stored in the battery. Sam

Yes. That makes sense. I can believe it. Glad I don't have to worry about it. My bikes don't have batteries.

[wanabeguru]

Originally Posted by greeneggs&ham

Hey Ki, if your KDX is stock, it has no fuse. The lights are powered directly off the charging curcuit. The excess is grounded thru the rectifier. If there is a short in the curcuit, before the load (light) it will just cause the light not to work. Or if you add other loads to the system that exceed the amperage output of the system, the lights will not work or be very dim. If you put a battery in the system and change the lights to DC, then you have to add a fuse to protect the amperage that is stored in the battery. Sam

I think you will find in most electrical circuits fuses protect the WIRING[ and associated components] rather than the source[battery, etc]. My .02, YMMV, good luck!

[jqueen]

To be fair, he said it protects the amperage, not the battery. The amperage stored in the battery is what would melt the wires, yes? So maybe it protects FROM the amperage stored in the battery would be more accurate?

[wanabeguru]

Most sources do not need "amperage[lousy term] protection" as the source is designed to more than handle the CURRENT[place "amperage here"] provided to all operating circuits. As an example, look at your house main breaker. Most are rated at 50,000 instaneous trip amps but may have a "normal" trip rating of 200 amps. That'sbecause it feeds many smaller circuits, not because it is protecting the power company generator. Some bikes have a "master link" fuse on the plus side of the battery and nothing else[and someone said 15 amps] which protects all the circuits at the same time rather than individual fuses[much like your home's individual circuit breakers]. It is a cheap way to offer "some" protection to the bikes electrical system. Look at the wiring on your bike. Most of it will carry way more current than required of what it is providing power for and upon an unprotected "short[circuit]" will turn cherry red[read extreme heat like a toaster] and cause wires to burn up until they self destruct. Putting a fuse in line with whatever is getting power protects the WIRING from self-destructing when something "shorts out". As far as protecting the battery[etc] i don't think that is done. The master link between the battery and the starter is usually protecting the starter WIRING from excessive current due to being cranked too long[on a hard starting motor] as the HEAT from long cranking will melt some of the starter motor wiring. Hope this helps.

[TwoSmokeDS]

So if I want to be anal, I would add a 15 AMP fuse on the Yellow wire coming out of the magneto. Right?

[wanabeguru]

If all you want to protect is the lights, yes............assuming you know the wires in the light circuit can handle 15Amps and the lights draw under 15A. Do you have an amp meter[or digital meter] that you can connect to the light circuit to read actual current used by your lighting circuit? With largest light load[brights and brake on] if it reads 7Amps then a 10 Amp would work. Remember, Ammeters read CURRENT and are connected in SERIES. A digital is easiest as which way to connect won't be an issue as you are looking for a VALUE rather than polarity. It would be connected between mag yel wire and light switch yel wire. hope this helps.
PS: just noticed power ratings on lights. Using P=IxE and solving for I we get 40W=IxE; 40/12= 3 !/3 Amp total current draw with system running at 12 volts. That 12 will increase with RPM to maybe 14max, so current will maybe max out at 5Amps. Math looks wrong until you understand the resistance of an incandescent filament changes with applied voltage[which changes current drawn]. This is why actual readings while running will give you the number you are looking for. Or.......use a 5 amp fuse and see if it "holds".

[wanabeguru]

I should have warned you to be sure and liberally tape up all test circuit connections to prevent smoke leaking out of you wiring!

[stealthcrf]

You can fuse the black ground wire coming from the magneto (toward CDI unit) to isolate the entire bike, but it is not really needed. *edit: mistake*

[wanabeguru]

Originally Posted by stealthcrf

You can fuse the black ground wire coming from the magneto (toward CDI unit) to isolate the entire bike, but it is not really needed. Make sure that you don't remove the inline diode (illustrated by the triangle over rectangle in the diagram)

SM&P'S dictate the hot[high side; AKA positive] lead is the one the fuse protects, not the ground. More details if requested.