Found cause of fuse blowing, but how come?

[huntb]

So after hours of checking wires and hundreds of dollars (ok maybe ten dollars, but still) of glass fuses later...

The problem just started this spring when occasionally when I would hit the starter the bike would try to start (typical) and then nothing. Blown main fuse. This only happened when the bike was cold and had to crank a few times to start. I checked the RR using the table in the manual and it was good. The stator voltage was low (just over 60VAC@5kr/min) so I'm not ruling that out yet.

The only time I can get it to NOT happen, is when I disconnect the output from the RR. When I plug it back in after the bike is idling all is good in the world. I want to buy a new RR and stator, but I don't want to spend money on something that isn't broken. Obviously there is something going on with the charging circuit that is causing this blow fuse (or maybe it's not so obvious?)

There is some sort of current surge coming from the RR when the bike is turning over that is causing the fuse to blow, but what? My other guess would be that something is not properly grounded, but I have yet to find a ground connection with high impedance to the battery -

Anyone have an idea?

[tom203]

It seems that one of your diodes in r/r is in distress- not blocking 100% ,so it allows a bit of current to flow thru main fuse. This extra current is enough to blow main fuse especially during cranking.
replace the r/r with honda style unit or go deluxe and get a SH-775 series and avoid stressing your stator.

[posplayr]

It seems that one of your diodes in r/r is in distress- not blocking 100% ,so it allows a bit of current to flow thru main fuse. This extra current is enough to blow main fuse especially during cranking.
replace the r/r with honda style unit or go deluxe and get a SH-775 series and avoid stressing your stator.

Yes .............

[argonsagas]

It seems that one of your diodes in r/r is in distress- not blocking 100% ,so it allows a bit of current to flow thru main fuse. This extra current is enough to blow main fuse especially during cranking.
replace the r/r with honda style unit or go deluxe and get a SH-775 series and avoid stressing your stator.

I will agree with that, too.

Also....clean the contacts on your fuse box. This is a common place for corrosion and the increased current loads it causes can blow the fuses..

[huntb]

Thanks guys, does this look ok to buy?

eBay Link

[JTGS850GL]

It's the right one.

[posplayr]

I will agree with that, too.

Also....clean the contacts on your fuse box. This is a common place for corrosion and the increased current loads it causes can blow the fuses..

Fully agree that a spray can of DeOxit would be very useful in cleaning out all electrical contacts.

However, with respect to something blowing a fuse, I think the causality is that
Corrosion, creates heat ==>Heat creates melted plastic ==>Melted plastic allows contacts to short.

You would need to allow a ground to come in contact with a hot to blow the main or other fuses.

I can't see how a moderate amount of green crude on a fuse will cause a blown fuse, without the physical deformation the overheated fusebox causing the short.

[robertbarr]

That's the creature you're looking for. As with anything else, there's a risk involved with a used electrical item.

I found my SH775 on eBay and it worked just fine. I'd personally take the risk with that seller. Some will argue that new is the way to go.

[huntb]

Well it's on the way. Who knew a $500 bike would end up costing me so much money. Wouldn't take it back though.

Anyone have tips for wiring the SH775? Will it mount in the same place as my 650E RR? Should I buy waterproof connectors for it or make my own?

[posplayr]

look at how much room you have; the connector is large. The alternative to the connector is a direct solder of a pigtail.

[argonsagas]

Fully agree that a spray can of DeOxit would be very useful in cleaning out all electrical contacts.

However, with respect to something blowing a fuse, I think the causality is that
Corrosion, creates heat ==>Heat creates melted plastic ==>Melted plastic allows contacts to short.

You would need to allow a ground to come in contact with a hot to blow the main or other fuses.

I can't see how a moderate amount of green crude on a fuse will cause a blown fuse, without the physical deformation the overheated fusebox causing the short.

Those are good points, as the increased load causes heat. ...BUT.....I am only speaking from experience as that problem was found on both my 1100 G and GK and it caused a couple of fuses to blow while I was a few hindred miles from home. I guess I should have paid attention to it sooner, as the heat melted part of the fuse box, as well as blowing fuses. After replacing the box, ...and cleaning the contacts on the replacement....no more blown fuses.

[posplayr]

Those are good points, as the increased load causes heat. ...BUT.....

The point of my comments are to correct this notion. The corrosion creates increased resistance at the junctions between copper and brass and where that current has to flow through oxidation and corrosion to continue the electrical circuit.

The increased resistance actually reduces the current flow somewhat; it does not increase the load. However, even with somewhat less current, the increase resistance also brings more heat to the locations of the corrosion as per ohms law and P=I^2*R=I*I*R.

It is only when this heat causes some type of breakdown in the plastic or insulation that a short can occur, and that short is an increase in the load causing causing blown fuses.

The point may seem somewhat academic, but when notions are bandied about that are clearly counter to the most basic principles of electrical circuits, I feel compelled to comment.