Honda Accord LX alternator intermittent - ECU or Alternator?

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
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How many miles are on this 91 Accord LX?
Like you are thinking, and from my reading, I would change the brush assemblies next. I understand a life of around 100k miles is good for brushes.
You can buy a brush assembly for about $21+ about $7 for shipping at www.slhonda.com . Click on "Parts and Service," then "Buy Parts Online." They're on the West Coast. The alternator is under the "Engine" section. I have used them several times. They give good service and have competitive prices. Add a few oil filters or other maintenance items to milk the shipping charge for every drop.
Consider changing out the two bearings, for another $35 total or so, too. Though I think the brushes are the first thing to suspect.
Unless the car's been in a flood, then based on reports here I would not suspect the ECU. The brushes or maybe bearings are much more likely to be the problem.
Stay tuned because people have said the brushes are easily purchased locally at electrical shops. But I don't think this is the same as the "brush assembly" you'll see at Honda OEM parts sites.
I am looking to do this as preventive maintenance in the next year or so.
My 91 Civic's alternator died after eight years and 106k miles. I didn't know any better and took it to the dealer. It may have only needed new brushes.
Updates are welcome.
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snipped-for-privacy@polyphoto.com wrote:

have em run it on the tester for at least 10 minutes. youve covered every other single possibility, (including a couple id never even consider like redoing the ECU solder joints!) so id bet on the alternator being flaky after it warms up.
especially if its 16 years old.
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alternator internals are not the classic arrangement, where generated voltage from the windings are used to self-excite the regulator and the light is in series with the regulator supply from the ignition switch. In those, brush failure is silent - the light doesn't go on because the regulator doesn't draw current to feed the field through the brushes.
If the manual is right, the light is a warning light from the regulator and comes on when the regulator can't (or doesn't believe it can) set the alternator voltage correctly. That covers quite a bit of ground. The ECM appears to come into play only through the "FR" lead - your guess is as good as mine what that is, but my guess is that it is a "fault/regulator" indication to the ECM.
You can troubleshoot in the car when the light is on with a DVM; the voltage should be a shade under 15 volts DC (depending on just how cold it is under the hood) and the AC voltage should be under 0.1 VDC. If the DC is above 16 volts or the AC is in the half volt range or above, the regulator or diodes are bad respectively. If the AC is between 1/10 and 1/2 volt, you should try it with another battery.
My advice (assuming the battery isn't at fault) is to bite the bullet and get a new OEM alternator or at least an OEM alternator from a wrecking yard. Whatever's wrong is very likely in the alternator and yours has given you faithful service for more than a dozen years. You will probably need to replace the alternator once before the car reaches the end of life (more than once if you put in an aftermarket rebuilt!) and this looks like the right time.
If the AC voltage is okay and you want to try brushes rather than springing for a new alternator, you can look in the "specialty hardware" drawers at your local Ace hardware store. They won't have the assembly, of course, but they have a modest selection of brushes that may be adaptable. Other than that, I dunno. I presume you can recognize critically worn brushes - when the assembly is removed the brushes barely protrude enough to make contact.
Mike
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Thanks for all the tips! This alternator is probably the original so it's got a lot of miles, well over 100K.
Got it opened up. Brushes so worn they barely stick out when removed from the slip rings.
Dang. One of the slip rings is very deeply grooved, can't be much metal left. I weighed the constraints of time, money, etc. My wife's car, so I've been driving her to work and home, 14mpg vs 28-30mpg and double the distance, vs the difference between $100 rebuild or $20 brushes plus $35 bearings plus $? slip rings... So I took it into a local starter/alternator rebuild shop today.
Thanks, appreciate the help!
As far as resoldering the ECU, I have been an electronics tech for quite a few years so I've had lots of good soldering experience. Not for the faint of heart. Although it has no surface mount, it is fairly dense and your average Radio Shack soldering iron is not up to the task. And Radio Shack solder (at least in the past) uses a rosin that is rather to aggressive for my liking.
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I wanted to mention- One of the alternator rebuilders I called set warning bells off in my head. I looked in Superpages.com, started going down the list.
I called one, found out I was actually calling a call center 3 hours away in Portland. That they have locations all across the country, all under different names.
Two names listed for the same address in Tacoma, but the name on the building is different from either of the listings. A listing for an address in Lakewood that doesn't exist, they send you to the Tacoma address.
No way was I going there. The only reason I can think of -not- to use the same name is if they get a bad reputation, you think it's an isolated location and the stink on one name can't carry over to differently named shops in other locations. And from that one Tacoma location with 3 name changes (at least), changing the name can escape the stink from previous names without even moving the business.
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