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It could be a malfunctioning internal voltage regulator of the alternator(presume you have a 3G alternator with the PSD engine).
I had that happen to me. The battery light was always on, but had 14.2 volts and good amperage out of the alternator. Ran for a few years till a bearing failed, put in a 2nd alternator I had and light went away unless the alternator is not putting out power.
Thanks Eddie... Wouldn't the tester have picked that up? Yes, PSD. I replaced it with a lifetime unit, but unless it tests bad or fails I doubt they will swap it out.
Unless the tester is designed like the Ford's charging system, probably not.
The LG/R wire that goes through the battery light also energizes the voltage regular. If the regulator senses a charging problem, it will ground that energizer(LG/R wire) line and light up the battery light. Under normal operation the 560ohm resistor feeds voltage to the regulator and the lamp will not light up.
Another possibility is a intermittent short to ground in the LG/R wire in the engine bay. I would start checking the wire harness from the alternator. That's a good place to start as a wire could of melted/rubbed onto something.
The idiot light goes out when there is voltage on both sides of it. One side is coming from battery power fed thru the ignition switch, the other comes from the alternator itself. Technically the "battery" power feeding thru the ign switch is alternator output when everything is operating correctly but let's not got there for simplicity's sake. With V+ on both sides of the bulb, it won't light.
If the alt fails to output voltage then the battery will power the lamp on one side and it will ground thru the regulator on the other.
And if the battery is somehow shorted and has a lower voltage than the alternator output, the alternator powers the lamp with V+ and the lamp will "ground" (again not technically) thru the battery with it's lower voltage.
So if you lose output for even just a second, you will have the light flicker on.
The ECM does not control the battery light and I'm fairly certain that your Aeroforce gauge does not read battery voltage from the ECM either, it is reading from whatever source is powering it. Hardly matters here BUT if you were to use an analog meter you just might see the voltage dip when the light comes on.
I would not trust the parts store alternator tester in this case. For one we do not know what the sampling rate is for output voltage... it may be ignoring the brief drops in output voltage or not recording them at all. An oscilloscope will give you the real story if you happen to have access to one.
Alt is no more the 2 years old. The rear bearing in the previous (and I believe factory) alternator self destructed.
No access to an O scope but I may try to find an analog meter. Digital readouts are nice but the needle can tell a story.
The idiot light only flickers at lower RPMs and when there is a load like headlights or the fan motor. As y'all have described, this is likely the case where the alt is just not putting out as much power as the batteries already have in them, making it look like the alt is failing. When inn reality the batteries are jut picking up the slack momentarily.
Until the idiot light starts to steady burn more regularly, I'll just keep an eye on it and not fret.
Interesting observation about the gauge not reading voltage FROM the ECM. Because if it was I presume it would be regulated and likely at a much lower voltage…5 volts seems to come to mind. And as the gauge reading changes, it is seeing the voltage TO the gauge. Seems to make sense!
