With 5 seconds left, Bell took the direct snap, ran left and was buried in traffic before diving for the end zone and getting the ball across the line as Donald Butler dragged him down.
Thousands of Pittsburgh fans waving Terrible Towels went nuts as the Steelers improved to 3-2. San Diego dropped to 2-3.
Vick, having an awful game until the fourth quarter, kept the drive alive with a 24-yard scramble up the middle and then a 16-yard pass to Heath Miller one play before Bell's big run.
Copyright 2015 by The Associated Press