Ran into this recently and it kind of stumped me for a few days until I had a brain flash one evening and and the oddball answer, which turned out to be correct, hit me.
This is mostly for techs and DIYers to have bit of fun with and to stir up the brain cells.
Vehicle is a late 80s Mustang with a 5.0
Problem is that the alternator will not charge; at all.
Here’s some relevant info.
Reman alternator. (not installed by me and tests fine with no doubt whatsoever)
New belt/new belt tensioner. (no slipping at all)
Battery excellent.
All battery cables and fusible links fine.
Red dashboard alt. warning lamp illuminates. (exciter circuit)
Exciter wire at the alternator plug shows 12 volts with the key on and the light illuminated.
Alternator pulley (steel) becomes magnetic with the key on. (sign the fields are being energized as they should be)
Anyhoo, for those that like a brain challenge have at it and I’ll fill in the blank tomorrow night.
While the odds of someone running across the anomaly are very slim it’s something to keep in mind. I know I learned something from it. Have fun!
And pardon the typos.
Ignore what I had written here. I misread the puzzle.
I’m scratching my head over this one although I’m not super familiar with the ford setup. The longer I think about it, the more curious I become.
So, although the field is energized, you only read BAT voltage on the alt output but it works correctly on the bench…
I assume the indicator is fully illuminated so the remote voltage sensing is working, fuse intact, as noted field energized, etc…
The pulley is the right size so it’s spinning at the correct speed and belts are not slipping…
Battery is good and not dragging it down…
All of the other considerations I can think of are eliminated by the aforementioned.
Weird. I will be very curious to hear the solution! In the meantime, I’m still pondering…
As I remember, Ford products of this period had a separate voltage regulator that was not built into the alternator. My son had a 1989 Mercury Sable and had the these came conditions. The alternator was replaced and quit functioning a week later. The problem the second time was the voltage regulator.
I too scratched my head over this one.
If I understand it correctly, we have:
1: The alternator tests fine (I assume means it passes a bench test)
2: On the car, the alternator does not charge the battery.
3: Alternator shaft is magnetized (implying brushes and field circuit
are working. It also means the regulator circuit is exciting the
field, though it’s not a measure of sufficient excitement).
4: The dashboard warning lamp is illuminated, both with key on and
when the car is running.
I wonder if there’s a fault in the wiring between the dashboard light and the alternator? Perhaps a short?
Defective Engine To Body Ground Strap?
CSA
If I were to guess, the fusible link from the alternator to the battery was blown.
Oops! You said you checked the fusible links. Then how about the ground between the engine and body?
Tester
Tester, I’m Guessing That OK4450’s Statement, “All battery cables and fusible links fine.”, Disqualifies That. Give Us Another Guess, Please.
CSA
this is fun. you state theres a new belt and tensioner. im unfamiliar with 5.0s, but how many belts and tensioners are there? question: can the car start?
I also had a Ford from this period (Mercury Marquis), and was thinking of the voltage regulator. Hooking up the battery terminals incorrectly will make the voltage regulator smoke, and then when you hook them up correctly, you will get nothing. (Don’t ask me how I know!)
Ok4450 said he tested the cables to the battery, but what about cables between the voltage regulator and the alternator?
Did you test the cables when the car had been sitting or did you test the cables after the engine had been running? Sometimes the cables will heat up when they are in use, which will affect their ability to conduct electricity.
In regards to the voltage regulator comment, the alternator has an internal regulator and with the belt/tensioner situation there is only one belt driving everything. And the tensioner and all accessories are fine with no dragging bearings, water pump bushings, etc.
For a hint, Joe Mario is hovering in the area of the problem although it’s not a short.
In case no one gets it, I’ll provide the answer this evening. This thing did give me a bit of an education after a few days of stewing I must admit.
Here’s the area I’m focusing on.
OK says the dash warning light illuminates when the key is turned to the “on” position. That means the circuit from the battery, to the key switch, to the light, to the alternator, to ground is complete. So far so good.
However, when the car is started, a good working alternator will then produce 12 volts on that same circuit. This results in the dash warning light having 12 volts on both sides of it - and hence it turns the light off.
What’s puzzling is that the dash warning light remains lit after the car is started. The only way for this to happen is for one side of the bulb to be connected to a ground (rather than both sides being at 12 volts).
Why the above paragraph puzzles me is because if one side of the bulb were accidentally connected to a ground, shouldn’t it either cause a burned wire or short out the alternator? (And he did say the alternator tests out fine).
This is a good one.
The circuit for the dash warning light has to have sufficient resistance. If the resistance is too low, the alternator may not turn on hard enough to charge the battery. This is one of the reasons why I mentioned the lamp is fully illuminated above. Perhaps it should have been asked as a question. I suppose there could be a situation where the resistance is too low…
No, I don’t think so. Resistance too low would result in an overcharging condition. Resistance too high would cause an undercharge. Since I don’t have a schematic, and don’t know the exact details of how the warning light circuit works beyond the general sort, I’m guessing a broken wire between the dashboard and the alternator, or possibly a broken ground connection in the dash.
My vote would be for the wrong wattage bulb being in the indicator socket i.e. too low of a wattage and too high of a resistance.
Well, here’s my thinking, perhaps it is wrong.
The indicator lamp wiring also supplies the sense voltage for the regulator.
In essence you have a voltage divider that forms between the lamp side and regulator side. The system expects a relatively stable and known resistance on both sides. If the lamp side resistance is lower than it should be, the voltage drop across it is also lower. That results in an increased voltage drop across the regulator side, fooling the regulator that there is more voltage on the battery than there actually is and therefore little to no charging.
Is it possible that the alternator housing is not grounded?
Well, guess I’ll throw the cause of this oddball problem out there and several of the comments were hovering in the area anyway.
The problem was caused by an alt. warning lamp loose bulb socket (the plastic twist-ins) in which one side of the bulb socket terminal was barely making a contact with the printed circuit film.
The connection was apparently good enough to pass current, provide a steady 12.6 volts, etc., but not good enough to keep the fields energized.
I hate getting beat on things so this chewed on me on and off for a few days until one evening the possibility of a weak circuit occurred to me.
The next day with the engine running (no alt. output and warning light on) I probed the exciter wire with a test light instead of a VOM. The light went out and alt. output went to 14 volts.
Remove the test light and back to no output with the lamp on again.
There should not be much current required at all to excite the alternator and apparently the 12.6 volts being shown at the alt. plug did not carry enough current to excite the alternator. The additional current flow (tiny as it is) through the test light was enough to keep the alt. energized and working normally.
One would have thought a poor circuit like this would have shown a high resistance and a voltage drop.
At this point, since the diagram showed no connector in line, I figured it had to be in the cluster or the cluster connector. The problem with the bulb socket was discovered when the cluster was removed and the car was fixed by bending the socket tang out on one side. This snugged the socket up and it worked fine.
While the odds of this kind of problem occurring are slim (near zero in my opinion) it’s something to keep in mind if all else fails.
A weak circuit is nothing new to me but what was throwing me on this one was that there was no discernible voltage drop, no flickering or dimming alt. warning light, etc. Any checks of voltage, resistance in the circuit, etc. were all shown to be normal with the VOM. Probably another reason to not put all of your faith into a VOM reading.
The last time I ran into something like this was on a VW and the fault in that case involved a car that would just stop dead now and then. When towed in it would always run fine, no trouble codes, etc. and the problem was eventually traced to a wire breaking down in the harness.
Anyhoo, that’s what happened and hope this post is beneficial if you happen to run into one of those headache cars. This is the first time I’ve ever run into something like this so it did provide a bit more education after throwing me for a loop. 
Well, it certainly was entertaining and informative. Probably one of the more interesting topics I’ve seen here in a while. Thanks for sharing!!