TCM error code 13

I have a friend whos trooper just developed a problem, he has the CT light flashing continously, the tach does not work, the vehicle does not drive when u shift the gear to D, but will drive manualy (L-2-3-D). It seems to have gone into backup mode. The TCM error code is 13, this has to do with the engine speed. What could be responsible for this??
where can i find the engine speed sensor?

Thanks

JM1