1993 lumina 3.1 engine. shuts down while

1993 lumina 3.1 engine. shuts down while driving,sometimes bucks at 50 m.p.h. checked plug at fuel pump harness and one pin had about 5 volts other 3 dead.swapped fuel pump relay and same thing.jumped relay and checked plug terminals again,found terminal with approx 5 v and one with 12 volts. reinstalled relay and plugged pump harness together and works fine now,need to know what controls the relay.(fuel pump was replaced about 30k miles ago.I know it's possible it's bad but I'm leaning toward a "control" problem for the relay) Thanx for your help. Bill