Charging issues, problem with stator?

[ChunCopter]

Hello everyone,

After seemingly figuring out issues with my ignition, I have discovered that my bike does not really charge the battery, which is likely what was causing my ignition timing to go all over the place as the battery gets slowly discharged.

I went about trying to diagnose this.
Pulled the stator wires. Those measured the listed 0.7 Ohms between two of the three connected windings, but about 1.5 Ohms between one of the pairs.
Just to make sure, I ran the dynamic test to check how much voltage I get out of the stator and sure enough the pair that reports a higher resistance has about half the voltage than the other two pairs (good pairs show about 20VAC at 1500 RPM, bad pair shows 12VAC).

So I figure it must be the stator, right? So I order a replacement stator, but when I measured that, it also shows 1.5 Ohms between the same pair!
My questions are:
1. Is that normal and the problem is in the regulator or rectifier?
2. Why are there four wires coming out of the stator but only three are connected to the rectifier?

Thanks.

 

[CaptChrome]

There should be five wires coming from the alternator - three white wires for the stator and one black along with one green that connect to the field coil.

The resistance is a problem. I don't remember what bike you have, but the spec for my '81 SOHC engine is 0.72 +/- 10% ohms at 68deg F (that is the measurement you should have between any of the white wires). The measurement for the field coil (between black and green wires) should be 4.0 +/- 10% ohms at 68deg F.

 

[ChunCopter]

I got a 1977 XS400D, should probably add that to the signature or something.
Both of my service manuals list 0.7 as the spec.

I have eight wires total. Four white wires coming out of the stator, two green, one blue and one yellow for the field coil.

Field coil seems to check out on my bike, around 4 Ohms there between the green wires. Stator on the other hand...
It just seems weird that the replacement that I ordered has the same 1.2-1.5 Ohms between the same pair of wires, so I wanted to see if that is normal or if by some miracle the replacement has the same issue that the one in my bike does.

The replacement looks fine visually.

 

[CaptChrome]

Well now, you are correct! The 400D has four wires coming from the stator (image below from the wiring diagram). One is from the "center tap" where the three coils of the stator are connected together. The center tap is not led anywhere else after the connector plug. But, that will certainly confuse matters when you try and test the resistance of the individual coils of wire. So, make sure you are only testing the wires that actually lead to the rectifier (you probably have made sure of this).

Buying used parts that are bad is par for the course when it comes to owning a vintage bike, but there is another test to do just to be certain that the two different alternators are bad. Find the center tap wire (the one that doesn't get run anywhere else after the connector plug) and measure the resistance between it and all of the other three wires. Those reading should be the same and somewhere in the neighborhood of 0.35 ohms since you are only measuring through one of the wire coils instead of two.

The other thing I would personally do is degrease the crap out of the wires leading from the stator coils and do a detailed visual inspection of each white wire looking specifically for worn or cut wires and insulation. Cut the plastic harness tubing off if you need to in order to visually look at everything. Re-solder any nicked wires and rap questionable insulation with good electrical tape and then retest the stator. The location where these wires exit the engine are in one of the most challenging environments as it is at the very bottom of the engine and the chain throws huge amounts of grit and chain lube on the harness. So, clean it up good and give this another go. If this turns out to solve the high resistance re-wrap the wires with some new overwrap and go with it.

 

[ChunCopter]

Thank you so much, that was extremely helpful.
I went through the wiring underneath and did find a few sketchy spots.
After re-soldering and redoing the wiring, the stator now seems to report reasonable numbers, both resistance and when running a dynamic voltage test.

However, the bike still doesn't quite seem to charge.
Just to rule out the reg/rec issues, I replaced those with a combo unit.

I do get a voltage reading on the battery that's a bit above what it is when the bike is completely off, but it does not rise up above 12.7V at ~3000 RPM.
I thought maybe the combo unit has a bad regulator, so I picked up one of these Dodge pickup ones that seems to be recommended on this forum. Same problem.

What gives? Seems like I've checked through every single part of the charging system.

 

[Lexi]

I was having a very similar issue. If you have a wet-cell battery, I would check the water level in that. My battery was almost bone-dry, filled it up with distilled water and it fixed my charging issue.

 

[ChunCopter]

Thanks, Lexi!

Regular AGM battery, seems to check out well with a tester.
Maybe I should try swapping to a known good battery though.

 

[Lexi]

Definitely could be worth a shot to try a fresh battery.

The only other thing I can think of is that it could be a corroded connector somewhere between stator, regulator/rectifier, and battery. But I'm sure you've already checked that if you've been testing leads!

Hope you get it figured out, electrical issues are so frustrating.