Hyundai Repair: 1995 hyundai accent, transmission control module tcm, transmission control module

Question
we bought this car and hauled it home on a trailer. when i drove it at the lot, it run fine and seemed to shift fine. now the transmission wont shift into overdrive. it probably didnt when i test drove it but i didnt notice it then. took it to the shop and they drove it and put it on the computer and said it might be the o/d switch and want to charge $90.00 to do that but said they didnt know if that was it for sure. they told me that when the o/d button was in it showed it wasnt working, when it was out it showed it was working, but they werent sure. any ideas? is it the o/d switch or something else. they said the computer didnt show a code when they had it on there.

Answer
The overdrive switch is a high likelihood.  Everything you mention is consistent with a faulty overdrive switch if gears 1, 2, and 3 work properly.  The switch wires sometimes break in the shifter.  There are three wires to the switch.  One wire supplies power to the switch.  The switch then determines which of the other two wires will have power applied.  One goes to the O/D OFF lamp and the other goes to the transmission control module TCM).  It sounds like it's already been verified that the TCM isn't receiving the signal from the switch to turn on the overdrive.  To check and be certain, you'll need to disassemble the shifter and check the output from the switch.  If you're getting proper output from the switch but it isn't reaching the TCM, then you'll need to check the wiring from the switch to the TCM.