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Discussion Starter · #1 ·
1981 Honda cb900c is not charging at low rpms. There are two issues I've found that could be contributing factors but could use some feedback before I invest more money in parts. The first issue is the resistance on the rotor is only 2.9 ohms instead of 3.6 to 5.2 required. And the other thing is voltage is dropping a volt through the ignition switch which means that voltage to the rotor is low. No charging at low rpms and at higher rpms the voltage doesn't get much higher than 13 volts. Probably just need to replace both rotor and ignition and go from there but wouldn't mind some feedback. Thanks.
 

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Where are you measuring the Voltage - directly across the battery terminals, or another spot? Also, did you measure the stator outputs to ground/engine case?
Often, poor charging is due to dirty contacts, like the battery ground cable at the chassis/engine or the battery terminals. The connector at the rectifier/regulator is also suspect, and between the r/r and the stator.
 

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The first issue is the resistance on the rotor is only 2.9 ohms instead of 3.6 to 5.2 required.
Your bike has a field coil, or do you mean the stator? (Rotor spins, stator stays stationary)

Is there a schematic online we could look at? Is it a combined rectifier / regulator?

Machines that were built before USA mandated daylight headlight use, ( which I think was in 1981) will generally have lower powered charging systems. They were designed to maintain proper charge when the machine was operated in a variety of conditions, like 65% daylight (no headlight), 35% nighttime. Simple shunt type regulators, have no field coil, and just dump excess power to ground. The charging system may not be designed to charge the battery at idle. With the headlight running all the time, the stock charging system may be unable to keep the battery charged especially if your use is city type driving with plenty of low speed and idling mixed in. Switching to LED or a lower powered headlight can help, or regular use of an external charger.

2.9 vs 4.4 ohms suggests that 20-30% of the windings are shorted. However, most handyman type ohmmeters are notoriously imprecise at low resistances. Do you have a precision resistance to calibrate your meter with? Did you account for the resistance of the clip leads?

I'd bypass as many switches and connectors as I could, with known good jumper wires, before condemning any expensive, and difficult to replace components, like stator and regulator. Connectors and switches on older machines develop a little oxidization and may make contact, but still steal a bit of voltage at each junction. A good trick is to check voltage drops all the way through the charging system wiring, to see where the resistances are. Voltmeters are more precise than Ohmmeters. Disconnecting and cleaning contacts all along the way can't hurt. At the same time look for any wiring that has been compromised by flexing or overheating. Comparing AC voltage at the alternator to specification is another worthwhile thing to do to isolate which component might have an issue.
 

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1981 Honda cb900c is not charging at low rpms
How low of RPM's?
Many Japanese machines have what is called a break even speed as it relates to charging of the battery when running.
Most of the time that speed is around ~1500 RPM or so. Meaning above 1500 RPM the battery is being charged, 13 ~ 14.5 DCV, and below 1500 you might see around 11.0 ~11.5 DCV at the battery.
When running right at break even speed you would see roughly battery voltage of around 12.0 ~12.5 DCV measured at the battery.
Your charging system may be working okay but as already mentioned bad connections somewhere might be bringing voltage down some.
What voltage are you seeing (at battery) running at these four engine speeds:
. On idle.
2000 RPM.
3000 RPM.
4000 RPM.

S F
 

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Discussion Starter · #6 ·
Where are you measuring the Voltage - directly across the battery terminals, or another spot? Also, did you measure the stator outputs to ground/engine case?
Often, poor charging is due to dirty contacts, like the battery ground cable at the chassis/engine or the battery terminals. The connector at the rectifier/regulator is also suspect, and between the r/r and the stator.
I'm still having trouble with charging system. Can you tell me if stator to ground resistance if 100-300 John's would indicate bad rotor?
 

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The resistance between the stator coils and ground should be near infinite. The only connection between the stator and rotor is the magnetic field produced by the rotor, so your question confuses me.
 

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Discussion Starter · #8 ·
The resistance between the stator coils and ground should be near infinite. The only connection between the stator and rotor is the magnetic field produced by the rotor, so your question confuses me.
That's my question, is 100-300 kohms considered near infinite? Or should it not measure anything on the ohm meter. Maybe I misspoke. I meant between coil and ground.
 

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For a stator, 100k-300k Ohms to ground would be close to infinite, with the grime that collects being the conductor. My meter reads about that when I just touch each lead tip with my hands. A good cleaning, especially at the connector, should send that number even higher, but that reading shouldn't be a problem. I read your previous post to say 100-300 Ohms; I see now it was a typo. Was the reference to a bad rotor meant to be to the stator? Or were you referring back to the original post about the rotor having lower that spec resistance?

Values less than 5 Ohms are often not reliable in consumer-grade meters, so the difference between 2.9 and 3.6-5.2 could be an artifact of the meter, or it could mean the rotor has some shorted coils, which would reduce charging. If you set your meter to the 10A (or higher) scale, and connect it between the battery (at 12.6V) and rotor, with the other side of the rotor connected to the other battery terminal, you should read 3.5A or less; if it reads 4A or more, your Ohm meter may not be lying to you, and the rotor has too low resistance. This can be a tricky reading to make, and sparks can fly, so it may be good to have a helper, and only connect the rotor for a few seconds.
 

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Discussion Starter · #10 ·
The rotor ended up going out completely so I replaced it. New wiring harness, new ignition switch, new r/r. I've got about 8vac coming out of the stator but battery voltage out of rectifier. Rectifier tests out fine. I'm really at a lose and frustrated as heck.
 

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Is that 8VAC with the stator connected to the r/r, or not? Also, measured to ground, or line to line? If connected, and measured to ground, that may be normal, just because how it all works, but I would expect closer to 9 or 10. It could be an artifact of the meter, too, because with the r/r connected, you don't get a normal sine wave that the meter is set for. If you can break into the wire from the r/r to the battery and insert your meter, set to the highest Amp scale, the reading will tell you how well it is, or isn't, charging. At idle, it may be normal for the battery to stay at about 12.6-12.8V with a low current into it. Be aware that the current coming from the r/r also powers the rest of the bike, so it is hard to measure the actual charge current. That would be the difference between the output of the r/r and that going from the battery to the main fuse. If you remove any and all fuses, except for what is needed to run the bike, you will get a better charge reading.

Too bad you're way down in Texas; making these kinds of tests were part of my specialty before I retired.
 

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Discussion Starter · #12 ·
The voltage was probably somewhere between 8-10 on all of them connected to rr. But, the voltage out of the rectifier doesn't go above battery voltage so there isn't any charging even when I return the engine. I know it doesn't make any sense. I'll try doing some more measurements tomorrow. Maybe I missed something.
 

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A couple more things you might check. I have seen the brushes get past their wear limit and make it not charge correctly. Also make sure you have a good 12 volts going to the brushes. And most of them I worked on, just barely charged, if at all, at an idle, and you had to have the battery charge really good, and it would charge a little better then.( but at higher RPM it should charge good.)
To check the AC voltage on the three legs from the stator. I would unplugged the stator, and make jumper wires for the two extra wires going to the brushes on the rotor so it will have 12 volts on the rotor and it will put out ac voltage correctly. Start the motor and turn it up to 4500 RPMs. Now check across the 3 legs of ac voltage, it should be putting out 70 to 80 AC volts on each leg equally.
When checking DC voltage output, it should put out the same at the battery as it does at the rectifier regulator or at least close to it. I have put a jumper wire between the hot side of the battery and the hot on the rectifier regulator so it would charge a little better.
Stay safe out there hope this helps you out.???
 

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When measuring a stator output, connected or not, you should always measure leg to leg, not to ground. It does sound like you have little or no current going through the rotor, so make sure the brushes are good, as recommended. Just because you get a good reading through the rotor at the connector doesn't mean the brushes are working when the rotor is moving, so if you can have a look at them, make sure they move enough from spring pressure that they make firm contact with the rotor. A clean ground to the r/r is required, too, so the ground leg of the harness needs to be clean and tight, especially if it is separate from the heavy battery ground.
 
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