then,if you have 12 volts with the key in start at the S terminal, the wire going back up to the distributor is probably broken.... the wire coming off the solenoid is tied into the hot wire coming off the fuse block that goes to the distributor.... so there is a constant 12 volts going.  

This being a '71 car, the original coil power from the ignition switch will be a resistor wire, which will drop the voltage from 12v down to about 8-9v...not enough to operate the coil on an HEI set-up. In order to have voltage to start the engine at the coil while cranking, it uses a full battery voltage(12v) source from the starter solenoid "I" terminal, which is the outside terminal of the solenoid.

Any terminal on the fusebox marked "Ign" will ONLY be hot with the key in the "Run" position, so there will be no voltage on that terminal while cranking. It starts when you release the key because voltage is now on that terminal.
You should, as Jimmie points out, and you confirmed, have 12v on the wire coming from the "I" terminal on the solenoid while cranking ONLY. In order for your set-up to work properly, you need to have BOTH wires, "I" from starter solenoid AND the wire from "IGN" terminal on fusebox connected on the "Batt" terminal of the HEI coil/dist cap. This will give you 12v while cranking(from the solenoid) and 12v in the "Run" position(from the fusebox).
Your ignition switch is not causing the problem, since you do have 12v on the "Ign" terminal with the key on.
hth 



The "S" terminal at the solenoid is 12v coming from the ign switch to CRANK the engine...there will be NO voltage on that terminal in any key position EXCEPT while cranking. The "S" terminal is the small terminal closest to the engine block, and will be a larger diameter purple wire.


|UPDATED|6/26/2017 7:23:31 PM (AZT)|/UPDATED|

Ooops, forgot about there being a resistor..... ok, must admit, i didn't realize there was a resistor.   Although he was getting 0 volts while its turning over makes me think the wire is either broken, or if there is a resistor, it might be disconnected or blown....guess if it were me and could not trace the wire, i would be compelled to run a hot wire to a momentary switch and splice it into the hot wire off the key switch... yeah i know,  me bubba...you jane... :)  but then i could at least drive and pick up a milkshake ... or banana split... onion rings... great, i'm hungry now... 

It's all in the details, Jimmie. 
In the original post, he states that he has 12v to the HEI from an "Ign" terminal on the fusebox, so that means he is not using the original resistor wire for the coil feed, but has run another wire from the fusebox to the distributor....at least that's how I read it. As I pointed out, the "Ign" terminals) on the fusebox are only hot with the key in the "On" or "Run" position....not while cranking. Also, the label "Ign" on the fusebox doesn't mean those terminals are for the ignition system, but simply meant to say that those terminals are only hot with the ignition switch ON....otherwise they are dead, power-wise. He will need to have the two wires (from "I" on solenoid and "Ign" on fusebox) tied together somewhere to make this work. Both wires will not be hot at the same time, so no issues there with backfeed.
The only other way to make it work is to find a terminal on the fusebox that is hot in both "Run" and "crank" modes.....which would have to be something in the lighting, or horn circuits.

Good Morning It sounds like you still have the 8' ressister wire in service that was used for a points system relative to OEM points system.You will have to run a separate 12 gauge RED wire from IGN post on fuse block to BAT side of HEI distributor.If you still have a yellow wire then disconnect this as it will go down to the R side of solinoid and will not be needed. God Luck      nosal

|UPDATED|7/1/2017 5:15:28 PM (AZT)|/UPDATED|

   

Glad to hear ya got it goin!!

yippee!!  :)