Your original flood light is designed for 400 watt high pressure sodium lamps. The floodlight you replaced it with is for 400 watt metal halide lamps. You can not interchange the two lamps. You will need to replace the ballast or order a new floodlight. The “S” after the wattage in the mhs-0400s-268 catalog number stands for high pressure sodium. “H” is for metal halide and “P” is for pulse start metal halide.
Curt
