It has been a very long time since I have used the overdrive so I had to do a little googling. Google says the governor drops the transmission out of OD when the speed drops below 30MPH and does not allow it to engage until it is over 30MPH. I remember a kick-down switch on the accelerator pedal momentarily grounds the coil and drops out the overdrive when you floor it. I'm not sure how that is supposed to work with ECM. The addition of OD predates the ECM by 20 years. If I had to take a wild guess I am going to say the wire from the coil to the kick-down switch got connected to the governor instead.
