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!

 

17 or 18 volts would definitely be a problem. Make sure that your meter is accurate. Some cheap digital voltmeters read high when their batteries are run down (the battery in the meter).

You should have about 14 volts with engine idling and everything else turned off. It should not go above 15.5 ever. If you only have 12.5 the charging system is not working.

The light coming on intermittently is usually because the alternator brushes are about worn out, or the belt is loose.

 

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.