help with overdrive 94 elantra

Can anyone help me diagnose my 94 elantra 1.8? If the overdrive is on, the car will not shift to its final gear. It tries, but it's like the car goes into neutral and the engine just races. If I hit the OD-off button, the car works fine in its first 3 gears. Is there a fuse or relay that I can check?
J

I have the same problem and have replaced a few things and havent fixed it yet - i have been told its something to do with the speed sensor, so that is my next step in the investigation - I will keep you posted

I own a '99 Hyundai Elantra wagon (auto) and I've been having the same problem for the last 3 months. First I thought the tranny was dying a slow death but I realized that the only way to make the 4 gear (overdrive) work without doing anything to the tranny is by letting off the gas and pressing the overdrive button. Once the 4th gear is in just hit the gas pedal and go. In the city I drive with the overdrive off anyways. Several days ago I took my vehicle to a mechanic who took the 4th gear clutch and plates out and noticed that they were burned out. He put in a new clutch and plates in and took the car for a drive. It turns out that the problem was still there. Tomorrow I have to take the car back and see where the problem is happening. The guy told me that after the clutch and plates were changed to new ones and the problem still exists he will have to check the hydraulic pressure in the 4th gear. This might be causing the main problem. That's why you're tranny goes into neutral and the RPM's of the engine go up. I'll keep you posted on what happens asap.

Thanks all,
I appreciate your feedback. Please keep me posted as to what happens.
Thanks again,
J