I have been trying to figure this one out forever. If I am cruising on the highway at around 3500 rpms the battery indicator will come on. If I put in the clutch temporarily it goes away. It will come back a short time later. If I am driving around town with varying rpms it never comes on. I have a scan gauge attached on the OBD port which consistently shows I am getting 14.0-14.1 volts. When the indicator light comes on this does not vary. When I test at home with a multimeter it also shows no difference in voltage when the indicator is on versus off.

I just bought a clamp-on ammeter thinking maybe that would give me some new data and I found that that I am drawing about 50-60 amps with everything running (lights, heater fan, defroster). Seems ok to me. I think our alternator is 100A.

Anyone know the criteria for that light coming on? Or have any ideas on what to test next?

 

I had alternator issues in the past and after a couple of failed rebuild attempts and after my original overcharged and ruined a battery and several expensive electrical components I replaced it with a new one and the problems went away. The battery light on the gauges was pretty accurate in that it would only come on when voltage dropped below 12v but if you are not experiencing voltage drops then I am not sure. Try unplugging and cleaning all the connections and the vehicle grounds.

 

I've been thinking about this problem. Heartygrain, I have exactly those symptoms except sometimes my alternator light will come on when I start the car after it having sat for several hours.

Here are some things to think about:

- You say your alternator is pumping out 14 volts while this is happening. Doesn't sound like the alternator is failing, then. I haven't measured mine, but I do see no dip in lighting when this light comes on (nor any ill effects at all). I don't think this problem is an alternator output problem.

- You said a new battery seemed to fix the problem for a while. I know I have a crummy battery (it's only about a year old, but I bought a cheap one). It also discharges so badly that after about a week I need to jump it. Have you had any problems like that? I need to check for a draw, and I suppose an unexpected one COULD fool the computer into lighting the alternator light.

- I also need to replace my positive terminal because it was corroded really badly with the old battery. I have had problems in the past when my car wouldn't start at all, and if I jiggled the positive clamp it would start up. I took it apart and cleaned it and reshaped it and I don't have the starting issue but I suppose it could be bad enough to throw off the computer a little. I definitely will clean my positive terminal and replace the clamp. When I bought the car, that whole assembly was eaten away from the corrosion, but it's just metal plates and a fuse. Bad connections in there maybe could fool the computer...

- I looked at some drawings (attached) and it looks like the connections from the alternator to the PCM come of one of the windings on one side, and off the +14v output plus some circuitry on the other. It's remotely possible that that circuitry could be failing without the main functionality of the alternator failing. In which case I think I'd just forget about it. Anybody understand alternators enough to know what that stuff might really be doing?

- Because the PCM lights the light based on ALL its inputs, it's also possible that a certain combination of events NOT based on alternator output could be making the light go on. This would be a software bug in the PCM...

All things being said, I think a GOOD battery and a replacement clamp might do the trick. In which case, I think I can wait until my battery gets really bad. The probability of that is inversely related to the temperature, and directly related to the urgency of my reason to get in the car...

 

I left out the fact that the battery light does come on for 1 or 2 minutes immediately after starting up in the morning. So my symptoms are exactly the same as yours. Replacing the positive clamp and battery made the issue go away for 6 months (warmer months too) but now it is back. Finishline performance has the clamps available online, that's where I got mine. The design of that clamp seems pretty poor and my original was completely ruined. I see a lot of people have posted that they have this random battery light issue but nobody seems to have found the solution yet.

 

I replaced the alternator with a lightly used one and the light still comes on at startup and when the engine is held at a steady 3500 rpms. So it's not the alternator. Still stumped by this.

 

I had a buddy tell me that sports cars (just in general) tend to be sensitive to aging batteries. I haven't had any trouble starting my RX-8 lately, but when the battery finally needs to be replaced, I bet that'll take care of the alternator light. Until then, I'm just putting up with the light.

 

So...this is embarrassing for me to post but maybe it will help someone else. All this time it was my coolant level warning light illuminating and not the charging system light. I saw in another thread that the lights look similar, and I never thought about it being anything besides a picture of a battery. Apparently there is an issue with the coolant sensor in the overflow tank that causes this exact issue with an intermittent coolant level warning. No wonder I could never find anything wrong with the electrical system. Oh well, mystery solved at least.