Classic/Antique Car Repair: 1966 Thunderbird - Primary Ignition Problem, wire terminals, solenoid

Question
I've been working on a 1966 Thunderbird Primary Ignition problem.  I get good voltage when the key is on the ignition position, but when I try to start the engine, the voltage on the + side of the coil going to 0 Volts.   Any ideas?

Answer
There should be two wires on the + side of the coil. One comes from the ignition switch and will have voltage in the run posiition of the ignition switch. The other comes from one of the small wire terminals of the solenoid and provides voltage to the coil when the solenoid is engaged. The terminal connected to the coil should be marked "I".