electrical problems???

[XJfan1]

So one of those brushes being short will not let me get and readings out of the alternator at all?

[XJfan1]

Will a short brush keep the alternator from producing any current?

[Rasputin]

First thing to suspect is the 3 wire white plug under the seat behind the fuse panel. If that is good then I would guess that the brushes may be at fault. The actual stator assembly is unlikely to fail. Not that it can not happen but there is no wear on that part.

[XJfan1]

Im getting no voltage at that plug with the bike running. I unpluged it and checked each wire with the engine running. No voltage at all. I checked accross the battery terminals with it running as well. Its just under 12 volts and dropping. When I rev the bike the headlight does not get brighter. Though ever now and then the tach needle will jump when I rev it.

[Rasputin]

I do not know how long the brushes should be. As there are 2 different numbers for the brushes one may be originally shorter than the other. You should be able to see if the alt is charging by holding a thin feeler guage close to the alt housing when it is running. If it pulls the feeler guage to the cover it is charging but the brushes are not conducting it to the charging system. I am far from knowledgable on electrical issues so someone else will have to advise you further if that does not show the problem.

[crazy steve]

You should be able to see if the alt is charging by holding a thin feeler guage close to the alt housing when it is running. If it pulls the feeler gauge to the cover it is charging...

That test doesn't work for the XJ; different type alternator.

Again, make sure you have good connections at those plugs, and do it by checking for resistance (ohms, with the motor stopped) not volts. If any of those are bad, what the rest of the system is doing won't matter. While a worn short brush might be the problem, verify the connections before deciding it's the brush....

[Rasputin]

I do not know for sure that the test will work for an XJ. I do know any alternator I have be it on my car or truck does behave the same way. If you go out and start your car the center cap on the rear of the alt will pull in a screwdriver when running. Besides it is worth a try.

[crazy steve]

Im getting no voltage at that plug with the bike running. I unpluged it and checked each wire with the engine running. No voltage at all. I checked accross the battery terminals with it running as well. Its just under 12 volts and dropping. When I rev the bike the headlight does not get brighter. Though ever now and then the tach needle will jump when I rev it.

If your meter is set to DC volts, it will read zero; the output is AC....

You can check the rotor and stator coils by unplugging their plugs from the harness and checking for ohms (key off); look for .4 ohms for the rotor (black & green wires), 4 ohms for the stator (between any/all white wires).

IF you get a higher or open reading on the rotor, then you may have a problem with the brushes. Unfortunately, Yamaha doesn't give a spec for brush length...

[BA80]

Im getting no voltage at that plug with the bike running. I unpluged it and checked each wire with the engine running. No voltage at all. I checked accross the battery terminals with it running as well. Its just under 12 volts and dropping. When I rev the bike the headlight does not get brighter. Though ever now and then the tach needle will jump when I rev it.

If your meter is set to DC volts, it will read zero; the output is AC....

To clarify what Steve is saying, the output at the plug from the alternator will be AC. That's what the tach reads. The reading across the battery will be DC. If your not getting at least 12.5 volts DC across the the battery it's not charging.

[CaptonZap]

QUOTE=XJfan1;371667]So one of those brushes being short will not let me get and readings out of the alternator at all?[/QUOTE]

The Yamaha service manual calls for brush replacement every 6000 KM
(10000 mi), which is obviously another in the list of errors in the manual.
Let's assume that they meant 10000 km, which would be approximately 6000 mi.
How many miles on the bike, and do you know its service history?
The brushes connect the field winding to the voltage regulator. When the regulator supplies current to the field coil, through the brushes, the magnetic field produced interacts with the stator coils and supplies output current.
If the field current cannot get to the field coil, because one of the brushes wasn't making good contact, then there will be no output from the three white wires. (Each wire is one phase of a three phase AC output)
Brushes have a flexible woven wire connecting the brush to the end cap. A spring keeps the carbon portion pushed against the slip ring. When the brush is installed, the wire is slack. If the carbon gets worn to the point that the wire is tight, the connection between the brush and the slip ring is compromised, and the output is degraded.
The slip ring, (the brass strip that you see down in the bottom of the brush hole) should be bright and shiny. If it isn't, that is an indication that there is a problem with the brush, or some sort of contaminate getting in there.
Generally, brushes wear at the same rate, (the one furthest from the center will usually wear a little faster, due to the higher surface speed, but anything over a ten percent difference in length would be cause for investigation as to the cause of the difference.)
The springs should both be the same color, and exert the same pressure against the carbon. If one spring is blue, and weak, that is an indication that there was an over current event, which got the spring hot, which degraded it's tension, which means that the pressure against the slip ring is less than optimal, which reduces output, etc. etc.
You can check the field coil part of the system by unplugging the 2 pole connecter on the regulator, and measuring the resistance across the brushes, at the alternator. Should be 4 ohms. If it is, wiggle the brush holders and see if it changes. If it doesn't, measure the ohms at the regulator plug. You should get the same response.
If you don't get 4 ohms, take the brushes out, and put the probes on the slip rings. If you get 4 ohms there, the brushes are bad.
Dirty slip rings can be cleaned with an ink pen eraser, the kind with grit in it.
Be gentle, you're only trying to remove the tarnish, not metal.
Good luck, and let us know what you find. CZ

[XJfan1]

Ok I didn't know that the alt out out on the three wires was ac. I will do some checking tomorrow.

[crazy steve]

Check for ohms, don't bother checking what the AC voltage is. Yamaha doesn't give a voltage spec for this anyway....

[XJfan1]

Check for ohms, don't bother checking what the AC voltage is. Yamaha doesn't give a voltage spec for this anyway....

I will tomorrow. Where exactly do I check with my leeds? Im guesing unplug it and test the 3 terminals in the plug to eachother. And do the two terminal plug the same way.

[crazy steve]

You're looking for .4 ohm between the green and black (two terminal plug) and 4 ohms between the white wires. Also make sure the plugs are good and have low resistance through them when plugged together.

[XJfan1]

I did check for continuity. But that would only tell me if there was a broke wire. I had continity on all three wires but they could still be shorted. Some of the winding loops were loose and rubbing aginst eachother. Im thinking thats where my problem lies.there were chunks and small peices of that coating and the insulators laying in the bottom of the cover. The winding im talking about is the one that bolts into the cover with three screws.with the part with the brushes screwed into the cover in the center of the winding.