I have a 1985 Ford E350 motorhome chassis with a carburated 460. The in-tank electric fuel pumps will run only when the start circuit is energized (key in start position with starter engaged). Starts and runs for 10 or 15 seconds untill the float bowls are drained. I have changed the ignition switch and the fuel pump relay with no positive results. Any mechanics advice or links for wireing diagrams will be much appreciated!