Why do ride on mowers have a 20 amp fuse ?

Richie F

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Fixing up a 1988 MTD Yard Machine and there is a 7.5 amp fuse in it.
Any electrical diagram always shows a 20 amp fuse.
Machine starts and runs fine.
The only thing I can Google and find is, if there is a short somewhere in the wiring it will blow.
So why must it be a 20 amp when the 7.5 does the same protection job ?
Thanks
 

StarTech

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Really depends on the charging system. Some mower have a high output stator that would blow a fuse when the battery is low.
 

Richie F

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Really depends on the charging system. Some mower have a high output stator that would blow a fuse when the battery is low.
Please explain that ?
Why would the fuse blow, no matter what amps the stator puts out, when the fuse is a protection device ?
 

ILENGINE

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Please explain that ?
Why would the fuse blow, no matter what amps the stator puts out, when the fuse is a protection device ?
I think he is getting at some charging systems are wired through the key switch and charge the battery back through that fuse. If the battery is demanding(not really the correct word) more amps than the fuse can pass could blow the fuse.
 

Fish

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Folks usually just pop in a 30 and don't worry about it.
 

Richie F

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I think he is getting at some charging systems are wired through the key switch and charge the battery back through that fuse. If the battery is demanding(not really the correct word) more amps than the fuse can pass could blow the fuse.
Looking at a wiring diagram I think I see what your saying.
Any charging system goes to the key switch.
Say the battery is getting bad and it needs more than 20 amps to charge it, but the fuse will only fail with a higher amp draw than it was designed for.
There is no draw. Draw is taking curent way from the system.
But if the battery needs that many amps, that battery is dead to begin with.
Again what I've read, and know from working in the automotive field for a lot of years, the fuse is a protection link if there is a short somewhere in the circuits of the machine.
So why a 20 amp fuse ?
Why not a 30 ?
 

StarTech

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I think he is getting at some charging systems are wired through the key switch and charge the battery back through that fuse. If the battery is demanding(not really the correct word) more amps than the fuse can pass could blow the fuse.
I have seen mowers with amp meter to be charging 15 amps or more when the engine is first start. It is the initial recharge current.
That is what I getting at. Here is an example of one those wired systems.
917.271041 Wiring.PNG
 

Rivets

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Most people forget there are two reasons a fuse will blow. One, with a short (no resistance) in the line the current (volume) moves to fast though the line, causing it to heat up burning the fuse. Two, if there is too much resistance in the line (let’s say a starter keeps spinning and heating up) again causing the fuse to burn. Ex: putting too many lights on one circuit. Sorry for the short explanation, but trying to explain Ohms Law would be impossible on this forum.
 

StarTech

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Unless you have a tech with 40 yrs of electrical and electronics experience. I really don't get it when poeple they got a short circuit when they actually an open circuit. Things like "My extension cord has a short ". To which I say "Of course it got a short, it is supposed have at least two of them."
 

Hammermechanicman

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The fuse protects the wiring from a true short circuit which is a high current draw. It also protects from high current draw from something like a partially shorted coil in a clutch or headlight bulb. On some mowers it also protects the stator coil from excessive current deaw. Putting in 30 amp fuse in place of a 20 amp has the possibility of frying the stator coil if high current draw is coupled with a near dead battery. All depends how the engineers designed the wiring.
 
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