There are plenty of posts relating to these problems,and I found useful information, but was hoping for a "more specific to my scenario" response.

1985 TPI 305 700r4 -
I left the car yesterday with a battery that was beginning to loose a little power, and seeing as the car doesn't stay running without constant throttle feathering, it required an external charge. I had it charging for some time this morning, and not thinking it was a problem turned the key while everything was still connected. It did attempt to turn over but stopped very quickly. I got the impression there was a power surge.
After disconnecting the charger (the battery read fully charged) and turning the key, I only heard clicking coming from the starter. I looked at the connections at the battery and starter, which all appeared normal, and tried again, to which I received nothing. not a sound and all power was gone. Repeated attempts resulted in sometimes getting clicks, and sometimes losing all power (by which I mean the voltage drops to zero, all gauges drop, dummy lights and other lights go out.)
After checking again, and grabbing a hammer, I've determined I can always get the starter to click after banging on it a few times, but it only last for a few seconds then all power goes as described above.
I also attempted to jump the solenoid ( i believe i did this on old fords with the fender mounted units). when jumping the solenoid, all power (as in the power loss previously described) returns to the car, but no difference in the starter motor behavior...
I have also intermittently heard pressure release from the starter area, which i assume to be the plunger disengaging?? not sure on this, and it has happened rather randomly. While swapping batteries, with banging on it, while turning the key, etc.. but not guaranteed in any scenario.

Of note, I have tried other batteries that are definitely working, so that is not the problem. I assume starter, and it seems like the obvious place to start, but If anyone has the knowledge of the inner workings, I was hoping to understand what is going on.
Why when i jump the two terminals on the solenoid do I always have power, but only intermittently otherwise and no other noticeable effect? why do I only randomly get clicks? why when i do hear clicks does the engine not crank? And also, could this be ignition related? If ignition related, can I hot wire the starter to check before replacing parts?

Thanks in advance.

 

First thing - you're going to need a good battery to eliminate that out of the equation.

Then, you need a helper. When you turn the key, and get a click, you need to leave the key turned and check voltage on the single purple wire at the starter - if it's not getting 12v, then starter will click but not turn.

This purple wire traces back through the main harness driver side to the neutral safety switch, which is in your center console attached to pass side of shifter - see if it's getting 12v there. The neutral safety switch gets the 12v from a large yellow wire on it - if the yellow wire gets 12v when key turned, but purple does not, then you have a bad NSS. If yellow wire is not getting 12v, then you have a bad ignition switch (not the key cylinder) - the ignition switch is on top of the steering column - you have to drop the steering column down to get to it.

http://www.austinthirdgen.org/mkport...ine_wiring.gif

 

Thank you for the response and assistance.
I did get a helper and began tracing wiring, but rather than being anything complicated, the connection on the solenoid had fried/melted/broken. Cleaned and fixed that and all problems are gone.
I assume that the banging allowed enough temporary contact to initiate the starter but once it turned and vibrated the contact loosened.

As a side note, I have been toying with this car for a long time, and finally have it starting with a single key turn and staying running without feathering the throttle (also excellent sound, throttle response, etc.. and no excess engine vibration) It isn't running perfectly, but now that it stays running on it's own, I will set the timing and move from there. It currently likes to surge and sit at about 2000 RPM. Sadly, I still can't drive it.