I couldn't figure out what this solenoid was for until I changed the filter and got a good look at it. Anyhow, if your Racor fuel-water separator has a solenoid attached, its purpose is to switch the filter's heater on. The un-optioned Racor filter just has a plastic bowl at the bottom which you use to drain out the water. Mine has a heater inside as well as a water...
Topic: Fuel Filter Solenoid