overcharging problem

[650mark]

sorry if this has been addressed before. my all stock xs650special has just started severely overcharging. found out today when I hooked up a digital vom to the battery and got 12.5 volts at idle when first started but the voltage started climbing and got to 18 after about 30 seconds and I shut it off. thinking it was the regulator/rectifier I tried 2 other known good units and the bike does the same thing. probably not the reg/rec,, huh? can anybody steer me in the right direction as to what to check next?
my clymers manual has been of no help. thanks

 

[XSLeo]

Check the ground for the reg/rec. Always check grounds first. Also check the green wire from the brushes to the reg/rec for shorts to ground.
You don't say what year it is but you say reg/rec so I assume it's an 80 up bike.
On these bikes power goes to the brushes on a brown wire. The green wire goes to the reg/rec. The reg/rec reads battery voltage off the brown wire. If low it grounds the green wire. When fully charged it ungrounds the green wire.
A short on the green wire bypasses the reg/rec, letting the alternator put out a max charge.
Leo

 

[fredintoon]

Hi Mark,
meanwhile most XS650 owners hope their Voltage will climb high enough to charge the battery, eh?
There's only the reg/rect between the alternator and the battery and you tried two "known good" alternatives for the same result.
I'd try hooking the bike's charging system to a car battery just to see what happens.
Then I'd try another VOM.
[edit] Leo's post leapfrogged mine. Yeah, check the green wire. It's far more likely the green wire got grounded than your VOM is hooped. [end edit]

 

[650mark]

thanks for getting me going in the right direction. with the reg/rec off the bike I just connected my meter to the green wire in the harness side of the plug and I get continuity(my meter buzzes) when I touch the other meter lead to any metal on the bike. sounds like a short to ground in the green wire? electrics are NOT my strong suite......

EDIT: as usual you guys were right on the money. the green wire is definitely shorted to ground somewhere. I connected a temporary jumper wire from the green wire connection at the brush and to the temporarily unhooked green wire connection at the reg/rec plug, fired up the bike and I'm back to 12.8vdc at idle and 14.8 at 3,000 rpm. now which do I do: try to find the short somewhere in the oem wire harness(which is very brittle and hasn't seen the light of day in 35 years) or make a jumper to replace the green wire and string it from the brush connection, through the bike, and then to the reg/rec side of the plug? I really hate to disturb the harness for fear of causing even more problems. any experienced advice please? and thanks so far for great info

 

[TwoManyXS1Bs]

A likely area for the green wire to find a short to ground would be in the serpentine routing of the alternator harness around/under and near the drive sprocket. If it is exposed and grounding, then the other wires in that harness may be approaching a calamity. I'd search it out, see if others are damaged...

 

[fredintoon]

the green wire is definitely shorted to ground somewhere - - - I really hate to disturb the harness for fear of causing even more problems. any experienced advice please? - - -

Hi Mark,
if the green wire is damaged you can't be sure about the others in that area and they may also be compromised by whatever grounded the green wire.
Best you check them out before the next little problem manifests itself.

 

[650mark]

thanks all od you guys, I've examined as much of the wiring harness as I can see without taking the bike apart and can't see any problem areas. behind the chain no way the wires are rubbing, under the gas tank and seat all appears undamaged. how the heck would you find a short inside a wiring harness without tearing into it? anyway, the jumper I installed works great, I'm back to 12.8-14.8vdc from idle to 3,500 rpm so I think I got it under control. at least for now. fwiw, a couple of my previous xs650's had some major wiring issues that I resolved by replacing the harnesses with new ones from mikesxs. but since I had the bikes completely disassembled for restoration it was a no brainer. thanks again for the help

 

[XSLeo]

The jumper, as you see works, as others have said tracing the green wire, looking for the short you might find others wires at risk.
Using a pair of scissors to cut the original casing off the wires works well.
To narrow down the are to search, unplug the wire bundle coming up from the stator. A large plug. Check for the short from the green wire in the stator side of the plug. If it shows the short there, the short is between the plug and stator.
If it doesn't show the short it's between the harness side if the plug and the reg/rec.
This cuts the search in half.
The short can be in the sprocket area as mentioned or on the stator.
Leo

 

[TwoManyXS1Bs]

...To narrow down the are to search, unplug the wire bundle coming up from the stator. A large plug. Check for the short from the green wire in the stator side of the plug. If it shows the short there, the short is between the plug and stator.
If it doesn't show the short it's between the harness side if the plug and the reg/rec.
This cuts the search in half.
The short can be in the sprocket area as mentioned or on the stator...

Excellent isolation test, Leo. Here's a pic of my old alternator and main wiring harness, unwrapped to reveal the regulator wires.

View attachment 58973

The green wire in the main harness, going from the big plug to the regulator, can be seen. On the early bikes, it just loops inside the main harness, and doesn't go very far.

My alternator harness, however, has seen better days. About 2-3" down from the big connector is a seriously chafed area, with exposed wiring. Chafing was caused by vibration rubbing against the downtube's wide nylon cable tie.

From there, about halfway to the alternator, at the lower/left bend, is another seriously chafed area. You may be able to see fuzzy insulation stuff coming out at that bend. That area wraps around/behind the sprocket area. Age/heat/hardening and vibration chafing is involved there, too.

That alternator and harness were pulled from my other XS1B back in the late '70s, boxed, and been in there since. That's just after a few years of being in-service, and look at the degradation. No telling what the internals of a 40 year old harness may look like...

 

[westonboege]

I read through some of the posts...

I just had a very similar problem on one of my other bikes.... not xs.

Was running great for months. had replaced the entire charging system less than a year earlier.

Started overcharging one day all of the sudden. Key on 13v normal. As soon as i would kick start it volts would jump to 17-18v and fry the headlight.

I started looking at everything. Someone told me to get a new battery. I COULD NOT understand how the battery could make it overcharge. Checked out all my electrics and even called the company i bought the charging system from to see if they would warranty it.

Before i sent off my electric system i decided to put in a new battery. Everything went back to normal. I was more than surprised....


Not saying its def it, but it wouldnt hurt to throw another known good battery in and give a shot. If your bike was running and charging good before this that is...

 

[XSLeo]

westonboege, was this other bike a PMA system by chance? On a PMA a good battery is needed to help control the reg/recs output, same as with a cap, if the cap fails the reg/rec can go crazy.
On our XS650's with a field excited alternator It less apt to happen that way. On our systems the regulator turns the current flow through the rotor on/off. With w low voltage it turns on longer than it's off to charge the battery, once charged it turns on for shorter intervals.
A weak battery won't have as much current to charge the rotor, this lowers the output. So the regulator turns the rotor on for longer intervals trying to charge the battery. If weak enough the rotor may be turned on almost continuously.
This can overheat the rotor, burning it out.
It won't really spike a high voltage like an out of control PMA.
Leo

 

[westonboege]

This was on a 69 shovelhead. The generator was replaced with a moder alternator setup by Cycle Electric. Im not too sure how similar the system is to the xs...

I was surprised as hell to find out that the battery was actually the problem... Just thought i would offer it up as a suggestion to try... haha