I think the OD Solenoid is electrically charged at all times pulling the rod into itself until needed. When you push in the cable shift this allows the rod to go down engaging the the overdrive as long as you have past the 26mph for the governor.   
I have installed a switch to disengage the circuit thinking this would release the system until I need it.   My system works perfectly as is
Is my thinking correct?
Thanks Richard

http://www.fordification.com/tech/overdrive.htm
This is the best explanation of how the system works that I've read to date.

Durham Missouri

On a factory overdrive, the solenoid is electrically charged only after the governor has engaged.

Lorena, Texas (South of Waco)

The governor grounds the relay which energizes the solenoid pull-in and hold-in coils at the same time. The pull-in coil is de-energized as soon as as the armature extends, the hold-in coil maintains the armature position until driving torque is released by the driver lifting his foot and the pawl engages the OD drive plate, i.e., the hold-in coil is hot all the time. You can install a switch to turn off the circuit when OD is not needed/wanted. If the cable is pushed in and the circuit is turned off the car will freewheel at any speed when you lift off the gas (no engine braking).

I did that for awhile, but went back to the stock setup. Freewheeling all the time is great if most of your driving is city/residential, but having to manually switch when you need OD soon gets old. While switching out the OD may well extend the life of it's components, you will just wear out out the syncros faster, since you can no longer shift by kick down only. The OEM system was well designed and works great, but like any complicated mechanical/electrical system, it requires more service attention. I had my 6V relay converted to solid state, I make sure the solenoid seal is not leaking since it will fill the solenoid up with lube and cause it to fail if the drain gets plugged (or if it is installed backwards), I also built a test jig to facilitate solenoid rebuilding/evaluation, etc., etc. I DO plan to splice a hidden switch into the kick down circuit soon as a theft deterrent though, so the car cannot be hotwired.

6 VOLTS/POS. GRD. NW INDIANA

Thanks everyone.  Mine does not freewheel unless I push in the lever with my safety switch off. Most of my driving is around town so the system sits idle until I hit the highway. I have a light that goes on when I toggle the system so I know its happening. The overdrive electrical components get expensive so for my tastes I'll keep my switch.  In addition I will add a switch to prevent theft. Good idea
Off topic I also use switch for the electrical clock. When the car is not in use off goes the clock. Yes I have to set the time, but its something I enjoy. My clock rebuilder said that the clocks do wear if constantly in use. 

Solid state relay sounds good, who does this.  

Richard

John Sailors 503 648 7250.

6 VOLTS/POS. GRD. NW INDIANA