I don't know the make of the alternator, it has just one wire. I will try to attach a photo of the alternator.
FYI the voltage across the battery terminals w/ the engine off is 12.55v with the engine at idle speed it's 14.62v. I found no draw on the system.
The Alt meter is showing - charge as it should but shows little or no + charge when it should.

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Your Picture came thru fine. Your Alternator looks like a Delco (GM) and popular for 1 wire conversions. Would you Look at the back of the Alternator for a stud the looks like it is insulated like the one wire that is coming off??? See the little window on the Alt at about 3 O'clock, are there two little spade terminals in the window??? They may be the answer to making your Alt light operate. AT THIS TIME I'm not sure how to make you Alt Light go out/work properly, we'll figure it out.

OK, single wire alternators have a built in regulator and are what you call, self energizing. If the Alt gauge shows a slight charge it would be normal if the battery is fully charged, as it does not require more input from the Alternator and the gauge will show almost no movement to the plus + side. You said your Battery had 12.55 volts, almost full charge

Comment, Rule of thumb, 12.60 fully charged battery, 12.50 3/4 charged, 12.40 1/2 charged, 12.30 1/4 charged, 12.20 and below is a dead battery.

Comment, After charging a battery and you check the voltage and it's say even over 13+ volts, it has a surface charge on it. To test for true voltage, place a load on it, like your Hi-Beam headlights on for 20 seconds. If you watch the voltmeter it will go down, down. Shut off the load after 20 seconds and watch the meter climb back up until it stabilizes, then you'll read the True Voltage.

Did you find the wire that feed,the whole truck??? Was it or did you attach it to the other wire from the gauge???

Two Tests,
1- Engine OFF, ign ON, headlights on Hi Beam, heater on High, look at the Alt meter it should be reading, Discharge -, thinking at least 15-20 amps. If so, all those loads you put on the battery should be going THRU the Alt meter. ( just to say it once the ALT meter IS an Ammeter) If not, thinking you didn't find and hook the whole load wire thru the meter. You didn't say if you did all I asked you in the previous post.
2- Start the truck rev the engine, like 1000, 1500 RPM, immediately watching the ALT meter, it should read charge + for a bit and the start tapering back to 0 or the middle with slight to + as the battery charge goes back up.
3- Oops a third test, everything OFF. While watching the Alt gauge, blow your horn, it should NOT move. The horn is too much of a load to run thru an ammeter,

A lot of writing I know, just want you to get what you want, with NO glitches. Cap

 

iicap,
I don't know the make of the alternator but it has just one wire and an internal regulator. I found no draw on the system.
With engine off there is 12.55v across the battery terminals at idle there is 14.62v
The Alt meter shows - charge when it should, but it shows very little or no +charge when it should be showing something.

I tried attaching a photo but some how it became "page 2" if you can look at that page there a nice picture of the alternator .
Thanks for your time.

 

cap,
I have looked but there is not another Insulated stud off of the back of the alternator, and no blade Connectors in the window.

I have made all the connection you asked and each of the test in the last 2 messages, everything has happened exactly as it should have.
Except the test with the engine running. While the started was turning the engine there was significant discharge soon as engine fired the meter
Returned almost to zero still a bit to the discharge side and when I revved the engine the meter show a little more bump to the Negative side.
The test with the key on, engine off, the lights and heated on, the meter showed a discharge of approximately 25 amp.

FYI everything off, across the battery terminal 12.59, with engine at idle approx. 13.35v with the engine revved up 14.25v ish.

 

OK,
First, because the Alternator single wire is going to the battery side of the solenoid, it is putting the charge directly to the battery (why you get a 14.25 voltage reading at the battery) and not thru the Alt gauge. To put the output of Alternator thru the gauge, it would have to join the wires, whole body feed and the one from the gauge, which you have joined already.

I Have To Search, but thinking Custom Cab with Alt and Oil gauge option MAY HAVE HAD an insulated stud just near the solenoid to mount these 3 wires together. Separating whole body wire and one from gauge and putting ring terminals on them would allow you to add the wire from the alternator, which you would remove from the solenoid. Joining the three together.

Comment, Note, it is the alternator or generator that runs everything on a vehicle. The Battery is just a storage device. It runs everything when the Alternator or Generator cannot cover the load. The A or G operates all Plus keeps the battery full.

We still need to find if your A can operate the light. In the picture, looking in that window, if not spades, am I seeing like aluminum fins??? Any where else around or back of A is the an opening that may have spade terminals???

 

You're right all that's in the window are some cooling fins, I will give the A a closer look.
I don't understand why the meter was showing discharge when it should and show a even more
discharge when it should have been showing positive charge.
I see what you're saying, hooking the three together should be easy enough.

 

I have just finished looking at 65-66 wiring diagrams, YouTubes, till I'm dizzy. Yes there were/are junction block(s) 1 or 2 involved and hookup about the same as I'm suggesting to you. The 3 wires hooked together. Your choice, ring terminals with a bolt and nut, an Insulated stud to stack them on, etc.

Again, ALWAYS caution, Battery Disconnected while making changes and the test light procedure with the Neg Batt cable and Neg post to make sure no draw before hooking Neg cable up.

The Ford Alt/ Regulator system used 4 wires. On the regulator the I terminal controlled the light. We'll go back to that after Gauge is working properly.

Will wait on You about more Alternator details, and gauge progress, Cap

 

I have good news and I have bad news.
I have checked every surface of the alternator and there are no spade connectors.
On the other hand after connecting the main feed wire and one ammeter wire with the wire
from the alternator, my truck's ammeter reads both + charge and - charge just as it should.
Frankly I don't think I would have ever connected those three wires together but clearly it was
the correct step. Thank you for your help this issue.
I don't have the Idiot lights but I still need to track down those wires, the gauges are far more important to me. The lights will just have to wait.

 

Good deal on the Alt meter working as we think it should. Don't give up on the idiot light(s) yet. just for now the OE volt regulator would have been along side the radiator on the drivers side. I will or you might have the wiring diagram to show you the color of the wire and we can test it to see if the light itself will work. It would have been hooked to the I terminal of the V Regulator.

OK I stand corrected on the terminal on the old VReg. It would be the S terminal and the wire there is 18 ga, Green with Red stripe, wire #904 in the diagram. As it makes it's way to the idiot light it becomes Yellow with a Black stripe and wire # 85 on the diagram.

If you locate #904, Red,green stripe, hopefully just taped off some where. With your volt meter or test light check it for power/voltage with the key on. You MUST have a good bulb in the idiot light socket, as the power with key on goes thru the bulb. Key on, someone watching the idiot light, touch the wire to ground, it should light. If it does, you know the circuit to the bulb is good. Tape the wire up for now. Cap

 

So I went looking and yes there are single wire Alternators that DO NOT have away to control the Idiot Light. If you will open this link below in a new tab so you can toggle back n forth between this forum and the link. This link is from Jegs and you will see that your Alt and the one in the link appear the same. Read the short article. DO YOU HAVE the rubber plug that would have the two terminals under it, the left R terminal for the idiot light. No rubber plug, I guess you Alt is not capable. Cap

https://www.jegs.com/InstallationInstructions/500/555/555-10100.pdf