Yet another charging system mystery. Please help!


XS400 New Member
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Vancouver BC

New member here, I just got a 1979 XS400F and love it but (no surprise) the charging system is not working. I have done all the resistance tests according to the Haynes manual and everything passed. The stator does magnetize when I start up the bike. Battery is new and in good condition. Cleaned all the grounds.

I did an AC volt test on the three white stator wires. I read abut two different ways to do this test. The first way I tested each white wire with one Meter lead on the wire and the other to ground. Got a very low reading this way. The other method I used was to test the white wires in pairs (one meter lead on one white wire and the other on the second white wire.) Doing it this way I got about 25 AC volts at 2000 RPM for each pair. Is this good?

The main thing that is confusing me is that when I test the volts at the battery with the rpm's at 2000 or higher the volts do rise but very slowly. I actually held the rpm's at 4000 for about a minute and got the volts up to 13.5 and I'm pretty sure it would have kept going but I didn't want to keep revving the motor like that. I did the VR bypass test and the stator did magnetize but the voltage did the same thing - only crept up very slowly.

Wondering if anyone has any insight into what my problem might be given the slow voltage increase? Or what other tests I could do before pulling what's left of my hair out and blindly throwing money at a new VR, rectifier and stator. Thanks!
Most charging issues is one of two things, seems you have yet too rule out the actual voltage? pending that discovery, it will tell you your voltage reg is shot, or I like the OHM test of the 3 white wires. If you do the OHM tests, you might notice some bad numbers, then if you do the touch and feel (cool engine) I suspect you will find the two bundles of wires running from the underside of the engine, to the center rear, and running up to the rear of the H air tube, to the area immediately infront of the battery. Right at the area where the two sets of wires come out from under the sprocket cover, they get toast right there. The wire any where else is flexible, and moves as expected. But at that point, its crispy, crackle-ly, and if you re-test the stator OHM test, they will have gotten significantly worse. The first half of Drewpy's re-wire is all you have to do. Only thing I did this time, that I didn't do the first time, and time will tell if this was a better idea, I coated the wires with an enamel spray paint, then I put heat shrink tubing on the connectors. You can find Drewpies procedures under TECH tab, on top of the page, or go to the Home page, and scrow down a little bit. (it was worth while to post on the FRONT PAGE!!!!)((For a reason))
Hi Arfstrom,

"seems you have yet too rule out the actual voltage?" Sorry but I'm not sure what you mean by this?

I did do the OHM test of the 3 white wires as it was one of the tests in the Haynes manual. The readings for the test were all in spec. I did visually inspect the stator wires and they are not dry or cracked. (The bike only has 3500 km's on it). I definitely am no expert in anything electrical but it seemed strange to me that the AC volts test on the stator showed reasonable AC output. Anyway thank's for getting back to me so soon. I will check the stator once again.

On the multimeter, with the setting on DC volts, check at the battery, what the volts are coming in to the battery, (red wire on the plus, and the black on any ground, or the neg post/connector.) So check before start, and after start. At idle, and at 2k RPM's. Generally expected is at rest just above 12v. when over 2000k RPM as much as 13.5v (DC) if the battery is shot, you will not achieve those numbers, and everything is fine. At about 2k RPM's the VR/Rectifier kicks the power up to the 13.3 zone for charging the battery. So after a good trip your bike should be fine on battery charge.

In the thread I posted just a bit ago, check out the links, I will post them here too. it will give you some great ideas to start with, Generally speaking, cause the web page wasn't created with our bike in mind, but inspiration can come from anywhere! Dans MC page has been a great help, the other one I just found today. Good luck Great FAQ, not for the our bike, but the info is vary valuable. Apply as it make sense.

extra useful info (for CDI) not the 80-new bikes.
When the bike is started and revved to 2000rpm, the voltage measured across my battery is usually around 12.5Volts. As I get riding on a straight stretch it will get up as high as 15.0 volts. At a stoplight at idle it wants to die and kinda runs like garbage? Do you think it's not getting charged enough? Like maybe not enough output?
I think that is an idle setting. The VR isn't suppose to allow the charge to get up to 15v. somewhere near there, will cause your battery to overheat, and burn off electrolytes. I would rule out issues with the Carb. Balance the carbs, use the manometer, and ensure that both plugs are firing. Look at the state of both plugs, is one darker than the other. (more wet = not firing). if everything else is fine, the thottle can be adjusted on my bike while at intersections. Since good bikers wear gloves, you should be able to find your idle adj screw, and turn it slightly. until you have the performance your looking for. somewhere between 900-1200 rpm.