No spark when cranking

What could draw system voltage down so low in start that you can't get a spark ?
How about the starter ?

Ignition one / blue wire.. In the later models they branched that wire to whatever crap they added ( electric choke assist for example ).
What some don't realize is ignition two / brown wire is on the ballast resistor just like the others. That resistor will pass current in either direction so any time one wire is hot all the wires at hot to some degree ( pardon the pun ) in both start and run.