'97 Elantra won't go into OD
 

Hey, I bought a Elantra wagon with an AT over the weekend. At the time I bought it, I hadn't noticed the O/D button on the shifter, and on the test drive just thought it was a 3-speed.

I pushed the button while I was driving the other day and it won't go into overdrive. When it gets to where it should shift the transmission will disengage or shift into neutral, I'm not sure which. End result is in "overdrive" the engine will just rev with no reaction on the other side of the transmission. First three gears shift fine, no problem, and the trans fluid is full, looks clean on the dipstick, and doesn't smell burnt. I was thinking it could be something electronic but don't know where to start. Any ideas? It's only got 75k miles, and I don't want to spend $80 on new transmission fluid, pan gasket and filter if that's not a likely solution.

Also, it doesn't come out of park without some finesse, although I'm thinking that's just a issue with the whatever safety lockout that won't let you shift without the brake applied.

Any advice on either issue would be greatly appreciated.

Update, today was the first cold (45*f) day I've driven it, and until the tranny warmed up it was also slipping in third gear. The CEL isn't on, how do I check to see if the tranny is throwing any codes?

EDIT: I've never rebuilt a transmission before, but I do have my truck I can drive while figure it out, do you think this would fix it? According to the website, it works for my vehicle?
http://www.autopartswarehouse.com/sk...%26+Drivetrain