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Alternator throwing 0 volts on 1978 CB750K

PortlandThomas

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Hi there,

I have an alternator issue. I first started to work on it because my battery wasn't keeping the charge. So I got a brand new battery, brand new regulator/rectifier (wanted to get those small lithium battery anyway). But I realize that I have no Volts coming out of my Alternator (3 yellow wires coming from the starter coil)
I checked the starter coil and the field coil with my ohmmeter and they seems to be in a perfect shape. So now I feel that the issue could only come from the Rotor but I'm not sure. And I don't really know how to test it. All I can find online is about DOHC rotor testing.

Does anybody would have more clue about the cause of my alternator having no Volts?
Do you also think that it must be the rotor? Do you guys know how to test it? (I can't pull the rotor off the bike currently, I'm waiting for a Rotor extractor in the mail)

Tks in advance. This shit is driving me nuts :)

thomas
 
There is ABSOLUTELY NOTHING to test on the rotor on these, it simply is a piece of steel that gets magnetized by the field coil. All it does.

I would not even bother pulling it, your issues are elsewhere. When you test a DOHC rotor you are testing the field coil, which spins along with the rotor on those, the ONLY difference and why they shell out so fast. A spinning field coil is more likely to go bad from rpm inertia shorting the windings out.

You are very obviously overlooking something.
 
WHAT connection? Grasping at straws there. You need to check for the field coil making magnetism there close to rotor, there is NO physical connection between field coil and rotor other than they are close together with an air gap between them. The coil merely magnetizes the steel in rotor. The rotor by the way it is constructed then makes alternating polarity magnetic spikes that stimulate power in the yellow wire stator wraps.

You will not get one drop of power from stator yellows until you got magnetism at the field coil. Some test for that by bringing a steel screwdriver tip close to alt case, if magnet (field) working then the screwdriver will be attracted to case trying to get to the field coil inside.
 
Field coil is powered up by the white wire from regulator, check it for power at filed coil plug-in with key on. The other end of field coil needs a good engine ground. Field coil windings cannot be grounded at the coil steel housing, only at the very end of the coil. Green wire I'm thinking. The rectifier needs a good ground as does the regulator. Green wires again.
 
Here ya go:

http://www.hondachopper.com/garage/...ng_system/troubleshooting_charging_system.pdf

The first part pertains to the ignition system and ignition coils. Scroll down to the charging system section for a very detailed troubleshooting regimen.

amc49 is correct, the rotor is just a hunk of iron. Your problem lies elsewhere, unless the rotor bolt has come loose, but this is unlikely and is usually accompanied by some very odd noises.


Good info here too:

http://forums.sohc4.net/index.php/topic,1012.msg8345.html#msg8345
 
I am having similar problems. I had a "tested good" stator I bought that was reading .002 volts while running in the bike. I decided I should return that so I cleaned up my old one that tested with good resistance (.6 ohm) and cleaned the rotor as well.
This stator now puts out only .6 volts and doesn't increase much with higher rpm.
I have checked the honda chopper trouble shooting countless times and have tested everything else on the charging system. I have a newer regulator/rectifier combo that IS supplying power to the field coil through the white wire. Green grounds are connected.
My only guess now is to check my grounds more carefully. Any other thoughts?
 
If field coil is internal shorted you can apply all the power in the world to it to still get nothing, and risk burning out the regulator doing it. Testing parts for proper resistance is a waste of time as well if no test is done for a short to ground in them too. Part out on bench might show perfect resistance and be shorted bigtime.
 
With "power to the field coil" have you checked to see if the field coil is generating a magnetic field? This can be achieved by holding a wrench up to the alternator cover and see if it has any magnetic "pull" on the wrench with the ignition switch on and a fully charged battery. The field coil is just an electromagnet and does not draw nor pass much of a current load, making them quite durable. The business end of the alternator is the stator coil. The stator coil (the three yellow leads) actually "generates" the charging current and creates/passes the most current. This also makes the stator coil connectors susceptible to failure over time due to exposure to the elements, and especially due to the amount of current passing through these connectors. There are tests to check the A/C voltage output of the coil that will help eliminate the regulator and/or rectifier as the source of the problem. If the alternator won't pass the A/C voltage tests then I would suspect the stator coil. The stator coil is no longer available. CMSNL lists one but they want $850!

Rick's Motorsports Electrics used to remanufacture them, but the only thing they list on their site now is the field coil. If you do need a stator coil, you might contact Rick's and see if they would be able to remanufacture your core. The last time I found a new old stock stator coil I paid almost $150, and that was at least two (or more) years ago. The high cost of the stator coil makes it that much more important that everything else be ruled out first. You can keep trying used ones, and you may get lucky, but make sure all of the connectors are in top shape first. I've done some tutorials on cleaning up harness connections:

http://forums.sohc4.net/index.php/topic,137351.msg1549191.html#msg1549191
http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364


I will usually replace the bullet connectors to the stator, both the female connectors from the stator and the male connectors on the harness, especially if they are the 40 year old baked-hard original connectors. The multi-block connector further down the line also tends to suffer. Be sure and use proper connectors like those used in the links above. If you are going to use "dime store" electrical connectors then you will likely have problems again. Cut the old connectors off as close as possible to the crimp since there is not very much slack at the connections. I make sure to cut through the insulator on the old connector so that I am cutting right at the crimp on the old connectors.
 
The field coil resistance was tested to ground as well but I do not feel any magnetism with the bike on.
I have three stators and two field coils, all of which aren't working. I am thinking it may more likely be the regulator/rectifier unit so I will test that out next.
 
The field coil resistance was tested to ground as well but I do not feel any magnetism with the bike on.


I have never seen a bad field coil. Not to say it can't happen, just that they are pretty durable and I have yet to come across a bad one. All you need to do to test a field coil is connect one lead + and the other lead - and it should become a magnet.
 
Yes, a bad field on these would not be common as it simply sits there and magnetizes. It's the ones that spin at engine rpm like on the DOHC that gives such fits. The centrifugal forces wreack havoc with the windings. I would think on SOHC if everything proper you simply turn key on and field magnetizes, the regulator powers the field coil up.
 
I was able to get magnetization when I connected the field coil to the battery. So bad rectifier unit, right?
I am probably going to go back to the original system of a separate regulator and rectifier.
 
I was able to get magnetization when I connected the field coil to the battery. So bad rectifier unit, right?
I am probably going to go back to the original system of a separate regulator and rectifier.


All you have done is shown that the field coil is probably OK. Besides the regulator/rectifier, the stator coil has not been eliminated either. I still strongly recommend the A/C voltage output test of the stator coil.
 
All you have done is shown that the field coil is probably OK. Besides the regulator/rectifier, the stator coil has not been eliminated either. I still strongly recommend the A/C voltage output test of the stator coil.
I have tried three stators, all of which are about the same. Am I right that you have to have your field coil magnetized in order to have a proper stator output?
I will test that just in case by running the bike with the rectifier bypass.
 
Am I right that you have to have your field coil magnetized in order to have a proper stator output?

Yes. You must start with a fully charged battery. If you do not have a fully charged battery then the magnetic field will weak or non-existant. The magnetic field from the field coil is required to make the whole thing function. No magnetic field=no charge.
 
Whatever you do, don't let any a/c get into the rest of the system, best way to start blowing parts there is. To whit, the talk of 'bypassing rectifier'. That screams danger to me.
 
Whatever you do, don't let any a/c get into the rest of the system, best way to start blowing parts there is. To whit, the talk of 'bypassing rectifier'. That screams danger to me.


I, too, am confused by the terminology. The rectifier (diodes) converts the alternating current from the "alternator" (that's where it gets its name) to direct current (or DC) that the motorcycle's electrical system can use. I also would not recommend "bypassing" the rectifier.
 
I am bypassing the rectifier the by connecting the battery to the field coil, not the way you two are thinking. No AC volts would go in to anything. I am simply trying to test the alternator output as madmtnmotors suggested, with the alternator not connected to the system.
 
Then call it breaking open the system, to bypass means generally to go around a thing not using it. Careful putting battery directly to field, you may get some whopping a/c if you rev the bike up. Unregulated a/c in an alt can easily hit 75 volts in many of them.
 
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