Ford: O/D light, d light, scan tool

Question
What could be wrong with the O/D light that is on at all time in my 1999Ford Mustang. As expected, at higher speed 50MPH, rev is very high, much higher than usual.
Thanks for your time.  

Answer
MONETTE

the switch on the o/d goes bad quite often. before i had the switch replaced i would go to advance auto or autozone and have the car put on a scan tool for free. a garage will charge 50-100 dollars for this service.the reason for the scan is that it could be a computor problem also,and you dont want to spend any more money than you have to.hope this helps.

ken