Hyundai Repair: 1996 Accent Gear/transmission, gear transmission, auto transmission

Question
I have this 1996 Accent Auto transmission with 123.7K miles on it. In/near the gear shift there are 2 push buttons 1 of O/D on/off and other says Norm/Economy. When I drive it with OD Off it runs on upto 60/70 (did not push it further) but when the OD is on (No O/D Off lightened on instrument panel) it doesn't go beyond 30 mph. if I'm driving at 50 mph with OD Off and turn on OD, the car slows down to 30, it drives but if I press the gas, the engine just revvs up but won't go up. I'm not sure if the norm/economy button plays any role either as I'm only driving on highway (my work is right off a highway and I live less than a mile from highway as well) I'm trying not to stay between 50-60mph as its in OD Off mode. The milage I'm getting is 26-27.7 mpg. There are no codes when I got it checked 3 days back. Is it possible that the Transmission is bad and can die anytime? please keep in mind that in OD position the car does pick up and drive but doesn't go beyond 30mph. I purchased it last week and unfortunately do not have any history between 99K miles-123K miles. Before that there are no reported problems with transmission. Could you tell by the description, if this could be repaired or I've to go for a new trans?
Thanks

Answer
It sounds like fourth gear doesn't engage, making your car act like it's in neutral.  In a few instances, this can be cured by replacing the end clutch of the transmission, which is a relatively minor repair when compared to replacing the transmission.  But it's more likely that the issue will require overhaul or replacement of the transmission.  The reason you don't have difficulty when the overdrive is turned off is that the transmission computer won't allow the transmission to shift into fourth gear.

When everything is working properly, you'll find that the Normal/Econ button has minimal effect on tranmission performance.  All it really does is change the shift points a little.  "Normal" will get you better acceleration through tending to keep the transmission in a lower gear and doing more shifting, while "Economy" will get you better economy by tending not to allow downshifts as easily and shifting to the next higher gear sooner.