The bike is a 2009 KLX250s EFI edition.
I just noticed that that bike doesn't charge the battery. Checked with multimeter, there's no voltage change when the bike is turned on, not even on higher revs.
According to the service manual, I measured the alternator, it's working correctly, gives around 50V.

Then tested the rectifier, with much less success. The manual says there's should be resistance between certain legs, but I couldn't measure any. It did not show 0 ohm, much rather infinite resistance, like it was an open circuit.
But then I tested it according to this video:
And to my surprise everything seemed to be correct.

Now I am really confused, have no idea what to problem could be.
Any ideas?

 

See if you can back probe for 0 - 20V DC out of the reg/rec connector with the bike running. If you get good voltage there it's somewhere in the wiring back to the battery. Also check continuity of the negative wire to a known good ground and continuity of the positive back to the battery +

 

The 3 wires coming out of the stator cover are the a/c side of the charging system. When you did the test, it should have at least 41 volts ac when you measure all 3 legs. If that checks ok, you'll see 6 wires on the rectifier/regulator. 3 yellow wires are the ac inputs and the other 3 wires are the dc side. Black/yellow is ground-make sure you have continuity to the neg terminal on the battery. The other 2 wires(white & white/red) should have battery voltage(continuity to the + terminal of the battery). If all that checks out, then you have a bad rectifier/.reg.

 

Is it possible that if the battery has a shorted cell, that is causing the failed readings? Will the KLX run with the battery removed?

 

Since than I have made some measurements, and it came out the regulator works like a charm, it outputs ~13.9 volts, at idle. I measured this directly at the regulator, but at the battery I can only measure around 12.5 volts, which means there's a big voltage drop on the cables. I found an other thread which mentioned that it's a common problem that the stock wiring can't handle the amount of power the alternator outputs. They recommended installing an other ground cable from the regulator to the battery, which I did as a test, and it kinda solved my problem. I got around 13.4 - 13.5 volts at the battery, which is still not the max that the regulator outputs, but I think it's enough to charge the battery.

But I still have two problems:
- With the auxiliary cable I had 13.5 volt at the battery at idle, but as soon as I increased the RPM only a little, the voltage dropped down to around 13.2 volts. Is it a problem, or does it work the same on your bikes? Shouldn't it increase as the rpm increases?
-The other thing is, I don't really know what size of cable to use, since no clue how many amperes it has to handle. Right now I have only spare wires which are thinner than the stock wires, and I am not sure that it's safe to use those wires.

Has anyone done this modification? Any experience with it?

 

1. I think your regulator is going to fluctuate as rpm changes, forget about the little things.
2. Get a #10 wire from the hardware, it's good for 30A, in the elec dept. #12 is good for 20A.
So it is charging, what was the problem you were trying to fix?

 

The problem was that it wasn’t charging the battery because of the low voltage.

With this little modification that is fixed, I think.

 

It sounds like you are on the right track.
1. Some of your confusion with testing the rec/reg from the video could be that it is a generic test that probably tests the rectifier section but not so much for the regulator part.
2. Adding the extra wire will not hurt anything but while you are in there inspect all the connections for corrosion. The high current connectors can be prone to corrosion and high resistance. Adding the extra wire is attempting to lower the resistance back to normal. If there is any corrosion at the connectors, battery terminals or frame grounds clean them up and maybe retest.
3. It may be possible the battery has internal issues but from your volt readings it is most likely OK.

 

The battery is almost new, so it shouldn’t be faulty.

Can tell me where the most important ground points are on the frame?

 

Check frame ground connections where rec/reg connects, battery terminal, and where the neg battery terminal connects to the frame.