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Ok, here's the deal. I'm driving a 4-door Civic (2005). Yesterday when I pulled out of a parking lot the charging system light came on. It only stayed on for a few seconds, but long enough to get my attention.

This morning was about 0-degrees Celsius where I'm living. I'm not suggesting that was a factor in what I'm describing next, but I just thought I'd mention it

So, this morning we fire up the car and the charging system light AND the airbag light come on. I get out my voltmeter, and while idling (with everything turned on: AC, etc.) I'm pulling approximately 13.9 volts. Now, the battery is rather new (less than a year old), so the alternator might be dying on me and perhaps I couldn't tell from the voltmeter simply because the battery is new... I don't know. (I should mention that with the engine being revved by my wife that I was measuring around 17 or 18 volts). Then, I stop at a friend's and borrow a meter that he has that tells you if the charging system is working. According to the needle, it looked like the charging system was definitely having trouble (there's a section where it says "Charging System" and goes from green to red, and when I attached it to the battery and pressed the "Load" button, the needle would start at green and go down to the red).

This afternoon, I start the car after church and no warning lights come on at all. We drive all the way home (10-15 minutes) and no charging system light comes on at all. I get out the voltmeter, and while idling the meter reads just below 13 volts. I get out my friend's meter and it no longer shows a problem with the charging system (now it just shows the voltage for the battery).

Any thoughts about all of this? I hope it makes some kind of sense because I am a computer scientist and know very little about cars!!

Thanks for any replies!

 
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An update:

It is definitely overcharging. I've checked it with two different meters. No option but to swap the alternator. Guess I'll be doing that tomorrow morning.

 
Rep Power: 313

Before you change your alternator, check your grounds.

 
Rep Power: 812

be sure to check voltage only once car is warmed up. at cold/hi idle in the winter, ive seen good alternators hover around 15.5v normally. once warm, the ELD regulates output voltage between 12v-14.4v normally depending on load.

 
Rep Power: 0

Yes, I've definitely checked the voltage after the car was warm. I drove into town and back (25 minute round trip) and then measured the voltage. It was still overcharging.

 
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OK. How do I do that?

 
Rep Power: 313

Look around the engine bay and make sure they are not corroded or dirty or anything. You never know, it could be that simple. Rule out the cheap stuff first before going after the alternator.