thank you for the question hope this helps
the over drive light is blinking telling you there is a problem What I would suggest is that you have your vehicle check over by a Ford dealership or your local garage for fault codes in the computer. if you have a auto zone around they can do this for free. if you do this and post back with the code i can help you with the problem further. this could be something as little as a solenoid to the wiring to the transmission. but with out the code this is only a guess.
When the OD light flashes your transmission goes in to "limp" mode and will shift hard and will not shift past 3rd gear. This is to save the transmission so you can get it to a shop for repair.
Hope this helps,
good luck
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Thanks
David