First in order to either lock the OD in direct or allow it to shift into overdrive, you need a cable. Old vehicles had a handle marked overdrive, but anything from a PTO cable to a hood release cable will work. With the cable pushed in, a governor in the very back of the transmission would signal the relay when 28 mph had been reached. At any speed above 28 mph, you could let off the gas and it would shift into od. When you feel the shift, you step on the gas and its in od. The relay is a 2 part relay. A heavy winding causes the solenoid to shift the od unit and the light winding holds it in. If you want to want to shift back into direct, you step down on the gas and a kickdown switch interrupts the ignition momentarily and allows the unit to shift back to direct. On Chev, the kickdown switch was at the carb. On ford it was under the gas pedal. If you are driving around town and don't want to use the od, you pull the cable out, and the transmission will function exactly like what you have now. I usually leave my overdrives in the locked out position at speeds below 40 mph because they are free wheeling at speeds below 28 and at speeds below 40, the engine just doesn't turn fast enough in od. Here is a wiring diagram for an od
http://www.classictruckshop.com/clubs/earlyburbs/projects/over/od-diagrm.jpg
