The one wire plug coming off of the main 3 wire plug. Trying to swap alternators but the new one doesn't have this.

 

Its the white stator wire jumper, you need it

Whats the brand and model number of the new alternator?
You need to get a wiring diagram for it, it may have an internal stator connection but usually there wouldn't be a 3 wire voltage regulator connection if that was the case

 

Its a JS Alternator. I found the wiring diagram that goes with it, just not sure what to make of it though.
This is on their website:
Turn On Single Wire Alternators:
Follow this step ONLY if we shipped a pigtail plug with your alternator. You need to connect the excite wire (L) to 7v to 15v ignition power source and connect sense wire (S) directly to positive post on alternator.See picture of the Pigtail Plug below.
PIGTAIL PLUG

 

That red alternator looks more like a Mopar alternator,

The case, the oval shaped 3 pin plug and the missing stator connection, make me think that.

Hopefully there is a model # on that one so you can verify it's correct for your truck.

 

Its a 370 amp alternator and from what I can tell it ditches the factory plug. One wire goes to the positive post, another to switched ignition and the other gets spliced to the factory wiring. I emailed the company so I'm waiting to hear back. It was specifically listed for the 4.0 Ranger.

 

I looked and could find a diagram

Just from the description which is NOT all that clear
Ranger side Light Green/red stripe wire on 3 wire connector is what they call excite wire (L) to 7v to 15v ignition power source
That would be correct

The S wire they call the "sense" wire, not the stator wire, the "sense" wire is the Yellow wire on Ranger 3 wire connector, it is used by Voltage regulator it can be hooked to B+ (battery wire, large connector)

I would get a wiring diagram specific to your YEAR Ranger from the seller

 

Customer service from this outfit is basically non existent after the sale, go figure. Hopefully I got this figured out right based off the above schematic and the vague instructions from the website. Splice in to GRN/RED wire with the (L) from regulator. Sense wire which is not using the factory wiring from the plug but instead going directly to the positive output post. Only thing what I don't understand is the other wire (F/I). BLK/ORG is the battery cable and the other?

 

So heres the stock plug A S I


According to the stock alternator A is yellow, S is white and I is green. From what I have been able to piece together A has battery voltage at all times, S is the wire that goes back to the alternator (which would be to the post on the new one), I is the switched ignition wire.

 

Yes, you are correct on all of that for the Ranger charging system

Still can't say on the new alternators wiring

 

I got a message back from them, the F/I wire is not used apparently although looking at youtube videos of other installs its hooked up. Once I get the coolant line moved I can test it and see if its working correctly and not giving me any warning lights.

 

"I" wire is the Battery Light wire, also the ON/OFF switch for the alternator...........................

If you leave an alternator on all the time it will drain the battery

They can make "1 wire" alternators, these have an RPM switch inside, when alternator spins faster than 1,000RPMs, it turns ON, if lower it turns OFF, so only the B+ wire(big wire) is needed