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.

 

up ^^^^^^^^^^^^ THANKS!

 

Besides speed sensor, OD solenoid could be sticking. It should be giving you a code in the OBDM. Read the code, and go from there.

BTW, it does your engine good to rev it up to 4K rpm or so at least once per month. Just do part of your driving in 2nd gear for a few minutes.

And be sure to engage that 4x4 at least once per month on some loose surface to keep it well oiled and good to go.

 

thanks for info, the obdm code is the dealer test? went to autozone they tested at hookup below steering wheel and it passed . so the obdm requires a different tester and hook up point? is it in engine compartment or at tranny? thanks

also if od solenoid was sticking- why once restarted and od NOT even on (I tried it this way too) -would light start blinking and it force itself INTO od if rpm exceeds 2500+ (passing gear) (doesnt make sense to me ?)- that doesnt impy its sticking? it implies that it reading an incorrect or faulty part and/or making itself go into overdrive? totally confused.

 

Autozone testers will not pick up transmission codes. My suggestion is to go to the dealer and ask them to check it out, and if a problem exists, to give you an estimate for repair. I had a flashing o/d light combined with an occasional RPM spike during the 2-3 upshift. Diagnosis: bad spring in 2-3 shift acculmulator valve. Repair cost at dealer: $250