3406b peec

i am having problems. the engine when started reves up by itself to 1000rpm and stays there. i step on pedal and only goes up to 1500 rpm and when i let off it dies. i notice that if i let it idle30-45 minutes and warms up. the problem goes away. Some people and even cat say its the speed sensor on back of the injection pump. does anyone know how it looks like? some say that on some vehicles, it was relocated to the flywheel housing. is it the long threaded sensor that also runs the tach? if so thats easy to replace. please help out