Hyundai Repair: Hyundai Excel having trouble shifting gear, throttle position sensor, hyundai excel

Question
Hi,

I have an Australian bought Hyundai Excel from -96 with an automatic transmission. In November I put in a brand new gearbox since the one I had burned out. Now, I've been having heaps of problems with it lately, and one week ago it stopped going in reverse. It would act as if it was in neutral. Driving didn't work too goo either, the car would have problems going into second year and then jump to third, and then back to first again, racing terribly. I called my auto mechanic whom replaced the gearbox since I had warranty on it. However, that didn't seem to have solved the issue. It got better, but it still races sometimes when shifting gear. Reverse works fine so far, and it doesn't have as much problems as before.
In addition, it has been leaking transmission fluid, but I checked the level and it looked fine to me. But then again, I know nothing about cars...
My mechanic claims it could be an electric fault, but I don't know... I don't want to put a lot of money into it, since it's a shit car and I just need it to work so I can get to work. Please, if you have any suggestions that I can take to my mechanic, I would greatly appreciate it!

With thanks,
Sofia

Answer
Hi, Sofia.  There's no single problem that will explain all your symptoms except an internal problem in the transmission requiring overhaul or replacement.  

The not driving in reverse can be caused by a misadjusted or defective range switch, but more frequently the problem is in the transmission itself.

The erratic shifting can be caused by a faulty throttle position sensor, but that, too, can be caused by an internal problem in the transmission.

As for the leak, that's a simple matter of finding the leak and repairing it, but if there's an internal transmission problem, there's no point in fixing a leak in a transmission that needs to be replaced.

Has anyone checked the engine and transmission control modules for diagnostic trouble codes?  This should be part of the diagnostic process anytime a transmission behaves erratically.

As for suggestions for your mechanic, I don't really have any.  A mechanic presented with a car with a problem present at the time he's inspecting it should be able to follow normal diagnostic problems to find the cause (or at least develop a strategy to find that cause).  If your mechanic cannot do this in this instance, he's not qualified to be doing this type of diagnosis.  A qualified mechanic physically checking the vehicle should always be able to do a better job than me, since I'm reduced almost entirely to analyzing the symptoms presented and listing potential problems.  The mechanic in person can actually run tests and receive meaningful results.