Voltage drop when in gear
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  1. #1
    Member bick54's Avatar
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    Voltage drop when in gear

    I bought a new battery for my k7 cb450 over the winter and Iím having some voltage drop issues when I put the bike in gear. The battery reads 12.6v with the bike off. I start it and it stays right around 12.6 off and increases with revs as it should. Once I put the bike in gear it drops to 11v and stays there. It doesnít fluctuate with revs anymore and will slowly discharge regardless of engine speed. Iím assuming my battery is a dud or I might have wired something incorrectly. Anyone have any ideas where to start looking? I just think itís weird that the charging system works normally in neutral but not in gear.


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  2. #2
    Senior Member WintrSol's Avatar
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    Yes, very weird, as the only relation between Voltage and gears is the neutral light switch. Where are you measuring the Voltage? Directly between the battery terminals, or somewhere else?
    Rick

    Mine is a mostly 1970 CB450K3

  3. #3
    Member bick54's Avatar
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    Voltage drop when in gear

    Wintrsol,

    Maybe thatís the problem? Iím measuring at the headlight. I have a little mini led voltmeter that I have tapped into the power feed to the headlight. I had to rewire the whole bike from scratch as the old wiring harness was beyond repair. I wanted something to watch the voltage to make sure my charging system was functioning and not gonna leave me stranded while Iím trying to get in some runs to break in the fresh top end. Does the voltage normally climb when your riding? I realize it will slowly charge the battery but will it fluctuate wildly with revs when youíre riding like from 12-14.5v?


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  5. #4
    Member bick54's Avatar
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    Voltage drop when in gear

    Update:

    I measured the voltage at the battery. At idle I was around 13v Iím neutral and at revs it went up to 18v (yikes!!!). I have the sparck moto reg/rec but Iíve clearly messed something up. It still drops to 12v immediately once it goes into gear and doesnít fluctuate with revs it stays a steady 12v. Iím not an electrical expert but if anyone has any ideas Iím happy to hear them.
    Last edited by bick54; Yesterday at 08:18 AM.

  6. #5
    Senior Member ancientdad's Avatar
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    Sounds to me like the neutral switch is somehow involved in a ground situation that changes when the ground goes away (neutral light goes off), but it's hard to understand just what it could be without... some pictures of the components and wiring maybe and you definitely don't want the battery getting beaten up with 18v for very long - might actually be what happened to the new battery if it turns out to be bad. Is it possible you connected the green from the rec/reg to the green w/ red from the neutral switch??
    Tom

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  7. #6
    Member bick54's Avatar
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    I donít have the neutral switch attached. So it shouldnít be something with the neutral switch. I didnít know if the engine being under load changed the stator output somehow? I didnít think it would but Iím not sure. Could it be a bad ground on the reg/rec that makes it lose its regulatory function. That wouldnít explain the voltage drop in gear but might that explain the high voltage at the battery?


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  8. #7
    Senior Member WintrSol's Avatar
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    While the Voltage does vary with rpm (up to a point), your readings are way wrong. Measured at the battery at idle, in or out of gear, it should be around 12.6V; a little lower if the headlamp is on. This is with a fully charged battery. As the rpm rise to about 3000, the Voltage should rise to about 14.5-14.7V, and stay there, because that's where the regulator stops it. Measuring in the headlamp, you are also measuring the drop through the harness wires, connectors, and switches. If you apply the turn signals, you will see it bounce with each flash. Mine even bounces a bit at idle with each coil charging and firing.

    Tom is right about killing the battery - 18V will fry one pretty fast. His conjecture about the neutral light switch wire would explain a lot, too. You need to review the wiring, end-to-end.

    EDIT: I see you didn't wire the n switch. But, putting it in gear doesn't load the engine enough to cause what you see, unless it is shifting the large ground terminal at the frame/transmission, making it lose contact. In that case, just bouncing the bike or pulling on the engine should have the same effect.
    Last edited by WintrSol; Yesterday at 09:45 AM.
    Rick

    Mine is a mostly 1970 CB450K3

  9. #8
    Senior Member JamesPal's Avatar
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    Some more info would be helpful. What is the model and year of the bike? What regulator and rectifier are you running? What is the wattage of the headlight? Are these voltage readings being made with the headlight on or off? 18VDC seems like a lot from a Honda twin.
    Jim Palmer
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  10. #9
    Sensei 66Sprint's Avatar
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    Quote Originally Posted by bick54 View Post
    Update:

    I measured the voltage at the battery. At idle I was around 13v Iím neutral and at revs it went up to 18v (yikes!!!). I have the sparck moto reg/rec but Iíve clearly messed something up. It still drops to 12v immediately once it goes into gear and doesnít fluctuate with revs it stays a steady 12v. Iím not an electrical expert but if anyone has any ideas Iím happy to hear them.
    Phone me while at bike with multimeter....
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  11. #10
    Member bick54's Avatar
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    Quote Originally Posted by JamesPal View Post
    Some more info would be helpful. What is the model and year of the bike? What regulator and rectifier are you running? What is the wattage of the headlight? Are these voltage readings being made with the headlight on or off? 18VDC seems like a lot from a Honda twin.
    Thanks Jim.

    Iím outta town for the 4th but when I get back to town Iíll get some better info for you. I was shocked that the charging system would have been capable of that much judging from what Iíve read here on the forum. Just for reference itís a 1974 cb450k7 with about 4K miles running a custom wiring harness (which I probably buggered up) the stock headlight (35w?) the last set of readings I posted were at the battery with the headlight unplugged but the led running lights on. Iím running a sparck moto single phase rectifier. I have a theory that I might have messed up running the sense (black wire) on the regulator. Which might account for my overcharging issue. However the voltage drop when the engine is under load is still confusing me.

    Happy 4th!


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