The idiot lights are wired with battery / harness voltage to the bulb - and then the lead out to the alternator functions as the ground. If that lead is grounded - the lamp lights. However - on the alternator the post for that function has voltage when the charging system is working - and if that voltage (potential) exceeds the dash voltage, current can flow backward through the bulb and cause it to light. So - placing a properly directed diode in that wire will block that return voltage - but allow the lamp to ground back toward the alternator when the system is dead.


Hopefully you are describing a system that has a volt meter - not an amp meter - and the voltage should read battery voltage and maybe a volt above when charging. 


 
 Steve Metzger       Tucson, Arizona

Ok ,so I think the alternator is working .  ( Haven't burnt to the ground yet  )  Where should the needle be on my amp gauge or how can I be sure it's working? And that dang gen lights still on on the dash.  

converted
After I converted my ignition system to use a 4 pin HEI module the engine would not shut down with the key until I put a diode in the GEN warning light wire. (I am still using a generator)


"FREE SAMPLE"

The diagram I published before - shows a 3 wire alternator with a jumper added to place the sensor on the output lead. That would still need a heavy output cable to get a good reading of system voltage. The third wire and diode act as the ground for your dash "gen" light.

That pin goes to ground when the alternator has failed to charge - and completes the warning light circuit. If the diode isn't in there, the system will back feed the dash light and try to make it flicker as the voltage at the alternator pin exceeds the dash voltage.

 
 Steve Metzger       Tucson, Arizona

It's a 3 wire napa Part # 213-3014, what I'd call a Gen 1.

Well, if that is the part number you got ...... did it come in a small box? My google of the number came up as an oxygen sensor...... The oxygen sensor on my T-bird is my face hanging out the driver's side window! 
If it is a "one wire" alternator of some other part number - there will only be a single stud on the back side. That stud needs a heavy gauge connection run to the same stud that your battery uses on the starter solenoid. I would recommend a very flexible piece of multi-strand copper wire - minimum 7 gauge for a low power outfit - and welding cable as the amp size or distance increases. Crimp on copper lugs, properly seated and shielded up to the eyelets with heavy heat shrink tubing should do the trick.
My  T-bird uses #6 welding cable - and I bought eyelets of two sizes - one to match the alternator and one to match the solenoid stud. It will cheerfully carry all 100 amps of the alternator current with little problem. 

 
 Steve Metzger       Tucson, Arizona

Thanks for the info guys . I ended up buying a napa 2133041 alternator but now I'm not sure how to wire it up , Help !

get the Motorcraft (Ford) 3G.. 
http://www.oldengine.org/unfaq/3ag/3ag.htm

wreaking yard they run around $20 - 30... 

Reman  around $70  + -  

New   $$$

pull one from a  3.8 V6   need to get a V pulley for it if you don't have one
and you might need to put a spacer behind it...

I did this swap on my 351c years ago, and just now doing it on my 
56 Packard... 
  I used the pulley of the 2g, but had to make a spacer, to  get the pulley off
the Alt body  I got a washer with a 5\8 hole, then ground it down to fit the 
recess that the pulley fits down into. You will not be able to use the normal 
lock washer. Also the upper mounting bolt I think is metric so if you get one
from the junk yard, take both bolts... you can figure out the rest
 OH and get one with 180 deg  opposed ears, they come in 3 configurations.
Best

Steve explained that a lot better than I did. My problem is I’m usually trying to fix a problem on someone else’s car, and running a new #6 cable is not in my work description.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

It is usually easier to get a "three wire" GM style alternator - and a simple "jumper" modification will rig it for use with the idiot light. In factory installations the third wire is a "sensor" lead that detects voltage present over in the wiring harness and adjusts the alternator regulator to boost voltage to suit. On a "one wire" installation with a heavy gauge lead to the battery, there is not much (if any) voltage drop between the alternator and the battery and the system works well. I have a chunk of #6 welding cable on mine - virtually  zero drop. Trouble can develop if you use too small a cable to the battery and the "sensor" is not reading the same as over at the battery connection. Digital volt meter will catch this. 

 
 Steve Metzger       Tucson, Arizona