The battery (charge) light in my CRX seems to randomly turn on and off, and not related to rpm, except it won't do it when driving constantly above 2000 rpm. This has been going on for the past several weeks. The car has had no problems starting and since it is off most of the time, I thought the warning light output pin on the voltage regulator was malfunctioning. I finally caught it with the light on in my drivewy where I could open the hood and get my voltmeter out. Measuring on the battery terminals, when the light was on, it gave 12.6V and when off it would climb to 14V.
The manual says the voltage should be 13.9-15.1V when the engine is running. Most cars I've had behave like that, not fluctuating between battery voltage and charge voltage. Seems to me my voltage regulator is acting up. Time for a new alternator?
check the belt tension/condition. Also the alternator might just need new brushes (and the rings cleaned up). If the dealer doesn't have brushes you may be able to find some at the hardware store that you can sand down to fit and solder the old and new pigtails together.
The belt it tight. The light doesn't fade, it just turns on and off. With the voltmeter, the voltage changed pretty fast. Surprisingly, the alternator is one of the few things the former owner said he replaced and by how clean the thing is, I believe him.
Decided to watch it a little closer this morning. On the freeway (over 2000 rpm), no light. After the 40 mile freeway drive I'm sitting at a left turn light with headlights on, left turn signal on, idling about 600rpm. The light is flashing on with the turn signals, meaning it comes on with the current draw of the turn signals. Turn the headlights off, (turn signals still on) and the light goes out.
So at low rpm, under a current load, the alternator quits charging. Also, the few times I tried EOC and go to start the car with the starter, the charge light will come on for 4-5 seconds.
Seems to me, the voltage regulator doesn't know when to turn on.