[HO455]

Here is a quick description of the current path to energize the solenoid and a basic diagram to help illustrate the number of connections involved.

Starts at the positive battery post connector, passes through the fusible link. (an undersized wire to protect the harness from overcurrent situations.) to the insulated post on the fender.
From there to the factory crimped splice inside of the engine harness near the horn.
From there through the firewall connectors to the factory crimped splice inside of the under dash harness.
From that splice to the ignition switch connector and the switch contacts.
Out of the ignition switch to the neutral safety switch connectors and the switch contacts inside of the switch.
Then back through the firewall connectors and down to the solenoid.

As you can see there are a lot of connections that have to be perfect. Everyone of these connections can add a small voltage drop to the circuit. Add in corrosion and heat and that voltage drop increases. The end result is that when you turn the switch if the voltage at the solenoid drops to 8 or 9 volts the solenoid is unlikely to operate.

To check for this you can put the positive lead of a voltmeter on the solenoid coil post and the black lead to the negative post on the battery. Then have someone start the engine. If the voltage is below 10 volts you have a wiring issue to locate.

Heat causes resistance to increase in the wiring. Less wiring less resistance.

The Ford solenoid conversion is one of the ways to bypass all the connections and get full voltage to the solenoid coil.