Is 5 ohms on an XJ550 rotor too much?

Has anyone definitively determined what "high resistance" means when it comes to the rotor as measured between the two brush contact surfaces? My Clymer manual says Yamaha doesn't specify a resistance.

My bike isn't charging properly, although it is still rideable. It barely puts out 12.8 volts all the way up to about 5000RPM, where it will top out at 13.9V.

I decided to check the resistance on the stator and rotor. The stator appears to be ok, not perfect, but ok. I read between .6 and .7 ohms between any of the white wires. The limit is .55 on the high side at 68 degrees, so since it is close to 90 outside, I am not going to condemn the stator just yet. The rotor field should measure about 4.5 ohms and it is reading 34.5 ohms between the green and brown wires. I also get 5ohms between the two brush contact surfaces.

My brushes are at 10.73mm and 11.2mm or so, so I am going to have to replace those. But I am wondering if the brushes being so close to the scribe lines is what is causing the rotor field to measure so high at the connector wires, or if I should just go ahead and replace the rotor also. The only way to know whether or not to go ahead and do that is to find out if 5 ohms is too much between the brush surfaces. It looks to be in great shape otherwise, and there doesn't appear to be any breaks in the epoxy, so I am leaning towards just the brushes causing the high reading.

Any thoughts?

Thanks to all that can help.

 

My thought would be to clean everything up real well and install a new set of brushes and then see how it works. If you know the brushes are getting down there, change 'em.

Also check the wiring and terminals on the big connector for the voltage regulator.

 

Well, I went back and rechecked the brushes after reading some of chacal's stuff. It turns out the brushes are well within spec as I was measuring them as they protruded from the holder. The only thing is, I pretty much destroyed the holder as it just about crumbled as I was taking it apart to get the brushes out.

I'm wondering if the really dirty face of the rotor is the reason the ohm reading was so high at the connector. I had already cleaned it with contact cleaner, but I took some scotch brite to it and that made a world of difference in the cleanliness. The only thing is, I can't read the ohms at the connector now because I have no way of holding the brushes to the rotor in the assembled position. The cleaning didn't really make a difference on the rotor contact points themselves, but that doesn't mean anything, as the ohms reading might be in range anyway. And that would make sense, that if you are supposed to get 4.5 ohms at the connector, you should get that at the rotor face also. So my 5 ohms reading may not really be that far off from perfect.

I am also beginning to suspect the regulator for the charging problem. I was able to check the rectifier side earlier, but I didn't check the regulator side yet.

 

You need to test for continuity between the two slip rings of the Rotor.
Fergit about the brushes for now.. they are either long enough or too short No measurements needed to spot tyhe difference.
Some cheap Meters cannot detect that low an Ohm reading So them simply applyt 12V batterey power to the 2 rings you will see a decent spark if the thing is not broken (and they Do break often , especially after 25 + years..

 

Please re-read my original post. I am getting 5 ohms between the slip rings. That is my question, whether that is too high or not. The book simply says high resistance. What is considered high resistance, between the two contact points, on this particular component?

I'm using a Fluke 177 for my measurements.

 

Checking alternator rotors: the resistance across the two lead wires (usually red and green) at the connector should be as follows. Note that worn or damaged alternator brushes can affect these readings, as can "dirty" copper commutator strips on the rotor face (where the brushes contact the rotor):

4.5 ohms +/- 10% for all XJ550 models.

4.0 ohms +/- 10% for all XJ650, all XJ700, all XJ750, XJ900RK, and XJ1100 models.


Your rotor is out of spec, but just barely. If you check it the "proper" way, thru the wiring harness lead, I'll bet you that the resistance is even greater.....




And just for giggles:

Checking alternator stators: the resistance across each pair (w1 to w2, w1 to w3, and w2 to w3) of the three white wires (w1, w2, and w3) at the alternator stator lead harness connector, on those three wires LEADING TO THE STATOR, should be as follows:

0.50 ohms +/- 10% for all XJ550 models.

0.46 ohms +/- 10% for all XJ650, all XJ700, all XJ750, and XJ900RK models.

0.37 ohms +/- 10% for all XJ1100 models.

 

Please re-read my original post. When I read the rotor field through the connector, I was getting about 34 ohms, instead of 4.5 ohms. While I understand that is completely out of spec, I would like to know if the 5 ohms I am reading directly on the brushes contact points of the rotor is out of spec. The book does not state a high spec, it simply states high resistance.

If the 5 ohms, directly on the rotor is not out of spec, then that would mean that the brushes are the culprit whether they are long enough or not, because the wiring checks good with no discernible resistance at all.

Also as I stated, I completely cleaned the rotor since I took the 34 ohm reading, but I can't check the resistance again in the assembled position because I broke the brush holder. You can't read the rotor field resistance without the entire assembly re-installed on the bike and that means the brushes touching the rotor as they normally would during operation.

 

About 2.5 - 3.0ohms directly on the commutator strips. This is on freshly rebuilt, unused 550 rotors.

 

Thanks for checking that for me. Now, I am still back at square one. Has anybody ever determined what is considered high resistance at the same spot. The commutator strips, the sliprings, the brush contact points. Whatever you would like to call them. What is considered high and out of spec?

 

OK,

I know you guys are thinking I just like asking dumb questions for the heck of it, but here is what I found out.

My rotor brushes are what was causing my charging problem, despite the fact that they were well within spec lengthwise. As I stated earlier, when I checked the resistance at the green and brown wires I was getting about 34 ohms when it should have been about 4.5 ohms. I got a different set of brushes, not new ones and as a matter of fact they are more worn then my original ones were, because I broke my connector and the replacement connector came with these brushes still installed. I wasn't willing to try and swap any more parts than necessary as I was afraid to break the "new" brush holder. With the replacement brush holder and brushes in place, the reading on the green and brown wires is now 4.5 ohms.

I also checked the regulator/rectifier and they are within spec and doing what they are supposed to. It still takes a little more than 2000 rpm to get the full 14.4V, but that rpm is still much lower than the 5000 rpm that the bike was requiring to just get 13.9V before. At idle, the bike is putting out about 13.3V. It wouldn't even do that before.

 

No dumb questions at all and you solved your problem which was the whole point after all.

The charging system isn't at full output at 2K, that's just the approx. RPM where it starts to actually do anything. (On a 550 Seca, you get to watch.)

I wonder WTF made those brushes read such huge resistance, or maybe somebody had replaced them previously with incorrect parts?

 

I think it was the actual wires that come off the brushes themselves. They appeared to be pretty green once I took a close look at them, and there is really no way to clean them once they are corroded like that because they are so fine and not very dense, so the corrosion just weaves through and through all the fibers.

 

I've got the identical issue going on as we speak, just not as bad as he had it.
Resistance at the rotor connector is about 10ohm, but checking between the rings on the rotor is 4ohms. And like BDM, my brushes appear to be fine, with a good 10mm above the wear marks.
Going to check the brushes wiring harness tomorrow and see if I can't find something...