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In for overdrive-out for straight 3 sp. Before pulling the gears from the car, I would drain / change the oil in both.
The electricals which must work are the governor which energizes an internal switch at 28mph (theoretically) and a solenoid which kicks the od gears in place.
The governor energizes the od relay on the firewall which gets voltage from the battery through a fuse sending it to the solenoid. A 'kickdown' switch under the accelerator also disconnects that circuit to drop out the od solenoid for auto-downshift. Some units also have a 'lock out' switch at the od. Hope this helps in your troubleshooting effort. Lastly, the manual warns if you remove the solenoid for some reason, always replace the rubber bushing which keeps od oil out of the inside of the solenoid.
The kickdown switch also shorts the engine ignition for about 4 revolutions to briefly reduce power. That circuit has nothing to do with the overdrive engaging.
GL