Rotor gone bad?

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xs650newb

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Hey everyone,

Just want some confirmation.

Went out for a ride, bike all of a sudden dies. Get back to my place and the battery is completely dead. I know before I left the charging system works because I checked it one or two days prior.

I charge the battery and check the current draw with the key ON and engine OFF. It is well over 10 amps so I immediately turn key off. I start by looking in my headlight buck for a shorted wire but don't see anything. Check current draw again with key ON and it is back to normal (2 amps with headlight off).

Go for a ride and SAME thing happens again. This time I check the charging system.

Results:

Resistance between three white cloth wires coming from alternator: ~0.7 ohms.
Resistance between the two slip rings: 1 ohm.

So the slip ring resistance is bad. I guess the rotor went bad? Can it go bad all at once? I mean, everything was great the day before. Also, the problem seemed to have fixed itself for about an hour or two between testings. Can something else be wrong? Maybe a frayed or pinched wire somewhere?

Best,
Adam
 
They (wire wrapped assemblies such as coils and stators) can heat and expand inducing a short that disappears when the cool down. Less likely that the same occurs with a single wire.
 
Ok. Quick update.

I measured the slip ring resistance incorrectly. It is actually 4 ohms which is not ideal, but I don't think that should be causing a +10 amp current flow with the key off.

Anyways, I have isolated the problem further. The short dissappears when I disconnect the cloth red and green wires which connect to the alternator brushes.

Any thoughts?

Adam
 
Ok. One more update.

When I disconnected the cloth red and green wires from the alternator brushes on the stator plate I measured their resistance relative to ground.

The red wire has 2.5 ohms while the green wire has about 1000 ohms.


Best,
Adam
 
Yeah pretty sure your rotor is toast. How is/was your battery? Is it new, has it been load tested?
 
Its a small battery but it gives 12.7 volts.

A quick update again.

The rotor giving 4 ohms between the slip rings shows I need to replace it eventually, but I dont think it explains the huge current draw when the key is on.

I have a question about the brushes. With the stator removed from the engine (but still plugged in) what should the resistance between each brush and ground be?

Best,
Adam
 
So I think I have found the problem, but I don't know.


When I remove the stator and disconnect it from the wiring harness the inside brush (brown wire) is shorted to the exterior casing of the stator while the outside brush (green wire) is not. Could this be the problem?

Best,
Adam
 
Be aware of the harness routing from the stator to the harness plug.....it travels past the front sprocket and I've seen them chewed a bit under the left side engine cover.
 
I had the same problem - 4 ohms but the battery not charging which meant the spark lasted only as long as the optimum charge in the battery (sudden death syndrome while riding). I am with gggGary and the charging system is toast. I installed a PMA this week, and wow!, what a difference - constant power flow, and a fully charged battery.

Anlaf
 
A PMA is not in my budget because I would also need the PAMCO.

I think the rotor is fine. Perhaps the stator needs to be replaced.
 
Newb You are correct; with the stator off the bike and unplugged on an 81, both brushes should show infinity to the stator housing.
Is this bike still TCI ignition? Just wondering..
It IS hard to get accurate meter readings at these very low ohms.
But 4 is pretty low

ohms law 12 volts 4 ohms is a 3 amp draw. So 10 is pretty high. But one time you had a 1 ohm reading so 10 amps would be in range for that.
Has the bike been charging right?

Has anyone used a regular puller to remove the rotor?
 
gggGary,

When I got the 1 ohm reading I was not measuring across the slip rings, so please neglect that reading.

The bike was working wonderfully until yesterday. Two days ago it was charging at 14.5 volts.

There is definitely a short between one of the cloth wires connecting to the brushes and the stator housing. Not sure if I'll be able to fix it or not without replacing the stator.

Can the stator be taking apart at all?

Best,
Adam
 
I should also mention that when I jiggle the alternator brush wires the ohmic reading will sometimes go to infinity, so that further points to a short.

Adam
 
Yeah it's easy, strip the black wrap off the bundle, the brush wires just travel in the same bundle they are not connected to the stator in any way. (at least by design) look by the bundle clamps and as NJ says back in the sprocket area, I have seen several fuggled wires in this harness back there. a brush wire could be shorted to one of the stator wires that could do it. Report what you find?
 
xs650newb said:
So I think I have found the problem, but I don't know.


When I remove the stator and disconnect it from the wiring harness the inside brush (brown wire) is shorted to the exterior casing of the stator while the outside brush (green wire) is not. Could this be the problem?

Best,
Adam
Click to expand...

Both brushes , green and brown, should have very high resistance to ground. Check all of the connections for the brushes and wiring around the stator area. You appear to have a short circuit from the brown to the stator frame...............that short circuit should blow the 20 amp main fuse when the key is turn on.
 
Hi rg,

The reason the fuse does not blow is because my batter is very small and can only suppy several amps.

I know that it was drawing more than 10+ amps because I have a power supply which supplies 12 amps and has a gauge.

I'll check the wiring now.

Adam
 
And in the post #19 of the thread the "rest of the story" May not be related to today's adventure but expect problems with the stock charging system and a tiny battery.
 

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