If you rely upon a single solenoid on the output of a pump for "shutting it off"....you'll eventually be sorry. My original wash had that setup. I arrived one morning to find that a solenoid had failed to close. Without a deadhead on the output....the pump ran wide open. A full barrel of
soap pumped down the drain.
Every pump now has a solenoid on the air input.
If multiple bays use the same pump (Foaming Brush, Tire Cleaner, PreSoak), the air input is controlled by a PLC. It wasn't hard to program.