I have a 99'automatic Tibby in which has been acting up lately. This is what the car is doing. I could be driving down the hwy @70mph & all of a sudden the whole car jerks, the rpm shoot up & it sounds like the car just went into a lower gear. I went to the Hyundai dealership here in Arkansas. They told me they thought it was the speed sensor. After doing some researching -it could be a number of things(eg. transmission,speed sensor, vaccum line, throttle position sensor). What do you think is the problem.


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Transmission control module, or the funny little vacuum solenoid that initiates the gear kickdown. These have both been reported to pull a similar stunt to what you're describing.

-Red-

 

Ditto to what RED said. If it was the speed Sensor, the ECU should read the RPM jump and shift again...so it would be jumping in and out of gears....

If it just drops down one gear and stays there till you lift off the throttle...probably the Vacuum solenoid. If it says in the lower gear even after you lift off the throtttle...then the TCM would be the best guess.

Either way the ECU should be recording this and the dealership should be able to diagnose it.