Well, I did some more screwing around.
Still pretty mysterious but ive got at least some repeatable results that make a little more sense.
Inside the ambulance, there is a dash module that controls siren, Thelma brake, lights, anti-theft, etc etc. For SOME uckfayed up reason, if I do not apply battery power to the cable I thought was direct from alt + post, this dash module does not power on. If the dash module isnt powered on, the alt will not output any volts.
Whats messed up is there is a little 2 wire harness coming from the back of the alt. (there is also two large gauge wires, one to frame and one is + output.
One wire is always hot to starter battery
one wire has some resistance to - post.... like 24.5 ohms....
If I had to guess, I would assume that the hot wire is the internal volt regulator reference... who knows what the other wire is?
But you would think, that if the above was true, it wouldnt matter wtf was going on at the isolator.... The alt should always crank up the volts until the starting batteries are up to 14.5V or whatever.
Very strange. I am going to call WORLD WIDE EAST SIDE starters and alternators and see if the dude can provide me a little insight as to how alternators work.