Input/Output sensors for automatic transmission

How do input/output sensors for an automatic transmission work and effect the drivability of the vehicle?

I drive a 2001 Hyundai Santa Fe with 150,000 (that might be the beginning of my problem). I bought it at 90,000, the day after purchase it lurched on the hwy and shifted down into 4th gear. I called the original owner and he paid for services which turned out to be a replacement of the input/output sensors. Last week (2.5 years later) it happened again, and again on the hwy, the engine light went on and later a littler lurch when the car had to shift from 1st to 2nd gear. Three days later the engine light went off. I took it back to the dealer, he tested the sensors, saw no problem and charged me for diagnostic (not before recommending a fuel injection service - which ticked me off). The day after picking it up from the dealer it lurched again on the hwy, engine light went on and it remained in 4th gear until I came to a stop off of the hwy.

Is replacing the sensors the answer?

Is this a larger problem associated with this make and model?

Additional material to consider with this problem:
I found a 2008 entry on a “lurching” 2001 Santa Fe in the CarTalk discussions -

I’ll look into this, but the replacement of sensor seemed to work 2.5 years ago and just wondering if it is the original problem.

Just an idea. I had an issue with a mitsubishi engine. I had stumble shift issues, took it to the dealer and told them just what i wanted them to look for. They did so and did not find a problem. Then I tore down the ignition and when checking the spark plugs found that the valve cover had a gasket at the top of each plug tube and they all had leaked oil so the plugs were swiming in oil. The stumble shift was due to bad engine vac/power. The electronics detected a bad torque/ power issue but threw no code. Run through the spark, vac and fuel system with a careful eye and you might get lucky.

You really need to get the computer scanned while the engine light is on. Dont wait 3 days for it to go out and then take it in. This is an OBD II vehicle so most local auto parts stores will scan the computer for free. Drive it, when the engine light comes on and the transmission sticks in gear (Probably limp mode) take it to the closest major auto parts store, LEAVE THE ENGINE RUNNING, and have them hook up a code reader. Write down ALL the codes they tell you and post back with the code/s numbers. We’ll go from there. Now, as far as your question on the input/output speed sensors go: The input or “Turbine” speed sensor reads the engine speed through the turbine shaft inside the transmission, the output speed sensor reads the output or “Wheel” speed. The computer reads both the input and output speeds, it then calculates both readings into a gear ratio which tells the computer when to shift the transmission. If either one of these sensors are faulty, the computer will be receiving wrong information which will confuse the computer, the computer then puts the transmission in limp mode which locks the transmission in 3rd gear (2nd gear in some transmissions) A scanner is needed to check the operation of the speed sensors. Below are pictures of both of the sensors on your transmission.