Where is the sensor that makes my car go into overdrive?

I have a 94 chevy cavalier that keeps dying at stop lights. It will run great in park and neutral but as soon as it gets in gear it dies. I think its just not downshifting from overdrive because it can put aroundtown forever but after i ge itup to like 45-50 and then stop it dies. I was told to pull the plug on the transmission to byp ovrdrive but where is that? is that the same as the vss?