audi a6 c5 2.8 1998 tranny problems

ok i just change tranny on my audi a6,and a try to drive car.. but is going into a limp mode, car stuct i 3 gear..but when a stop car and turn off engine a start again it going till 3 gear and stuct again..check engine light is on a give me code p0717 (input/turbine speed sensor circuit no signal) where can i find thet sesor,and what can be wrong??why car stuc in 3 gear? PLEASE HELPPPP anyone i dont know what to do anymore// :mad:/ Thanks