1991 Honda Accord LX, 2200cc engine, 4 door sedan with standard
transmission.

The charge light came on one day, went on and off again, finally came
on and stayed on.

Frustrating... after checking all the wiring, removing and retightening
the battery cables and the wires on the alternator, checking ground,
the light went out. Thought I'd fixed it. In the time it took to pack
up all my tools, went back and started it and the light was on again.

Removed the alternator and battery and had them tested at the local
parts store. Both test good.

Rechecked everything, I see on the wiring diagram there are a couple of
wires that go to the Engine Control Unit/Module, so I take the ECU out
and clean it's connectors by taking them off and on again a few times.
Charge light goes out again. I shut off the car and restart 3 times
(letting it sit a few minutes between), light goes out immediately when
engine starts as it has when things were fine.

Nope, restart car about 10 minutes later and the charge light is on
again. No wiggling of wires or tapping on anything makes it change.

I even took out the ECU and resoldered all questionable solder joints
(I've been an electronics tech for a number of years). Charge light
still on.

During one of the times when it was working, I took some readings. I
doin't know how meaningful they are except they tell me that it is not
a wiring problem between the alternator and the battery.

The only difference between charging and not, one of the two wires
between the ECU and alternator: normal operation, when first started it
is about 3.6V, dropping to about 2.5V. Later starting it and the charge
light comes on, the same line is now about 1.5V.

But without knowing which way this signal goes, I don't know if the
fault is the alternator or the ECU telling it the wrong thing.

I see some mentions here of worn out brushes. Where might I find those
in the northwest USA? I've checked a few places like Autozone and it
lists them but says unavailable.

Thanks,
Steve