One alternator I have (3G) has terminal marking as A for Armature, S for Stator, and I for Ignition. An alternator from a 90 351 has 2 of the same markings. Instead of an A it has a letter F for Field. Are these the same?
There is no field with the 3G. Go back to the regulator and take the "A" wire and run that to the alternator.
The "A" is often just tied back into the output post. But it should really be run back to the battery. It is the wire that is used to measure the battery voltage and adjust the regulator to get the desired output. The I turns it on, it needs to be switched or else the alternator will run the battery down trying to get output from a non-spinning alternator.
The alternator in the pics is a good old tried-and-true large case 1G externally regulated alternator. Same basic function as original, using the same regulator as original.
They were usually found on larger trucks and ambulances and police vehicles.
They were the heavy-duty version and came in ratings starting at about 65 amps and running to 100/105 amps in their stock forms, with some in the 150 range I think for the ambulance applications. Aftermarket companies made them in up to 250 amp versions.
It was a good first step upgrade from stock on a Bronco, as they fit the stock brackets and had killer output down at low rpm levels. I still run a 70 amp version on my '71 and a 100 amp version on my F350.
Good alternator, unless you want to get rid of the external regulator and tidy up under the hood.
Paul
Didn't even take the time to notice the bracket extension in your second pic before. Now that you mention it, it looks pretty stout and should do the job nicely.
Paul
...I have two more EBs I'm glad that the 1G will work with stock Broncos.
Yes and no. Works great and is a good reliable alternator and easy to connect to stock wiring with just a little modification. My 70 amp version lasted 15 years in daily driving, a few more after that, and then ended up on Mtfit's red buggy doing extra hard labor there as well unless he changed it out after getting it.
But looking at the numbers you posted I'd have to say yours is the 100 amp version. While it would probably work for awhile on the stock wiring, there is no guarantee it won't melt down your Black wire when you least expect it.
Anything over 70 amps is usually recommended to work with a larger gauge and much shorter charge cable than the Black w/yellow 10ga stock one.
With 100 amps and potentially even more depending on the build, you're better off with running a short length of 6ga cable from the alternator's BAT terminal directly to the starter relay and battery.
If you have 8ga wire laying around that would work fine as well. But if you don't it's easier to find 6ga to use.
We sell this kit: https://www.wildhorses4x4.com/product/Battery_Charge_Cable_AMG_fused/Bronco_Starters as a mega-upgrade to modern specs. Good for basically anything short of 200 amps (we fuse them for 175a) and easy to connect.
You can make your own, or buy one already made up like that.
Just don't run it through the stock 10ga wire. It's just too long for that gauge of wire running more than 70 amps.
Paul
...Being a retired High voltage electrician, I'm a firm believer in over kill.