Topic: CB650 Testing Rotor Resistance (fluctuating readings)

[lennlen]

Hi everyone.

Before I venture out to pick up a new rotor, I figure I would check in here first.  A friend of mine had recently purchsed a CB650, and found it to have problems charging the battery.  The PO mentioned that he had replaced the RR, and the problem should be fixed.  Aparently, it wasn't.  Had he known the 650's to have such a widespread issue, he would've probably passed on it.

Anyway, we have been testing the various parts of the charging system, and could not figure out what was going on when testing the rotor.  We first tried testing the resistance between the Black and White wire that goes into the RR, we found the readings all over the place, between .2 ohms to 18ohms, and could not find a steady reading.  We proceeded to open the alternator cover to test at the slip rings to find the same thing.

We tested the multimeter on a 10ohm resistor i had laying around, and it read true, so it does not seem to be a calibration problem.

Is this fluctuating reading at both the "slip rings" (where the brushes touch the rotor) and also at the RR indicative of a faulty rotor, or an issue with our method of measuring?

I tried searching for "fluctuating, ohms", "CB650 Rotor", "Test Rotor" etc. etc.  I couldnt find anything like this.

[Tower]

Are you trying to test for continuity in the Stator coil.  If so, the resistance should be 0.2 ohms.  The stator coil has three (yellow?) leads that run to the rectifier.  Or are you testing the field coil?  which has two leads (Black and White?)  The resistance in the field coil should be 7.2 ohms.
If you are getting continuity between lead wires and the coil ground or conversely if there is no continuity between the wires - then the coil is bad and needs replacing.

[750essess]

A 650 has the twin cam 750 style alternator right? If so is probably a bad rotor. I have seen two personally fail.

[TwoTired]

Probe contact resistance variability? Are your probe points clean?
What meter scale are you using?  If it has a diode test range, try that.
Do you know what voltage is being used for the meter resistance testing?
Are you disconnecting the RR from the rotor during the ohms test of the rotor?

From a conceptual standpoint, the black and white wires come out of the RR and into the Rotor.

Cheers,

[lennlen]

Probe contact resistance variability? Are your probe points clean?
What meter scale are you using?  If it has a diode test range, try that.
Do you know what voltage is being used for the meter resistance testing?
Are you disconnecting the RR from the rotor during the ohms test of the rotor?

From a conceptual standpoint, the black and white wires come out of the RR and into the Rotor.

Cheers,

Thanks for the reply TT, i've been reading all of your other replies to this kind of issue.

The probe + multimeter is brand new, everything is clean.
The multimeter is auto-calibrating/auto scaling, and jumps from .15 to 18ohms.  There is no "diode" test range to be set.
I am not too sure about the voltage being used, but it was tested against a known good resistance, so the meter is accurate
The RR is disconnected during testing.  And I have also tested from the rotor's surface, where the brushes touch (as well as the B+W leads that come out of the charging system)

Are you trying to test for continuity in the Stator coil.  If so, the resistance should be 0.2 ohms.  The stator coil has three (yellow?) leads that run to the rectifier.  Or are you testing the field coil?  which has two leads (Black and White?)  The resistance in the field coil should be 7.2 ohms.
If you are getting continuity between lead wires and the coil ground or conversely if there is no continuity between the wires - then the coil is bad and needs replacing.

I've tested between the yellow leads and found ~0.2 ohms, I believe the stator (if the yellow wires correspond to that) is fine.

I was fearing it was a bad rotor, but just wanted confirmation.  I also wanted to check to make sure I am testing the rotor in a correct fashion.  B + W leads that go from the charging system to the RR, and also testing the rotor's surface where the brushes touch.

What I dont understand is why we are seeing varying resistance.  Isnt the rotor just magnetized metal?  When not spinning, shouldnt this show a consistent level of resistance?!

[Tower]

Yes it should be consistent as long as your probe is clean and not jiggly when testing and the ohmmeter battery is capable of sustaining its voltage over the test period.  Varying readings would therefore suggest a broken/intermittent/loose/severely burned wire.  A severely burned/overheated wire might also show up as high, but steady, resistance reading.

edit:BTW this test doesn't tell you if the problem is in the windings or simply in the way the leads are connected.

[TwoTired]

The 650 has rotor windings, as this is the field coil electromagnet.  This it spins, and this differers from other SOHC4s that have a stationary field coil.
The rotor slip ring face can sometimes get a "glaze" on the surface that prevents or hinders current flow.  This may be what is giving you strange meter readings, as well and cause the alternator to diminish it's output.

I assume your meter is a digital type and what I was trying to determine is if your readings are an artifact of the meter technology employed.  Modern meters inject a very small amount of power into the circuit being tested, and then sample the returned power at some digital interval and duration.  The difference between what gets injected and what is measured is then placed on the display at yet another digital refresh rate.

Anyway, it wouldn't take much surface resistance on the slip ring face to make the meter display appear erratic.

Then again, you may well have a bad rotor.  It's just that your test equipment ought to give you a definite and consistent indication rather than quizzical flashes.  Take some 600 grit paper and clean the rotor slip ring surface.  Then repeat your measurements.  It would be nice to have a stable and repetitive reading before positively condemning the expensive rotor.


cheers,

[lennlen]

Tower~I held the probe very steady both on the black and white wires, and also when metering on the face of the rotor.  The Multimeters (I've used 2 different digital multimeters) both had enough battery to sustain its voltage as tested on my known 10ohm resistor.  As far as your edit, I have tested on the face of the rotor to rule out the leads.

TT~I'll give the sandpaper a shot this weekend when I have time.  I'll check back after I get a second set of readings off the face again.  I was under the assumption that a glazed surface would completely stop the current from passing.

My first thought was that the digital multimeter was playing tricks, as we used an older, less robust digital meter last week and was only able to get fluctuating results (again, varying wildly between .2 and 16), seeing that my friend (who owns the bike) is into his electronics as a sound engineer, he went ahead and picked up another multimeter primarilty for work, but tested with the same results.  Hence why I am posting this, as the varying resistance is boggling.

So just to confirm what I have been searching and reading, any other generation 650 rotor will fit, but only a 650 rotor will.

[Ibsen]

The resistance between the yellow leads from the stator should be 0.41~0.51 ohm. And between each yellow wire and ground it should be infinite.
There are no factory specified resistance for the field coil, but measured between the slip rings it should be very low, approx 0.40~0.50 ohm. And measured between each slip ring and ground it should be infinite. But I think measuring the field coil at the end of the wire by the connector might not be the best way since it also includes the carbon brushes.  If the reading you get are fluctuating with both meters you have tried, I would suspect that the slip rings either are contaminated with carbon deposits, or you have a bad rotor.
Cleaning the slip rings thorough and measuring directly on the rings is the best way to establish a good reading. As far as I know, a fluctuating reading, like no reading, indicates a defective winding.

And at least the 1981 CB750F and CB900F share the same rotor as the SOHC CB650, and maybe other year models share the same rotor as well.
It is Honda parts # 31110-425-005 now replaced with parts # 31110-426-832.

[Tower]

Both Haynes and Claymer manuals specify  Stator coil resistance  (0.2 ohms) and Field coil resistance (7.2 ohms).

The manuals also warn to ensure the voltage regulator is set up correctly before verifying the output from the alternator.  Output should be checked using a voltmeter and an ammeter.

[Ibsen]

Both Haynes and Claymer manuals specify  Stator coil resistance  (0.2 ohms) and Field coil resistance (7.2 ohms).

That's strange. Are you referring to the Clymer and Haynes for the CB650, or for the SOHC CB750? The alternator on these bikes are a bit different.

Both my Honda workshop manual for the CB650 and my Clymer manual (thirteenth printing) for the CB650 specify the resistance in the stator windings to be 0.41~0.51. And in the Honda manual the resistance in the field cold isn't specified.
Also my Haynes manual (printed in 1993) specify the resistance in the stator windings to be 0.41~0.51. And like the Honda manual, the Haynes manual doesn't specify any resistance in the rotor. Only the Clymer manual mention the resistance in the rotor by a specific number (approx 0.41~0.51 ohm).
And I can't see that there are any difference between the year models either in the manuals I have. However, there is a difference in the test procedure for the reg/rec unit for the 1979-1980 models and for the 1981 and up models.

[Tower]

@Ibsen, you are absolutely right.  The 650 manuals (circa 1982) don't show the stats, but my 750 manuals (circa 1978) do.

Wasn't there only one alternator up to 1978 for the CB750 and that one was also used on the first CB650s in 1979 (or so I understood - perhaps mistakenly)?  The specs show no variance, nor do the testing procedures up to 1978.

In 1979 the alternators changed - requiring a different testing procedure and the CB750 went out of production.  Did the CB650 got a completely different alternator as well in 1979, or was it not until 1980? 

edit: I'm guessing we're referring to the early years otherwise someone would have said Nighthawk.

[Ibsen]

Tower, the SOHC CB650 had the same type of alternator, combined with a transistorized reg/rec unit, from it's introduction in November 1978 (the UK Z-model) till the last model produced in February 1984 (the UK Nighthawk).
And like TwoTired pointed out:
"The 650 has rotor windings, as this is the field coil electromagnet.  This it spins, and this differers from other SOHC4s that have a stationary field coil."

This is the 650 alternator:

LINK

[Pinhead]

The field coil should DEFINITELY not be that low of resistance (.4 to .5 ohms). If they are, your rotor is shot. I know this because I bought three of them off eBay before I built my regulator/rectifier. The DOHC CB750, SOHC CB650 '79 to '82, (NOT DOHC CB650), and CB900/1000/1100's, (not positive on the years) all use the same rotor. The resistance CAN vary a little between 4 and 7 ohms. Maybe a little higher, but I wouldn't want to go much lower.

Cleaning the slip rings is definitely needed before you can get an accurate measurement. All three of mine needed cleaning before I could verify that they were good. All of Honda's three-phase charging systems are BASICALLY identical. Some of the components are interchangeable (with a little wiring rework). If your regulator/rectifier is bad, build your own using MUCH cheaper parts (see the link in my sig).

The rotor definitely has the highest failure rate of all the parts on the CB650's. Using the reg/rec in my sig reduces those failure rates by quite a margin.

[TwoTired]

There are no factory specified resistance for the field coil, but measured between the slip rings it should be very low, approx 0.40~0.50 ohm.

This cannot be correct.  I would expect around 7 ohm to be correct.
The regulator delivers 12V to the 650 rotor/field coil to create an electromagnet.  Ohms law states that 12v through a 0.50 ohm resistance will draw 24 amps or 288 watts.
This is far more than the alternator is rated to deliver out the yellow wires and would render the device useless.

A 7 ohm coil would draw only 1.7A or 20 watts which would allow the alternator an output to power things like lights and battery restoration.


Cheers,

[Tower]

Agreed TwoTired.
I believe a chart is in order:

Alternator Component Resistance Specs (ohms)                     SOHC          SOHC Component           750           650        Field/Rotor Coil     7.2       6.0 - 7.0 Stator Coil          0.2       0.41 - 0.51

[Ibsen]

I don't disagree with you guys, but I really didn't want to make a guess, not even an estimated one.
So, can anyone come up with the correct factory specification?

In Mike Nixon's troubleshooting guide he claims that if the resistance between the slip rings are less than 2 ohms there is a short in the windings:

http://home.earthlink.net/~trinomial/DOHCcharge.html

And a good reading should be somewhere between 2 and 6 ohms:

http://home.earthlink.net/~trinomial/chgfigure06.jpg

Another troubleshooting guide found for the CB650 charging system on the Oregonmotorcycleparts.com website states that the resistance normally should be 4.5~6 ohms, but, like Mike Nixon's guide, that anything under 2 ohms means that there is a short in the windings.

http://www.oregonmotorcycleparts.com/FAQ.html

So 7 ohms is a pretty good estimate, although it seems to be a little high.

[lennlen]

Thanks so much for the good information.  I have yet to go out there to test the Rotor again.  I was running into this exact problem when trying to garther information on what resistance I should be seeing at the slip rings.

Ibsen and TT are right, and I realized I forgot to mention that the bike is a 1980 CB650SC, which does not use a field coil.  I was a bit confused at first when I opened up the covers, where the diagrams/manuals I had on hand did not match what I was looking at on the bike.

Tower, I will look at the stator leads again this weekend, and test between the yellow leads.  Just to confirm, I am now looking for (as per your beautiful chart) 0.41 - 0.51ohms between each yellow wire.

Again, thanks for the great information.  And please dont hesitate to add to what has been said here.  I'm sure I'll need further guidance once I find time to fuss with the bike this weekend.

[Ibsen]

It's sometimes easy to forget that the 650 is different from the other SOHC models, and that it has got the same charging system (and electronic ignition system) as the DOHC CB750/900.
The funny thing is that I had the alternator cover off on my 650 only a week ago to check the state of the carbon brushes and the slip rings, but I never thought about measuring the resistance in the rotor. If I had I would have been able to tell you what to expect from a good rotor.

If it turns out that you need a new rotor you can get one for $139 +shipping from Electrosport:

http://www.electrosport.com/electrosport_press4.html

[lennlen]

It's sometimes easy to forget that the 650 is different from the other SOHC models, and that it has got the same charging system (and electronic ignition system) as the DOHC CB750/900.
The funny thing is that I had the alternator cover off on my 650 only a week ago to check the state of the carbon brushes and the slip rings, but I never thought about measuring the resistance in the rotor. If I had I would have been able to tell you what to expect from a good rotor.

If it turns out that you need a new rotor you can get one for $139 +shipping from Electrosport:

http://www.electrosport.com/electrosport_press4.html

Any sense in picking up a used rotor on ebay?  Or am I stepping into the zone of unknown reliability and service history?  My friend was hoping to spend far less than $100 on this part...

[TwoTired]

I've not owned one of these bikes,  But, I have helped a few through repair, and know a fair bit about electrical devices.
The failure mode is that the insulation of the wires wound on the rotor breaks down over time, heat, centrifugal force, and possibly oil attacks.  The wire winding insulation isn't very thick even when new.  So, there doesn't have to be very much insulation displacement to get connectivity where none is desired.  Given this, I wouldn't trust or pay for a used one if I intended to keep the bike, particularly if it came with no warranty.  It might work for a few years, or a few weeks.  A new one, or a re-manufactured one ought to serve reliably for 10-20 years.

In this case, the peace of mind purchased along with a new unit should play a factor in the repair outlay decision.

Bear in mind, I've only dealt with failed 650s on this forum, and I have no idea of the population statistics of 650 rotors still working since original manufacture.  But, my impression is... not too many.

Good Luck!

[Pinhead]

I agree. The rotor is always the first to go on a 650. It usually takes the regulator with it.

[Ibsen]

I agree with the others. And TT are spot on regarding the weak spots on these rotors. Also if a bike have been sitting for a long period the rotor will pick up condensation and start to rust, and the rust will also crack the insulation on the windings. Oil shouldn't be a problem though unless you get a leaking crankshaft seal.
A new rotor would be my choice as well. As far as I can recall, used rotors are seldom up for sale on Ebay (or maybe they are but I just haven't looked for them). And like Pinhead did, you might end up buying several used ones till you find one that is in working order.

The rotor problem isn't related to the 650 only, also the DOHC CB750 and 900 have got the same issue since they use the same rotor. That's probably why you can't get a spare rotor from Honda  any more (they are all sold out) and why Electrosport are producing them (there is a market for them). And if it is of any comfort, the OEM rotor is listed on the MrCyles website at a price of $273.  David Silver also have it listed, £199/$410.

[lennlen]

Hi guys, just getting back to all of you.  Thanks again for all of the help!

I was finally able to get a good reading off the rotor after hitting it with some 220 sandpaper.  I didnt know the rotor was originally supposed to look that clean!  I ended up with a steady 1ohm reading, which, as per this thread, means the rotor will need replacing.

Thanks again for all of the help!  I'll probably post again at the end of the day, we're getting through the carbs right now :p