Topic: '79 cb750f charging issues...solved

[tennesseebreeze]

I acquired this bike from an old friend of mine. It has high miles (48,000) and looks like a Mad Max starter bike, but he had full service records going back 10 years, a clear title, and my inability to pass up a bike for less than $500. I solved some idling and carburetor issues only to get some charging issues in their place. There is the chance that some welding I did on the bike contributed to these problems, but that seems to be less and less likely as I eliminate potential culprits.

At the battery I'm getting 11.25-11.85v while the bike is running. It does not change much in higher RPMs. The rotor is reading 5.0-5.5 ohms resistance to spec, the stator has continuity. The battery is good and holds a charge. I bought a cheap regulator/rectifier to replace the original that no longer worked.  I suspected that it was probably the regulator so I bought another, but I'm still having the same issue.

I'm diving back into it tonight where I'll check the following:

See if the brushes are damaged or worn,
check all grounds (something I should have checked first)
clean the rotor
check for burnt wires
make sure the welding on the case isn't physically interfering with the generator's fit

If there are other things to check, I'd love to hear it.

[tennesseebreeze]

Well this was unexpected. 

I started it up last night and noticed the generator case heated up very quickly, before anything else did. I took off the cover and discovered the rotor is rubbing the stator and producing a ton of heat, friction, and metal dust. There was a missing screw holding the stator in place to the cover. Not only that but the screws that were there were slightly longer than stock. That, coupled with the piece I welded on the case being slightly proud of the matching edge, served to screw up the fit and caused the rubbing. This is probably causing the weak voltage going into the battery. After replacing the missing screw and grinding down the matching edge, it fit better and my voltage went from 11.25 to 12.45. Still not enough, but I suspect the stator got damaged or possibly warped by the excessive heat. It still seems to be rubbing, just not nearly as much. I have a spare rotor that I will try to install tonight. I hope there isn't more damage than that, I'd hate to have to buy a new rotor.

[Hondawggie]

I bought a '79 CB750f after the original owner experienced a charging issue.

He bought a NOS (New Old Stock) rotor -- I think he told me $250 at that time (2011 time frame). 

I asked him what led him to the rotor as the cause -- he said "it lost its magnetism.  But the new rotor had the same problem."

He told me that when he put a wrench on the rotor and it fell to the ground instead of sticking magnetically to the rotor, he *knew* the rotor was bad.

He sold me the bike for $450.  The bike looked brand new cosmetically with 18k miles.

Some rotors are in fact magnetized; and some not.  The '79 CB750 rotor is not.

Turns out the bike had a bad regulator.
Much cheaper than the new rotor.

Follow a troubleshooting manual carefully on this problem, the parts are expensive.
And I was told the 1979/80 CB750F has a reputation for charging system problems.

[tennesseebreeze]

I bought a '79 CB750f after the original owner experienced a charging issue.

He bought a NOS (New Old Stock) rotor -- I think he told me $250 at that time (2011 time frame). 

I asked him what led him to the rotor as the cause -- he said "it lost its magnetism.  But the new rotor had the same problem."

He told me that when he put a wrench on the rotor and it fell to the ground instead of sticking magnetically to the rotor, he *knew* the rotor was bad.

He sold me the bike for $450.  The bike looked brand new cosmetically with 18k miles.

Some rotors are in fact magnetized; and some not.  The '79 CB750 rotor is not.

Turns out the bike had a bad regulator.
Much cheaper than the new rotor.

Follow a troubleshooting manual carefully on this problem, the parts are expensive.
And I was told the 1979/80 CB750F has a reputation for charging system problems.

Thanks for the info. I didn't read anything about rotor magnetism. The rotor got scored on the sides when it rubbed the stator. I didn't know how it would impact it's effectiveness, but I lightly filed and sanded the sides in case. I'm putting new brushes and a replacement stator in it since I have a spare. Stripped screws are slowing that down.  Right now I've got the carbs out to service them so I won't be testing anything for a couple days. I'm so anxious to ride in this weather that I'm having dreams about it.

[tennesseebreeze]

The saga continues...

The rotor was indeed rubbing the inside of the stator coil and causing a lot of heat which I suspect compromised the stator or some other component. I ground down more of the inside of the generator case where I had welded a piece of aluminum to repair it. This eliminated the rubbing I believe. I did some further tests and this is what I came up with- somewhat contradictory results:
The battery was showing no more that 12.65 volts at 3000 rpm and above. Something like 12.15 at idle speed, 1500 rpm. I checked the stator wires with a meter (engine off) and it read continuity between all the yellow wires, but not from the yellow to the ground wire. This checks out according to the manual. It doesn't seem to have a ground fault in the stator. The yellow wires read .9 ohms resistance between each of them. I don't know what it's supposed to read or if this is an acceptable amount.

The rotor reads a 5.1 ohm resistance, which is where it pretty much should be, 5-5.2 I think. I tested the battery with the generator disconnected and was getting about the same readings as I do when it is connected, so it seems like I'm getting no charge coming from it. I ran the bike and tested the wires coming from the generator and I was getting pretty much zero voltage. My best guess is that it is a bad stator.

I'd love to get some more feedback before I order a stator. I'm not super confident in my abilities in the electrical stuff, so if I missed something, let me know.

[calj737]

Here you go-

http://www.electrosport.com/media/pdf/fault-finding-diagram.pdf

[tennesseebreeze]

Thank you calj. I was still getting good readings out of the rotor and stator last night and the fault finding diagram had me looking closer at the brush wires (I had already replaced the brushes). Under the heat resistant wire cover I saw what could be the possible culprit- the three yellow wires' insulation had melted together and they were exposed to each other. I think what happened is when the rotor and stator were rubbing together it got way too hot for the wires and they melted. I separated the wires and taped them up. But by that time my battery was dead from all the testing and I couldn't get a read on whether I fixed it.

If the yellow wires were touching then I would presumably get continuity readings from them. And since the ground wire was not effected, I was not getting a short to ground. This may explain the good readings but the lack of charging voltage.

[tennesseebreeze]

It feels so good to figure out a problem and solve it. I'm getting 13.96 volts to the battery- a little low perhaps, but the battery is low and I may get more when it's fully charged. I'd bought a used generator cover on ebay to replace the scraped up one one the bike. It had stator in it I didn't know I'd need. After using an easy-out to remove the stripped bolts I replaced the brushes and installed it.

And don't you know, it's raining all day today.

[calj737]

That neglible amount of voltage difference can be down to corrosion in your wiring harness, or error in your meter. If your battery was low, the regulator on the bike should actually allow a higher voltage to be sent to the battery than a reduced voltage. So, you might double check the regulator sensing wire and determine what reading it gets compared to the reading at the battery.

If the battery was say, 12.6 and the regulator see 11.7 due to corrosion, your regulator will always allow an overcharging of the battery based upon the 11.7 (as an example). Best to insure that these two metering points are as close as possible to each other. And of course, different battery types behave differently (lead acid vs AGM vs LiFe).

[tennesseebreeze]

Great info. Thanks man.