Hyundai Repair: cruise control problem / speedometer flux, vehicle speed sensor, electrical circuits

Question
Is this a problem with input speed sensor?   I changed the output speed sensor, problem remains.  The 3 problems are: Cruise will drop out / speedometer will stop registering /  On level ground transmission will shift to passing mode but speed remains the same.

Answer
The first two issues are related to the vehicle speed sensor (located on top of the differential portion of the transmission) or one of the electrical circuits (power, ground, signal) necessary for the sensor's operation.  In some cases, the sensor itself has failed, while in others the plastic gear attached to the sensor is worn.  I typically replace both together simply because diagnostic costs are comparable to replacing both.  I have seen one car where one of the wires to the sensor was broken inside the insulation.

The last issue could be related or separate.  I cannot tell from your description whether the transmission is in failsafe-- this would set a check engine lamp and would stay in 3rd gear until the ignition is turned off-- or has simply downshifted abnormally and will later upshift to the presumably proper gear.  Either way, the issue could be related to the vehicle speed sensor, so unless you have a trouble code indicating otherwise, I'd recommend repairing the speed sensor and gear first and then rechecking whether the third condition remains.