1996 Yukon Fuel System

I have just purchased a 1996 Yukon- 4wd- 5.7L. The guy I bought it from told me he was having trouble with it quitting on him for no reason and put two fuel pumps in it so far. Last week my wife had it and was backing out of the driveway and it quit. Now, there's no "buzz" from the fuel pump. I have now changed the pump again- here's where my questions start. There is still no buzz from the pump. I put the enitre sending unit in- as well as a new connector from the main wiring harness as I know they're prone to corosion on the positive and negative connectors. Still no buzz from the fuel pump. I have power on one wire (for fuel gauge) and power for approx 3-4 seconds (to pressurize the system) on another wire on the connector from the warness. The fuel pump relay is clicking and coming on and off when it should. It won't start or run the pump. Any ideas would help. PS- the wiring diagram shows the oil press/fuel press switch in the same harness- has anyone heard of them stopping the fuel pump?