HATZIE, you sh ok uld put all of this information into a post in the tech section. I will be looking for this info when I put my TBI back to working dual tanks.
TBI fuel pumps run off the same Green wires that run the valve. It's actually pretty clever that they didn't have to switch the fuel pumps any further. The fuel sender and fuel pump share a ground that runs through the liquid tight grommet in the sender to the frame along with the fuel pump power wire and sender wire that run to the valve.
They shut off the not selected fuel pump as follows... When the LH tank is selected the RH fuel pump has two grounds. When the RH tank is selected the LH fuel pump has two grounds. You need power and ground to run the fuel pump two grounds will just shut it off.
GM did a poor job of making the 1987 RV series wiring diagrams readable. They're very difficult to follow and not miss things... like the Hot-Start fuel pump relay system controlled by what passed for an ECM in 1987.
Look over the 1988 -1991 RV wiring diagrams but keep in mind they also seem to be missing the fuel pump relays... Maybe that's a 1987 only thing but I doubt it.
1987 TBI is where the MOTOR year-specific wiring diagrams shine. Pages from that manual, that include the hot start relay etc, are posted here...
http://forum.73-87chevytrucks.com/smforum/index.php?topic=9072.msg198713#msg198713