I'm having problems with it kicking into overdrive and I talked to a local toyota mechanic and he said, the thermo is stuck open so the engine is abnormally cold not allowing the temperature sensor to register right, which has to be a certain temperature to allow the overdrive switch engage.

So I blocked off almost all of the radiator from receiving any air flow and drove pretty far on a decent day (mid 70's), and no change. What does this indicate?