1995 GMC Sierra 5.7L won't shift into 3rd

[LuLuMay]

I have a 1995 GMC Sierra 5.7L  the VIN indicates U.S. made but seems more Canadian the more I look at it. My major question is What is the required voltage coming in and out of the vehicles speed sensor buffer? I have this truck and it runs just fine until shifting into third. It won't shift at all into third, it just keeps revving. But I still have reverse and First and second shift fine. I've narrowed it down to a few things but have been having a hard time finding out where all of the grounds are and the voltage requirements for alot of the sensors.. does anyone know specifics? Or have any ideas on why this is happening and what I need to do to fix it?