1994 GMC Sierra Pickup, Fuel Pump Inop

I have 1994 GMC Sierra, the fuel pump will not come on when key is turned on but will work when directly connected to battery. We have checked all fuses that we know of and they are good. If you pour gas into throttle body the engine will fire and run for a few seconds. We have been trying for a week now to figure out what may be wrong even replaced the computer and fuel pump relay can you help?