overdrive flashes when passing gear kicks in -around 2500 rpm. when vehicle ignition turned off blinking light quits and will stay off unless rpm exceeds 2500rpm in drive. drive it 30 miles a day- has absolutely no driving effects (same shift points and always has overdrive gear- does not slip or shift slow or anything. just cant disengage 4th (overdrive gear). tried driving with overdrive off does same thing and forces overdrive to be added (when blinking) . Does this sound like its getting incorrect signals from speed sensor and safetly defaulting into overdrive to protect itself? and if so which sensor do I replace (wheres location)- heard of one on rear diff. and one on /in tranny ? Thanks for any help! also heard many incorrect factory diagnosis of problem and 100's dollars later problem not fixed- scared to take there since theres no driving problem whatsoever. this is a 1999 expedition xlt- 4.6L- 4X4. also does this at any engine temperature, not from overheating.. car can sit overnight at 20 degrees -pull out of drive (dead cold) put gas pedal down halfway (2500 rpm) and light will start up. checked fluid level. has only 65,600 easy driven miles- i've only floored it once when I first bought it back at 43000. tach rarely ever goes to 3000- 3500. this will be first repair to it needed since Ive owned it 21 months.