Can a bad rotor blow the main fuse?


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Custer, SD
Working on my '81 Special II. Bought it a few weeks ago (had one like it when they were new) and the first few hundred miles all was fine. Lost the charging system recently and after going through the checks I found on this site, determined that the rotor is bad but before spending hundreds I thought I'd clean up the rotor and install new brushes. Didn't solve the charging issue but now the main fuse will go after a short time with the key on. I found one thread that said a bad rotor could do this. I'm getting a reading of 1.4 ohms on the slip rings. Battery holds charge at about 12.4. Could the clean rotor and new brushes making better contact cause an overload on the main fuse? Will be replacing he fuse block and likely installing a PMA. Photo just to show the wiring I'm working with. Didn't have these issues back in '81:)
Hi SDLars and welcome,
1.4 Ohms is way too much draw and it's quite likely sucking enough juice to blow the main fuse after a while.
5.4 Ohms is what the rotor should read and 4 Ohms is the absolute minimum where it'll make more current than it sucks up.
Your least cost option is to buy a rewound rotor, last quote I saw for one was $130.
A PMA will cost you at least twice that and I much doubt that a PMA is an improvement over the stock alternator, anyway.
While you are at it, think about swapping those stock glass tube fuses for mini-spades.
Scallywag sells nice ones.
&BTW, I too have issues that I didn't have back in '81. Getting old ain't for sissies.
wut Fred sez, used rotors aren't that hard to find, hughes sells pull outs cheap You gotta get one with a magnet for the TCI pickups. 80 and newer.
Thanks guys. Confirms my suspicions. Blade fuse block is on the way. Think I'll go the rotor route first. Fred, the comment on different issues than in '81 made me smile.
On the 80 up bikes swapping to a PMA requires you replace the stock ignition for and after market ignition. The PMA has no provision for a magnet to trigger the TCI's pick ups.
So another large expense.
New rotor installed, charging fine, no blown fuses, back in business as soon as the new chain and fuse block arrives. Thanks for all the help.
New rotor installed, charging fine, no blown fuses, back in business as soon as the new chain and fuse block arrives. Thanks for all the help.

Is it a new rotor, or a used one? If it's original, you will face the same problem sooner than later. I hope you kept the old one and I'll get to that.

The issue with the alternator rotor is the insulation on the wires it's wound with. Wire insulation technology from 35 years ago was a very long way away from what it is now. Now that it's old, once exposed to heat and vibration, it breaks down very quickly and develops short circuits in the winding. Depending upon where the short is, you get 2, 3, or zero ohms. It won't make a strong enough magnetic field to induce current into the stator windings and your battery goes flat, right through the rotor with help from the voltage regulator. Yes, it can blow the main fuse.

If you send the old rotor to someone reputable for a rewind, the new wire has very much improved technology in the insulation. Custom Rewind enjoys a good reputation for that. The rewound rotor will likely outlast the bike. You just have to keep in mind the brushes are a service item that you have to look after. The voltage regulator feeds battery current to the rotor through the brushes at a rate that depends upon the demands on the system. It's really quite sophisticated for a motorcycle at this price point.

Unlike the XS650, most motorcycles employ a PMA (permanent magnet alternator). Instead of a battery current fed rotor, the rotor is a permanent magnet with no wiring to it. The faster it spins in the alternator, the more current is induced into the stator, regardless what the demand is. That is why you see the big heat sink on the voltage regulator. The excess current has to go someplace, so it is dissipated as heat.

I met Hugh of Hugh's Handbuilt just recently. He told me he now has a PMA system complete with ignition. He was running it on an XS650B. I didn't ask the price, but I'm sure it could be a great option for many, especially as ignition parts become scarce.

I hope this helps somebody, if only a little.
PM alternators present two issues. The first is fitment.The OE XS650 stator has full support on the radial axis; the two 6 mm. screws that hold it give only axial support. Some PM adapters allow radial movement, forcing the two long, skinny fasteners to support the stator on both axes. If anything loosens up, the rotor contacts the stator and the alternator becomes scrap. The second problem is excess current. Under sustained hard running, the heat sink on the reg/rec unit is often unable to cool quickly enough, and rectifier diodes fry. The result is AC pumped to the battery. If you have an unsealed battery you'll know the rectifier is shot when acid boils out. Sealed batteries can crack. Anyway, if you just have to have a PM system, don't follow the other growing fad and install LED lighting. Go the other way and install the highest wattage stuff you can if you intend to do anything but cruise around town.
What brand rotor did you buy, SDLars? How's it working?
Got it off ebay, not sure from who anymore. Had other issues after this post. I got two screws mixed up on the first one. Ran for a couple hundred miles before the long one trashed the rotor. (Dumb!) Got another one, found a good vintage bike mechanic. Ran for maybe 25 miles and it was like the rotor lost much of its magnetism. Mechanic JB welded a couple small magnets to it and it's been fine now for about a thousand miles. See how long it lasts.
A new one, supposedly. Another ebay item. I hadn't seen this thread update for quite a while as you can tell. We did go through the tests but I was really only in the background after I handed it off to the mechanic. Can't even tell you what the magnets looked like.