2004 Ranger, 4.0 liter, standard trans. A month ago, the dash display battery light would come on, but only after driving 5 or so miles. Alternator tested by knowledgeable tech shows diodes good, voltage at 14.2. Tech says alternator is good. Replaced the battery 2 weeks ago and light did not come on until yesterday. It is not constantly on; only lights at certain times. Goes off and on under all driving conditions. Battery does not dishcarge, stays strong on starting. Have read about "fusible links". Could this be the problem? Thanks for any help. Fordable

 

Battery light circuit is the ON/OFF switch for alternator, and it is a simple circuit to understand

First, how a 12v light bulb works
If you hook up a 12v bulb's 2 terminals to a battery's 12v and ground(0v), power flows thru it and it heats up/lights up
If you connect both terminals to 12v it doesn't light up because no power is flowing thru it
There must be a voltage difference between the 2 terminals or power won't flow thru the bulb to heat it up, light it up

Battery light circuit
Key on 12v-----------------battery light bulb--------------------------------------------------------------alternator/voltage regulator--------------ground

When you turn on the key the battery light bulb has 12v on one terminal, the other terminal is connected to alternator that is OFF, so lower voltage, like a ground
Battery light is ON
When you start the engine alternator generates 14volts, so battery light now has 14v from "key on" side and alternator is also 14v, so no power flows thru the bulb, its OFF

If alternator voltage ever drops below 12v(battery volts), then Battery light will start to flicker or come on for that moment, because one terminal on the bulb has a lower voltage than the other
i.e. if a 12v bulb has 12v on one terminal and 10v on the other it will light up, OR 14v on one terminal and 12v on the other it will light up

You have a 2004 Ranger which was the first year for the new Digital Dash, called HEC(hybrid electronic cluster), this adds another layer of possible causes for your symptom
2003 charging and 2004 charging diagrams below
A Microprocessor was added in 2004 and up Ranger's charging systems

You can get an inexpensive digital Volt Meter that plugs in to cigar lighter in cab, you can then watch to see if there is a voltage drop when battery light comes on, dash Volt gauge is not all that accurate unless there is a BIG voltage change

A Fusible link is a short wire with non-flammable insulation, and smaller wire gauge, it acts as a Slow Blow Fuse, but cheaper, it can tolerate momentary voltage/AMP spikes without "blowing" like a regular fuse would, if they do "blow" the insulation vaporizes and a wire separates without the smoke show of a regular wire that could cause a fire
Commonly used since the 1970's in many vehicle applications in place of slow blow fuses
Its possible you have a corroded connection where a Fusible link connects to a main wire, and when it heats up there is a momentary voltage drop, but would be a longshot because that's not how a poor connection usually displays, i.e. its not intermittent, its just a poor connection all the time

My best guess is you have a failing alternator, right now its intermittent, no test for that until it fails outright, but could be an issue with the HEC Cluster also no test for that as far as I know
I think monitoring the cab voltage would tell you more

 

Thanks for the explanation Ron D. Obviously you are very knowledgeable on the Ranger electrical system, something that has me baffled at times. After new battery and alternator test, I dug further and read about the fusible link in the system, or at least the device that works as the "high blow fuse" which it it sometimes called. Found it on the side of the underhood fuse box. Looked good to me, but cleaned up the terminals etc. and no change with battery light. Next, I followed the wiring harness and found the wire exiting the harness to fusible link, and then another wire exiting harness that went to a ground located on inner lip of fender just behind the battery. There were two wires attached by a self-tapping bolt. The infamous green corrosion was present on this connection. Pulled the bolt, cleaned it and the two terminals. Put a little die-electric grease on the bolt, and reassembled. Problem solved. In addition, I learned valuable knowledge from you on how this battery light system works. Imagine how long a diagnosis at the shop might have taken and the cost, or possibly taking a chance on a new alternator which would not have helped me. Once again, thank you.