My Grandfather has a 1997 F-150 4x4 with the triton 4.6 and an automatic overdrive. The pickup has 205000 miles and ran fine until about a month ago. The O/D light on the shift lever flashes all the time while driving at highway speeds and only is off when the vehicle is first started and up to about 40mph. It is shifting into overdrive from watching the tach and drives out fine will this continuing to flash cause any damage

 

The flashing OD light is an indicator or a fault code in the tranny, similar to the check-engine light.

 

Many parts stores will now pull & clear codes free.