Honda Dual-Mode Charging Making Charge Indicator Turn On

1992 Honda Accord - I just replaced the alternator with a new (not reman) AC Delco unit. The new alternator worked fine, except the charge warning indicator light intermittently turned on while driving (which is a new problem I didn't have before). I figured out that it happens when the computer commands the alternator to go into "low voltage" mode. If you're not familiar with Honda's "dual mode" charging system, it maintains voltage at about 14V like any other car...most of the time. Under certain circumstances, it'll reduce voltage to 12.5V to reduce drag on the engine and save gas. I attached a photo, which is from the factory service manual, that shows the criteria that has to be met to go into the low voltage mode. Sure enough, the charge light turns on the moment all of those criteria are met. There is a wire that goes from the alternator to the PCM for this purpose (the "C" wire). Under the normal, "high voltage" mode, the alternator keeps the voltage on the wire at around 2.19V. The PCM pulls it to ground when it wants the low voltage mode. I probed this wire and hooked up my multimeter and drove around while monitoring the voltage. I confirmed the warning light turns on when it switches to the low voltage mode. However, the warning light will often turn off after a few seconds, even if it's still in low voltage mode. 

The warning light is directly controlled by the regulator in the alternator. The wire goes from the instrument cluster straight to the alternator, and the alternator grounds it to turn it on. Since this happened the moment I installed the new alternator, I figured this was caused by a defective alternator. So I got another AC Delco alternator and just installed it today. I took it out for a test drive, and this alternator is doing the exact same thing! Is it just that AC Delco doesn't know how to build an alternator? Or is something else going on that I'm missing? I'm getting REALLY sick of replacing alternators over and over.

And yes, I realize I can just disable the "low voltage" mode by cutting the C wire and cap it off, but 1. It goes against my OCD to have something on my car disabled/not functioning the way it's supposed to
2. If I spend $109 on a brand new part, I expect it to work properly. I don't think I'm asking for much.

Should I try another brand alternator, or could this be caused by some other problem?

Unfortunately, I've been here/seen that with reman alternators on Hondas. I wouldn't hesitate try another brand.

If you want that 110%, you can depin the alternator connector and remove the 'C' wire (and tape it up somewhere safe). If the battery light stops coming on, you know you're on the right track. But I think you've got enough evidence as it is.

If you're ordering from RockAuto or the like, you might consider a local parts stored instead. That way, the impetus is on the parts store to get you a working part, even if they have to change brands or upgrade the part to do it.

I did some more research and saw that other people had the same problem and it was because of the alternator. Last night I took the regulator out of the old OE alternator and swapped it into the new one. I was able to swap it with the alternator still installed in the car (I really didn't want to take it out again). I don't know if this is going to fix it for sure, but I think it will. I still need to take it out for a test drive and make the PCM request the low voltage mode to confirm. Which means waiting for daylight, since it won't go into low voltage mode with the headlights on (too much current draw).

One interesting thing I noticed is the OE regulator keeps the voltage at about 8V under the normal high voltage mode, whereas the aftermarket one kept it at 2.19V. When I was learning about this system, everything I read said that 8V is normal, and some models keep it at 5V, but no mention of 2V. So that threw me off when I saw 2.19V, but I shrugged it off as just another voltage that wasn't mentioned in the articles I read, since both aftermarket units had the same voltage. I don't think that really matters much; it was more of an observation.

I was finally able to test drive the car yesterday. I monitored the voltage on the C wire and I saw it go into low voltage mode several times and the warning light never turned on once. So that's a confirmed fix.