I have a no start issue in my 1974 360 road runner. The starter relay isn't closing because it only has 10.80 volts. NSS is tested and working. I can jump the contacts and it starts right up. The battery voltage is 12.5 volts. When I'm measuring voltage at the starter relay, I'm measuring voltage between the yellow wire at starter relay (which is disconnected for all this testing) to the battery negative terminal. With the ignition bulkhead connector unplugged, I have 12 volts coming out of pin 19 engine side of the bulkhead (0.5V drop at interlock bypass control unit under dash).

If I disconnect the brown alternator wire at bulkhead and then plug connector back in, then the voltage at the starter relay is 12V. This tells me that the voltage drop is coming from the ballast resistor/coil/alternator/voltage reg circuit and not the starter circuit. Why would there be a voltage drop from the coil start circuit that drops voltage at the starter relay?

 

Weak Contact in the ignition switch maybe?

 

But I get 12 volts coming out of the ignition switch, the drop seems to be coming from something downstream.

 

Sorry... I had to go read this again.... I think the answer to you question is "Load Drop" When you turn the key to the crank position.
I'm assuming you have all good ground connections..