First up, testing the rectifier, because I'd not done that last May as intended. Pulled it off the bike, got out my handy-dandy free Harbor Freight multimeter, and started checking it according to this chart in the service manual:
To my dismay, none of the continuity connections were working. I went inside and got my expensive multimeter ($70 in 1991) and got the same results. Remembering that I had a spare rectifier module, I dug that one out, and checked it, and got the exact same results with both DMMs.
How likely was it that I had two modules which were completely fried? But then I stopped myself and took a look at the rectifier circuit diagram, part of the charging system circuit diagram thoughtfully provided two pages earlier:
I broke it down into the simplest possible terms. What was the path that current would take to get from the red wire (R) (not labeled, but coming from the battery) to the black and white wire (B/W) (also not labeled, but going to ground)?
There was none.
I checked it again, and a third time, trying to find any way for current to get from red wire to black wire--and there was none. And when I checked the yellow (Y) wire, the White/Blue (W/Bl), and the White/Red (W/R) wires, the story was the same: there was no continuity from those wires to ground.
So I started over, this time from the standpoint that whoever labeled those charts was a screeching moron and got them ass backwards.
(Not necessarily a Suzuki technical writer.)
And when I checked them that way, both modules passed the check, meaning both were just fine. The one which had been on the bike looked like it had been through a fire, so I put the other one on.
Started bike, ran it up to 5,000 RPM, and checked voltage at battery. 11.95v, a mere 3.05v lower than it should be. Argh.
Pulled the rectifier out of the system to do an unloaded check, in the process finding that I'd crossed a couple of wires. Rechecked with correct connections, voltage across battery still 11.95v. Removed it from circuit and checked unloaded voltage. Nominal is 75v AC at 5,000 RPM; I'm getting 6.
Checked that the stator is not shorted to ground--it's not--and that means in all probability that there's something wrong with the rotor. Magnet may have died. Well, the bike is 35 years old, and it's not unheard of for one of these things to lose its magnetism. Since I'd just been running the bike at 5,000 RPM, it's too hot to check now, but it's on the list for later this week. In the meantime, I know not to take the bike anywhere too far, because the battery is emphatically not charging.
Well, you know, I only paid $400 for the thing, with helmet, six years ago, and I have a spare engine; hopefully that rotor will be fine. The problem with the battery not charging has been an ongoing thing for a couple of years at least, anyway. This is what you do when you own a classic bike: something breaks, and you fix it. (New rotor might cost as much as $50 or so.)
Diagnosing the problem is half of solving it.