Odd, Intermittent Charging Problem - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 32 (permalink) Old 06-10-2018, 12:01 PM Thread Starter
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Question Odd, Intermittent Charging Problem

Here's one of the ways a dash mounted voltmeter can be a good thing. I'm open to suggestions if anyone has something, haven't got to check it out yet.

For a couple of seasons, I've been thinking the JB fan relay was pulling the charge volts down, because slow traffic with the fan on is when I noticed this. So I thought it was heat and the fan combined to cause this, but now I don't think so.

Symptom: Rolling onto the throttle, engine hesitates and backfires. At speed, won't rev over 4k. Every time this happens, I look at the voltmeter and it's down around 12-12.5v. It's random and doing it when the fan is off. It goes maybe a minute or two at low voltage, comes back for a few minutes, rinse/repeat. Revving it up makes no change, it just comes and goes on it's own. Once, I pulled the choke while trying to speed up and there was no change, just 4k rpm and bogging.

When the charge volts are 13-14.5v, it runs fine and revs to the limit. On a 50 mile ride it kept going in/out of the low charge condition. Battery is 5yr old agm, seems fine, always starts, stayed up all winter with no tender, but I wouldn't rule out an intermittent short, yet.. Seems like a short in the battery would drop a lot more voltage, the voltmeter looks like the charging system quits and it just shows battery voltage.

It's got a TPE stator and Shin. mosfet r/r. Looks like this might be hard to pin down. Need to find that ohm test for the mosfet. Anyone have it?

I suppose first I'll check connections, but the time it had a loose battery terminal, the charge pegged at 15v.

2003-21k mi
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post #2 of 32 (permalink) Old 06-10-2018, 02:06 PM Thread Starter
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Can't reproduce the problem in the garage, the fan keeps kicking on. Voltage drops to 12 at idle when the fan starts, then comes back up around 12.75v. With a rev it comes up to 13.75v.

Wondering if my multimeter is wonky. With the engine off, it sits at 12.80v, then I look a minute later and it shows 12.0v. Turn the key on (headlight on) and it jumps back up to 12.79v.

All the connections are good. Guess I'll go take a ride.

Maybe it's time for a battery.

2003-21k mi
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post #3 of 32 (permalink) Old 06-10-2018, 02:08 PM
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5 years is getting up there for a battery. I have an old AGM i use as a test tool that has one dead/short cell in it, charges fine but drops in voltage right quick under load.
It does sound like your charging system is erratic though, i would check the connections around the R/R thoroughly.

'99 Vulcan "Victoria"
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post #4 of 32 (permalink) Old 06-10-2018, 02:31 PM Thread Starter
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5 years is getting up there for a battery. I have an old AGM i use as a test tool that has one dead/short cell in it, charges fine but drops in voltage right quick under load.
It does sound like your charging system is erratic though, i would check the connections around the R/R thoroughly.
AGMs are new to me, I dealt with wet cells a lot, even used to sell reconditioned ones. So I'm not sure how the AGM is going to act. Was still sitting at 12.79v just now. But if that drop to 12v is real, it must be shorting intermittently and is on it's way out. It's all I can come up with.

Connections are all good and the wiggle test showed no problem. Putting my boots on for another 50 mile ride.

What voltage does your shorted battery show before you load it? It holds the charge?

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post #5 of 32 (permalink) Old 06-10-2018, 05:14 PM Thread Starter
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It's still doing it, but not as bad as yesterday. Only backfired a couple of times after I got back in town, didn't do it at all on the back roads today and the charge volts are staying higher most of the time.

We know a bad battery can pull the charging volts down, this one may be on it's way to being dead at the worst possible moment. It will be 5 yrs old in Nov. Have no idea how old it really is, have to check the code.

As soon as the voltage drops down, seems like the ignition just goes stupid. It cuts out, blasts back on, hiccups, then backfires. Really gets your attention in the twisties.

Going to leave the multimeter on the battery and see if I can catch it dropping to 12v again. Resting at 12.79v right now.

edit: Found the test for the mosfet r/r.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf

Last edited by Spockster; 06-10-2018 at 07:19 PM.
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post #6 of 32 (permalink) Old 06-10-2018, 08:21 PM Thread Starter
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Guess my meter won't do the diode test on the r/r. I set it on diode and got nothing on any of the terminals, and the meter shows .681 with the leads together.

Volts are 12.73 after sitting three hours.

2003-21k mi
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post #7 of 32 (permalink) Old 06-12-2018, 08:52 AM
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When my R/R died it was letting pulsed AC volts through to the battery. My cheap Chinese voltmeter mounted to the bike was unable to figure out what was happening and displayed a slightly higher voltage only. Killed a new battery and dumped me on the side of the road on my way home from work. In a cell phone dead zone.

'99 Vulcan "Victoria"
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post #8 of 32 (permalink) Old 06-12-2018, 08:17 PM Thread Starter
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Left the meter connected since Sunday, checked it today and it was still on 12.73v. Of course it started right up, that hasn't been an issue yet.

Guess I'll ride some more and watch it. ... and keep my cellphone charged.

Probably something in that goofy junction box.

Without that dash mounted voltmeter, I probably would've yanked the carbs by now. It does act like a fuel problem, but the volts are down every time it runs bad.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf

Last edited by Spockster; 06-12-2018 at 08:28 PM.
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post #9 of 32 (permalink) Old 06-13-2018, 09:26 AM
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I had a problem with charging and it turned out to be the battery box where its bolted to the frame ,it was really rusty but you could not see it until you pulled it , I cleaned it up and bypassed it with a sturdy earth wire to the battery, ok now !.

Vn 750 A2 1986 rebuilt engine, crank shaft, half casing,shell bearings,con rods,rear cylinder head,oil pump,water pump ,(had ceased bearing rusted up),tie bar upgrade,clutch basket drilled,front cylinder cam chains,toc springs,stator,shaved, rectifier,agm battery,solenoid ,starter motor (used) re-spray ongoing.
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post #10 of 32 (permalink) Old 06-13-2018, 09:48 AM Thread Starter
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Originally Posted by JETHRO View Post
I had a problem with charging and it turned out to be the battery box where its bolted to the frame ,it was really rusty but you could not see it until you pulled it , I cleaned it up and bypassed it with a sturdy earth wire to the battery, ok now !.
I'll look at that closer next time the seat is off, thanks.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
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