So I have been suspecting charging system issues for quite a long time but never gotten to checking it until today.
I followed Kerry's guide from here: http://gstwins.com/gsboard/index.php?topic=38480.0

It seems that my alternator is at fault. I checked for voltage between all three pairs of yellow wires (my bike is 2004 so I have 3 yellow wires coming from alternator) and at 5000 RPMs I get around 66 Volts. In his guide, Kerry mentions that I should be getting at least 75 Volts.

Here is image of GS500F alternator:


Three questions:
- what could cause alternator to go bad (bike has only 8000 miles on it)
- is there any way to tell if I am buying good alternator (I will be buying from eBay)
- do I have to drain oil from the engine when I will be replacing it? ( can't figure out if it is sunk in oil or not)

Thank you!

If you get the same voltage between all 3 pairs, you're probably OK. While 75 volts is a good value to look for, you won't always get it due to many factors. If one pair had a different reading, then the stator would be defective.

Most likely a charging system fault is:
1. Bad battery that won't hold a charge
2. Bad regulator

For regulator, check voltage at battery. I found  the break-even point where the bike generates enough electricity to offset usage is around 3000 rpm (varies depending on what stuff is on your bike using electricity). So check at 4000 rpm at the battery. Should be somewhere around 14.5 volts. It should or might drop a little below 3000 rpm because then it is not generating enough electricity to offset usage.

If it goes above 15 volts when revving the regulator is bad. If it never gets above 14 volts when revving, then regulator is probably bad.

Check battery. With bike off, it should be over 12 volts. More in the area of 13 volts. If you measure the voltage after turning off the bike and measure again in an hour or more and the voltage is dropping, the battery is not holding a charge.

I would check your dc out put from the regulator rectifier and make sure its 13.5 - 14 and not like 6v or something lol. The stator would be a pain to remove when you could just bolt on a new r/r

Oh and by the way, if your still set on replacing stator, its not broken or else the voltage would be much lower, you could just take it to bike shop or something and pay to have the stator rewound, its most likely cheaper than buying a new stator all together.

Quote from: kml.krk on January 21, 2013, 04:30:14 PM

If you are seeing a balanced 66V across each phase its unlikey to be the stator, unless this figure alters significantly if you turn every light on and sound the horn 

Its far more likely to be the reg/rect.

I will have a stab at your three questions

- what could cause alternator to go bad (bike has only 8000 miles on it)

A fault in the winding, ie shorted turns, earth faults, intermittent open circuits. These faults may only become apparant when its actually doing some work and seeing the 75VAC mentioned or when it reaches its normal operating temp, NOT when its stone cold and seeing the 3/9/12VDC the average multimeter puts out. The three circuit resistances MUST be of equal value to each other to within a few percent. You will need a meter capable of measuring very low resistances to determine this. Its a specialist piece of kit and chances are you wont have one. You can however test the insulation resistance by applying 12v and a headlight bulb across any lead and the metalwork, if the bulb lights the winding is breaking down. Also flat or cracked magnets in the flywheel which will reduce and upset field strength leading to a lower than normal output voltages.

- is there any way to tell if I am buying good alternator (I will be buying from eBay)

Not really, even with a good set of static readings the winding could have a fault that might only show up dynamicaly. The definative test would be to carry out a surge test but you wont see the sort of kit needed to do this outside a well equiped electric-motor or transformer repair shop.
See this ....... http://www.adc-electrical.co.uk/Surge_Desc-pg.htm

Unless the alternator is dirt cheap and you are prepared to take a chance on it being shagged you might want to consider a brand new pattern part with a guarantee. Plenty of them on the market.... http://www.electrosport.com/street-bikes/suzuki/gs500e-97-00/stator-suzuki-gs450-gs500e-gs550-gs650-gs750e-2.html

- do I have to drain oil from the engine when I will be replacing it? ( can't figure out if it is sunk in oil or not)

Yes....and you will probably have to replace the side cover gasket as well as it has to come off to get to the stator.......unless you can do it without removing the cover 

I seem to recall the 3 screws that hold the stator to the case are held with threadlock......but its been a while since I last changed one and I cant remember for sure. Use your JIS screwdrivers 

Quote from: jacob92icu on January 21, 2013, 06:07:18 PM
pay to have the stator rewound, its most likely cheaper than buying a new stator all together.

I doubt it very much 

Rewind a stator? They're not that expensive, easy to get and I've never heard of rewinding one of them. Kind of a complex 3 phase thing. By the time you remove the wire and try to rewind it, $30 for a used one has got to be cheaper. Or I'll just take one out of my box-o-stators. I haven't had one go bad. I have encountered two bad regulators so my bet is a bad regulator.

Quote from: adidasguy on January 21, 2013, 06:15:29 PM
Rewind a stator? They're not that expensive, easy to get and I've never heard of rewinding one of them. Kind of a complex 3 phase thing. By the time you remove the wire and try to rewind it, $30 for a used one has got to be cheaper. Or I'll just take one out of my box-o-stators. I haven't had one go bad. I have encountered two bad regulators so my bet is a bad regulator.

Hmmm, i just heard of people doing it them selves, and that in turn would be cheaper. But yea I would never try it my self either and would buy a new or used one so i couldn't mess it up by trying haha

Plenty of specialists out there who will rewind motorcycle stators.......but unless its very rare or very old and a spare is not available its not worth it. I guess the owners who do rewind them do it simply because they want to.

Why pay upwards of 100 to have a GS5 stator rewound when a new pattern one is about 65 and a used one substantialy less 

http://www.elliottmc.co.uk/products/99

Quote from: sledge on January 21, 2013, 06:33:38 PM
Plenty of specialists out there who will rewind motorcycle stators.......but unless its very rare or very old and a spare is not available its not worth it. I guess the owners who do rewind them do it simply because they want to.

Why pay upwards of 100 to have a GS5 stator rewound when a new pattern one is about 65 and a used one substantialy less 

http://www.elliottmc.co.uk/products/99

Damn really that spendy? It would be pretty cool to learn how to rewind one, learn how a very large transformer works haha

Few updates:

1. I was quite impatient about this so I went ahead and bought this: http://www.ebay.com/itm/150974775082?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Now, after reading all your replies, I kind of regret that, but it's too late. It didn't cost arm and leg so I guess I will just have to put the item back on ebay

2. The voltage measured at battery is close to 12.40 volts @ 5000 RPMs. It is almost 13 volts @ 1500-1800 RPMs but then drops to 12.40 as mentioned before.

3. The voltage is pretty much the same (66 Volts) on all three pairs of yellow wires coming from alternator/stator?

4. My battery showed 12.75 volts when I checked it this morning with bike turned off (the bike was NOT used for few months and the battery wasn't charged during that time, which means that battery holds charge pretty darn well - it is GEL battery if that matters)

Should I just go ahead and buy buy another regulator? Is there a way to tell for sure that it is broken?

EDIT: I am really impatient today... I just got this because the price was good: http://www.ebay.com/itm/130826124796?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

I will proceed to change the regulator rectifier and see what happens, if it is still not good, I will ask your opinion first and possibly swap stator. Hopefully regulator will fix it.

Since you have your multi meter you can stick the leads into the black and red connectors coming off the r/r and switch meter to dc voltage, as close to twelve as you can get. If that reads 12.4 or whatever your saying your battery voltage reads on the bike, than your r/r should be good. I just replaced my r/r and i was greeting about 80v ac  and only like 6v coming from r/r so it was clear that it was bad. Your getting low voltage from stator so you should expect low dc voltage from r/r. Its charging the battery that's for sure, just not as much as you would probably prefer.

Hmmmm are you saying that I just bought another useless part? I am totally confused...

Haha possibly but not necessarily. Worse come to worse you could replace both and get your desired 14.5 volts at 5 grand. Did you buy a used r/r?

Quote from: jacob92icu on January 21, 2013, 07:20:23 PM
Your getting low voltage from stator so you should expect low dc voltage from r/r.

Errr....I dont agree with that statement, the clue is in the terminology. It is a regulator not a transformer so output from it is not going to be directly proportional to its input.

The regulator looks at the DC-voltage across the battery-terminals and short-circuits a certain amount of power that is produced by the stator to ground. This is regulated constantly, so the output-voltage of the regulator-rectifier (which ideallyis the same as the voltage across the battery-terminals) stays at 14.4 Vdc all the time.

From here....... http://www.electrosport.com/technical-resources/technical-articles/how-motorcycle-charging-system-works

Quote from: sledge on January 21, 2013, 07:44:13 PM

Quote from: jacob92icu on January 21, 2013, 07:20:23 PM
Your getting low voltage from stator so you should expect low dc voltage from r/r.

Errr....I dont agree with that statement, the clue is in the terminology. It is a regulator not a transformer so output from it is not going to be directly proportional to its input.

The regulator looks at the DC-voltage across the battery-terminals and short-circuits a certain amount of power that is produced by the stator to ground. This is regulated constantly, so the output-voltage of the regulator-rectifier (which ideallyis the same as the voltage across the battery-terminals) stays at 14.4 Vdc all the time.

From here....... http://www.electrosport.com/technical-resources/technical-articles/how-motorcycle-charging-system-works

This is true, however if the diode inside is malfunctioning and only letting out 12.4v as in this case,  instead of as you say, 14.4, the r/r will need to be replaced. The reason I find this situation confusing is because, when a diode fails it completely fails, not partially. So you shouldn't even be getting 12.4v if the r/r is bad.

Quote from: sledge on January 21, 2013, 07:44:13 PM

Quote from: jacob92icu on January 21, 2013, 07:20:23 PM
Your getting low voltage from stator so you should expect low dc voltage from r/r.

Errr....I dont agree with that statement, the clue is in the terminology. It is a regulator not a transformer so output from it is not going to be directly proportional to its input.

The regulator looks at the DC-voltage across the battery-terminals and short-circuits a certain amount of power that is produced by the stator to ground. This is regulated constantly, so the output-voltage of the regulator-rectifier (which ideallyis the same as the voltage across the battery-terminals) stays at 14.4 Vdc all the time.

From here....... http://www.electrosport.com/technical-resources/technical-articles/how-motorcycle-charging-system-works

Whoever wrote the article doesn't know what a voltage regulator does these days. A solid state voltage regulator controls the amount of electricity going through it like a faucet - it does not short extra voltage to ground. Shorting extra voltage to ground could result in a near short circuit from the stator if no current was needed to charge up the system. Possibly some ancient voltage regulator methods did that, but solid state ones like we have do not do that.

Diodes convert the AC to DC. Diodes can fail by opening up and breaking the circuit or they can fail by shorting out and no longer being a diode but a piece of wire.

Once converted to DC, you have transistors that regulate the voltage. The battery acts like the capacitor in a standard DC power supply - a very big capacitor.

When the regulator fails, it can either not allow enough current to pass through (i.e. low voltage) or can't shut down  the current/voltage and  you get an over voltage situation (above 15v DC).

One odd or interesting thing about solid state things is they can be temperature dependent. They can be OK at normal temperatures and fail at high temperature, then be OK again when they cool down.

On anothger note: a diode generally has a 0.7v voltage drop across it. Figure you probably have about a 1.5v drop across the regulator/rectifier. Meaning, that if there was enough current, or low load, anything above 16v would give 14.5v output. Due good old Ohm's law, to make things work you would be fine with 20v if it had enough current. Conversely, you can get by with higher voltage and lower current. It works out the same. So out 50-80v from the stator is low current but high voltage. Sufficient electrons to keep us at 13.5v to 14.5v at the battery.

A diode just lets electricity flow in one direction. However, there is a zener diode that has a "breakdown" voltage at which above it conducts, below it doesn't. Without dissecting our RR, my guess is it uses a zener to control the gate of a transistor that controls the voltage. If the zener fails, it could allow the voltage to go too high or not high enough because it's breakdown voltage would change. So basically, the main transistor allows voltage through. When voltage goes above , say, 14.5v, the zener conducts and starts to shut down the transistor to keep the voltage where it should be. Without the battery to act like a capacitor and smooth out the voltage, I would guess we would see a pulsating voltage if looked at on an oscilloscope. The reason for the heat sink is the transistor is a valve and gets hot.

 haha jeez Patrick, you basically just explained everything we learn I'm electronic engineering classes. You remind me of my professor 

I should add that I was explaining linear power supplies. Be glad I avoided going into switching regulators.   

thanks a lot for extra detailed answer adidasguy!
So in conclusion, you think that it is the regulator that is bad? Therefore as soon as I replace it, the charging voltage measured at battery should be around 13.5 - 15 volts?