Joe, not sure what that can be if you don't have a computer... Sounds like it might be a thermal vacuum switch, but not sure... Where do the vacuum lines go? Also, where on the transmission does the one wire go? Should be 4 terminals on the transmission plug, are there any other wires?

That switch may be some kind of coolant temperature switch to prevent the torque converter from locking up until the engine gets hot enough... Hard to tell though. Is it installed into one of the water passages in the intake?

Is it the original engine in your truck? If so, it might be worthwhile to go to the library and see if they have the GM service manual for your truck. Might take some time to find out what that switch really does, but it might be better than guessing...

Sorry I'm not more helpful...

John