2000 Hyundai elantra shifts into overdrive but just slips.

DTC P0732 sets when 2nd clutch still slips after the PCM has commanded high main line pressure
DTC P0734 sets when 4th clutch still slips after the PCM has commanded high main line pressure.
DTC P0733 sets when 3rd clutch still slips after the PCM has commanded high main line pressure.
The powertrain control module (PCM) uses the crankshaft position (CKP) sensor, input speed sensor (ISS) and output speed sensor (OSS) to determine engine speed, input shaft speed and output shaft speed. These sensor readings are used to determine the current gear ratio and RPM slip across the torque converter. The PCM uses this information from the sensors to determine if the commanded gear matches the actual gear. During normal driving conditions, the PCM will adapt main line pressure via the pressure control (PC) solenoid to prevent slipping and harsh shifts. Shift solenoid 2 is used to apply or release second gear clutch. The PCM controls 2nd gear by controlling the ground for shift solenoid 2. The PCM will turn the solenoid Off (open the driver circuit) when second gear is commanded On to apply 2nd clutch. Shift solenoid 2 is supplied ignition voltage from the fuel pump relay whenever the PCM commands the fuel pump relay On.
Do you know how to test main line pressure ? Also , with the valve body off air check clutch packs . Do you know how to do that ? How many miles on the trany ?

Auto Trans Rebuild Chapter 8 Air Check
Making sure clutch packs are good .

to determine the condition of the transmission have a pressure test done first This will determine the internal condition. Find out what the codes refer to as I am thinking torque converter clutch solenoid and TCM . A new transmission will not be cheap so find out exactly what needs fixing and calculate the cost against a replacement unit. I am suspecting that the fix will be relatively simple and cheap like an oil change and a filter clean . band and clutch pack adjustment.

Have the fault codes read. Replace faulty items. Use only
genuine items as replacement as non-genuine are not always compatible with the OEM fitted CPU units. This results in the fault codes not cancelling out.

need to check for transmission codes to make sure you don't have a bad shift solenoid if it an automatic. if it is a manual shift check the clutch disc for wear.

Hello you will need to have a further assessment completed your car will need to be hoisted up preferably at a transmission specialist you will probably need to have your transmission overhaul this is a big job.

This code means theres a problem with the shift soleniod for the 3-4 shift, have this soleniod tested,its locted on the valvebody if the pan was dropped you will see the soleniod.

There should be an electric solenoid on the side of the transmission that controls overdrive, if it is not the cause then the friction plates inside the transmission will need to be replaced.

DTC P0734 is "4th gear incorrect ratio" - that's your torque converter 'LOCK UP Clutch" It's worn out (or you're using the wrong Trans Fluid required by Hyundai - SPIII OR Both). The P1624 is simply the Trans Control Unit (computer) communicating to the Engine Control Module (ECM computer) MIL - on request signal to ECM = asking it to turn on "Check Engine" light. MIL = Malfunction Indicator Lamp. When the torque converter 'lock up" clutch slips or doesn't engage, the trans automatically returns to 3rd gear and stays there (it's called 'fail safe' mode) until you cycle the ignition key, then everything resets until the malfunction occurs again.
You'll need to have an AT (Automatic Transaxel) tester hooked up to verify this. Hyundai has a Service Bulletin specifically for "gear incorrect ratio' codes. If when you changed the trans fluid, was it done at a Hyundai Service Center? If not, someone may have installed the wrong fluid.

well the low cost way to go first would be to try and change the trans fluids. If the problems still remains the it may be an internal problem

Hi Jackiaria

Unfortunately you have a hydraulic clutch, which is self adjusting on your car.

The problem you are experiencing is the excessive wear to your clutch, or oil contamination on the clutch plate.

Both these problems can only be fully diagnosed once the gearbox is removed and the clutch is inspected.
There is also a possibility of excessive wear, or scoring, on the flywheel surface, the longer you drive the vehicle with a slipping clutch, possibly requiring the flywheel to be skimmed.

Shop around for a best price, and a trustworthy garage !

Good Luck !

A FixYa rating would be Nice....Thanks.....John.