Alrighty. So my dad bought this 94 ranger a month or so ago from a guy, its had some work done. im a diesel tech, but I know how to work on pretty much whatever so i look it over and the trucks in great running condition, the interior is a little rough/dirty but whatever.. Well, the other day, I replace 2 bulbs in the dash, they were the correct size and all. i get the dash back together and all the sudden i have a battery light on in the dash, and my voltmeter is reading a little lower. Now, JUST previous to that, i drove it like 15 miles with no problem. i had been driving it all week, with no problem. so i test my batter while the truck was running, im getting 12 volts. i say ok, maybe the alt is just putting out a little bit low, no biggie. ill run it, whatever. two days later, im working on my car and i need the truck to make a run to the parts department where i work. its running worse, i test it, charging at like 9.9 volts or so. so i park it and start running some diag on it. i charge the battery and make sure it can hold a charge, i pull the alternator and have it bench tested, alt tests bad, bearing going bad, pulley seizing. ok, no problem. i put the new alt on, put the battery back in, verify my connections and check fuses to make sure nothing is blown. well i start the truck, battery light still on. i go through some more tests i find my signal wire from the alt to my first connector right before the firewall has reads OL on my DMM. i run a new wire using the same gauge wire and deutsch pins necessary. i restart the truck, and lookie there i have 14.3 volts. im happy. so i let the truck run for a bit to check everything, then out of no where my charging voltage drops to 12.3. and stays there. next day, i retest before i come to work, thought maybe its got to do with truck getting hot, maybe a connection getting hot. no dice, still 12.4/12.4 v. I decide to replace the fusible link, even though i had good continuity through it with very little to no resistance, i just figured id try it out since i have access to the parts for it. i make the new wire with new eyelets and install to the truck. start it, still at 12.4 v. then a few minutes later, all the sudden im at 14.14? ok ill take that. so i let it run. and it didn't drop any. awesome. i go back out maybe 2 or so hours later and start it, to check again, 12.4v!. now i spoke to a buddy of mine who said the charge will drop until the alt sees a specific voltage and then pick back up, but the its never dropped below 12.3. my next step right now is to pull the dash back out and make sure i didn't screw something up in there and mind you my battery light has been on and off during this whole process even when its charging at 14v. im going to pull the alt back off if i don't see anything obvious and have it bench tested to verify its not that. but im running out of ideas. im not seeing any grounding issues, ALL my fuses test good, i test them cold and after trucks at operating temp. any other ideas? other grounds that may need checked? battery has even been replaced because my dad decided to replace it, even though i told him it was ok.

YES THE AMBIENT TEMP OUTSIDE VARIED WHEN READINGS WERE TAKEN, VARIED FROM 65 DEG. F TO 95 DEG.F.
ENGINE NOT OVERHEATING.
THE SOURCE OF THE SHORT IN MY SIGNAL WIRE WAS DUE TO ROUTING OF THAT HARNESS, IT WAS LOCATED BETWEEN A HOT COOLANT PIPE AND TOP OF MOTOR, WIRE WAS DAMAGED INTERNALLY.
YES THE ALTERNATOR IS A REMAN FROM OREILLEYS
PULL TEST ALL PINS, 5LB, TEST GOOD.
CONNECTIONS TIGHT, NO CORROSION
NO PARASITIC DRAWS
NO AFTERMARKET ADD-ONS.

 

In spite of our best efforts, aftermarket rebuilt Ford alternators don't always perform as expected. If possible, try to get an output reading from the alt when there is a sizable current demand. If it puts out the rated amperage now I would suspect a possible high resistance in the charging circuit causing the alt to think the battery no longer needs any high output and drops output accordingly.
It may require voltage drop testing of all the system including the battery cables AND the connections coming from the alt to the final destination of the battery.
If all that checks out , it could be a problem with the plug at the alt or the alt itself.