Topic: 1975 cb750 charging issues

[phil71]

Helping a friend with his bike. He took it to a shop with low/no charging situation and they put in a combo reg/rect.
A misdiagnosis I think, but still not a bad thing to have. Anyhow, I started diving in, and the wires between the stator and the reg/rect were pretty beat up, and rigged with bad connections.
I ran all new leads from the stator and field coil, then I took the reg/rect output and home-ran it to the battery with nice thick wire.
  Prior to that, even with the new reg rect, it would idle around 11.7, and max out at 12.4 around 3.5k rpm.
Once the new wire was incorporated, I was getting a solid 12.4 at idle, and seeing a nice steady climb, maxing out at 15.1 at 3.5k and above.
  A day later, he's stuck again, with crappy spark from undervolt condition. Sure enough, it's down to 11.7 again at idle, and only coming up a little with rpms.
   I'm going to check it out again saturday, but I'm wondering.. IF i find that a cold bike again starts to charge in the normal range, has anyone experienced a stator or field coil that performs worse as it gets hot?
      Obviously I'll re-check my connections, but I'm a stickler, and I know my solder jobs probably are not at fault.
Thanks in advance for any info, thoughts, or musings.

[bryanj]

15 is a bit high but possibly within the range of meter error, the electronic units dont like over voltage, reverse voltage and bad groounds---all can cook them.

Not impossible for a coil to short hot but never heard of it in a lot of years

[spazz1984]

How is the reg/rec wired and where did it come from I just went thru this myself

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[phil71]

how is it wired? there's a fat green and red, they go straight to the battery terminals now. Three's a skinny green and white, they go to the field coil. There are three yelllow, and those connect to the stator. As for the brand, I didn't buy or supply it, but It looks to be a Rick's..
It's never been hooked up in reverse as far as I can tell.

[spazz1984]

All the greens should go to ground is there a black wire coming out of the reg/rec

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[phil71]

No black. Skinny green IS a ground, but it goes to the field coil.
I should also mention , both the field coil and stator resistance figures are exactly where they should be, and no open or shorted contacts.

[spazz1984]

U can just gound them to the bike but I have one from ricks your missing a wire this is how it should go
Off the reg/rec
2 greens -grounds
1 red - battery
1 black - ignition
1 white - field coil
3 yellows - stator


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[spazz1984]

Are you able to post a pic of it ?

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[phil71]

o right.. yeah, there's a black .Black is hooked to black from the oem harness, and is hot with key on.

[spazz1984]

I would pull that green off the battery and ground that's should be ground for the rec side hopeful you haven't fired it sorry for saying that

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[phil71]

Spazz, that doesn't make any sense. Thick green is ground to chassis, not to mention the body of the reg/rect is grounded. How is going straight to the battery any different. You're implying the other ground has some kind of resistance to it?

[spazz1984]

Sorry I readed your past post wrong. I was thinking you had that wire hooked to the pos side. I'm so sorry.  but I would try grounding the field coil grounds to the frame. if there's no change check the output voltage of the stator.
One other thing. from what I've been told if the battery doesn't give the field coil 12v it will not engage the charging system. I don't know what condition its in may want to check that too 

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[phil71]

I've tested both the field coil and stator, both on and off the bike. Both are well within factory spec, and neither part is shorted to ground, or open. The bike is charging well again.. so whatever this is , it's definitely only happening hot. I'll figure it out.

[spazz1984]

I have seen whinings in electric motors work fine cold then cut out same without load to have a load on it I guess testing when its at temp 

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[sandcastcb750]

You can't get much more simpler than a CB750 charging system. If you have spare parts, keep swapping until it charges. I know the three leads from the stator can be fried ( probably from high resistance due to corrosion  ) and I have had to make new connections using Vintage Connections parts. May have ruined the stator but I never had a bad '69-'78 one like the '79-"82.

You may have a bad ground too.

[spazz1984]

I'll like to know what you find out on this

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