1997 Hyundai Accent Transmission Issues.

Ok, so this has been an ongoing issue for awhile. At least 6 months, not too bad for myself cause its still driveable but intown, highway driving it Rev's way to high.

I drive a 97 Hyundai Accent 1.5L SOHC - 3 speed automatic transmission with Overdrive. I blew the transmission up long ago. Had it replaced with a used one. Mileage at the time of replacement was 110,000KM and transmission going in had about 240 to 260 if memory serves me right. When replaced I used ATF3 with the friction modifier to make it the right stuff for my transmission (Being as I didn't have a Hyundai Dealer closer. 800km away) - all has been going well. Transmission has driven me all over. But the last 6 months. This happened.

My O/D has stoped working, The shifter button when pressed in or out, would be blank on my dash or say "O/D OFF" - Now, the button does absolutly nothing, I replaced the switch, nothing. I have replaced the pulse generators on the transmission, basically the Input and Output speed sensors, while back, did not fix it.

So for intown driving the transmission holds up great, runs smoothly into 1st, 2nd and 3rd. But will not go into Overdrive. Reverse works also.

Another question, with a old transmission with no records of services, should I do a Transmission Flush, or drain and fill. Reason I ask is possibly when the filter is replaced underneath, could of got stuff into the Valve body. I've worked in the automotive Industry before, (Parts and Service) but I really know NOTHING about transmissions, some professional help could assist me in correcting this issue which would help me out big time.

If anyone knows anything please help me out!
if you need questions or more diag - feel free to message me.

Thanks - Tim