Water valve

The water valve, or water inlet valve is what lets water in the dishwasher. If it is filling and emptying at the same time, that is a different situation, and is most likely the drain solenoid. Take the bottom panel off the dishwasher and take a peek underneath. The solenoid may be simply stuck, but if it's an old dishwasher, it's most likely corroded or burned up and is stuck in the drain position. If this is the case, it should be replaced.