Do you know if the shifter knob has ever been removed. If the answer is yes, the small wires may have been easily damaged on replacement, now causing a intermittant short circuit that operates the overdrive button circuit. I have priced a shifter assembly at Ford and the price is $495.00 for the parts only. This service manager says the shifter and or shifter knob is not available separately from Ford. I intend to check another dealer.
