Voltage Drop at Increased RPMs - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 10 (permalink) Old 06-28-2010, 10:08 AM Thread Starter
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Question Voltage Drop at Increased RPMs

This is a new one to me....
Voltage DROPS as the RPMs rise!

I've been through the stator-RR-battery replacement a few times and I now have all that in good order--hasn't had a problem for about 3 years and a coupla ten-thousand miles.
RR unit on the front (heavier wires than stock).
Voltmeter wired into the headlight bucket (I calibrated this by a second voltmeter on the battery for a few months--but since removed a couple years ago, since it was in the way under my leg).

I start the engine, and volts immediately run up to about 12 even at an idle. (Remember, this is voltage in the headlight bucket.) Normally at an idle, they don't go that high, but I've it seen bounce up that way at times if the battery needs a charge. But here they run up and stay up there at an idle, although they bounce around a bit.

Now comes the catch.
When I bring the RPMs up to running speed the voltage DROPS! Drops down to about 10 (headlight bucket again). As long as the engine runs up, the volts stay down. Sometimes they drop below 10. And they seem to bounce around down there.

Kills the engine sometimes too--at a stop light where I'm trying to keep the engine running up around 2K.

It's like a short somewhere.
But what does it have to do with RPMs?

With the voltage drop at increased RPMs, might this be related to a short in:
*the coil or
*the secondary wiring or
*a plug or
*igniter?
Might it be a short that kills the engine if I don't watch it? Since the 750 has dual ignition, it can keep running with a bad secondary, but it pulls the voltage down so far it can kill everything.

But I cannot find the short--if that is what it is.

Oh! Almost forgot to mention this. I put a new RR unit on just to be sure. No change whatsoever. And the battery has good amperage and voltage. Holds a charge very well and cranks with headlight on for as long as I want to burn up the starter. Checked spark on the one plug that I thought might be shorting and it's blue and regular. This engine doesn't miss--starts quickly, runs smooth and clean with no staggering. New NGK iridium plugs.

If I didn't have a voltmeter, I wouldn't know there is a problem, except for the dieing at low RPMs.
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post #2 of 10 (permalink) Old 06-28-2010, 10:56 AM
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This is what I would do, from the junction box remove all the fuses but the main 30 A fuse and see if the voltage still drops at high RPM. Then what you want is to start putting in the fuses one by one and see if one of the circuits it's the one that has the short circuit and causes the voltage drop. This should help you point you in the right direction on the voltage drop you are experiencing.

Hope this helps

93 VN750
R/R Relocation to the front
Pirelli MT66 Tires soon to be mounted (only after 14 years...)
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post #3 of 10 (permalink) Old 06-28-2010, 10:48 PM Thread Starter
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Thanks, Mushu. A simple thought that just didn't dawn on my poor wore-out brain.

Still doesn't make sense, though, about the voltage drop as RPMs rise...
But I'll check that circuit suggestion out.
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post #4 of 10 (permalink) Old 06-28-2010, 11:05 PM
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It doesn't make any sense to me either, but in discussing the diagnosis I was doing on my electrical system, my dad happened to mention that his '92 Virago 1100 that my brother is still riding has always exhibited that symptom, since it was new and through several different regulators. My VN750 does not.
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post #5 of 10 (permalink) Old 06-29-2010, 06:48 AM
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Check for a poor (high resistance) ground connection.

- Safety First

Paul

'04 VN750
'75 CB360T
'08 Concours 14
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post #6 of 10 (permalink) Old 06-29-2010, 01:34 PM
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Volt Meter Showing Discharge

I'm experencing the same problem with my 04 @ 15K. My volt meter is also wired through the headlite and has been fine for four years until today. I pulled the 10 volt fuses one at a time and found that the 10 volt headlite fuse when pulled out returned the volt meter to green charging. Thats at idiling 1100 and warm. When I put the fuse back in the volt meter began to show yellow, discharge again. I will check for a loose connection in the headlite. Anybody else have any suggestions,thoughts or possibilities? Good Luck and Safe Riding!

2005 VN750 R/R relocation, Lubed Splines, Engine Guard & Pegs, Saddlemen Bags, Roush Triple Chromed Headlight & Blinker Visors, Plasma GT-XA-VIB Headlite Blub, Memphis Slim 17" Windshield, Airhawk Seat Cushion, Chrome Billet Mirrors, Interstate Sealed Battery, Iridium Plugs, Metz Tires
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post #7 of 10 (permalink) Old 06-29-2010, 04:58 PM
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Uhmmm... An extra suggestion is to measure the voltage at the battery. Better yet measure at the following points:

- Output terminal of the R/R
- The battery
- Headlight
- Rear light

You should see somewhat different readings but not that off (I guessing 1 volt should be normal loss in the cabling), if you see readings that are far off then that part of the circuit is faulty. I know it may sound strange but it may be charging all right and you may be seeing bad reading somewhere else in the circuit. This has to do with the voltage drop that happens at the nodes of a circuit.
One thing to keep in mind (and you may want to experiment a bit) is what you are actually measuring. If you put the positive and negative from the leads of the headlight bulb you are actually measuring the voltage drop of the light bulb not the voltage off the system. I know it's kinda confusing but it true
The right reading would be the positive lead from the light and the negative from the frame (or similar) then you would be seeing the actual voltage of the system. So I urge you to test several places to see where you get the better reading of voltage.

93 VN750
R/R Relocation to the front
Pirelli MT66 Tires soon to be mounted (only after 14 years...)
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post #8 of 10 (permalink) Old 06-29-2010, 09:47 PM Thread Starter
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Thanks to all

Thanks to all you folks who answered my question with a post.
I have still not resolved the matter (haven't had time yet), but I now have some good starting points.
Very helpful!
As soon as I get some kind of result off the bike, I'll post what I found.
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post #9 of 10 (permalink) Old 06-29-2010, 09:56 PM
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My batt voltage varies. If I run the low beam I can have 12.2 volts showing, but as soon as I run my high beam and the auxilary lights the voltage will run up to high 13 to low 14 volts. I haven't had any starting problems so I don't worry about it. I have a rather new r/r.

02 honda sabre 1100
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post #10 of 10 (permalink) Old 06-30-2010, 12:53 AM Thread Starter
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Okay! Got it! (I hate electrical problems!)

Thanks to all the input I got. Y'all helped me to get my focus back to the basics instead of all the crazy stuff. (This bike has had so many exotic electrical issues....)

When I started to wire my voltmeter to the battery post, I discovered that a connection that looked good was actually "hangin' by a thread". Just one thread of the hold-down screw on the + post was snagged on the post, but the nut on the other side of the post (underneath it so it can't be seen) was backed way off.

Best I can tell, at low RPM, the "connection", such as it was, would allow a charge. But at higher RPMs, the vibration shook the wire so much that the connection was barely there. So the voltage dropped.

At least that is my suspicion. When I tightened everything down, with my voltmeter across the battery, I got a good straight 12.5 volt charge. The voltmeter on the front--out of the headlight bucket--also showed a tiny bit over 12. Steady, at all RPMs.

I'm just really glad that a surge didn't scorch my diodes in my charging system and ignition system. At least they seem to be ok.


Sure glad we have this site to bounce our problems & ideas around!

One lesson to be learned from this. Make very sure to have a voltmeter on the bike. I would not have noticed this problem until I was out in the middle of the desert with a dead battery. Or a surge from a rattling connection burned up some expensive components.
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