Filling: Valves and Level Sensing

A washing machine has no pump for filling. The water main does the pushing: your cold supply is under pressure all the time, and the machine only decides when to let it in. The gatekeeper is the inlet valve, at the back where the hose connects. Inside it is a coil of wire around a plunger, a solenoid. When the control board sends current through the coil, it becomes an electromagnet, lifts the plunger and lets water through. Cut the current and a spring pushes it shut again. Dishwashers fill the same way.

So a fill depends on two things: enough pressure arriving at the valve, and a valve that opens when it's told to. That's why the first question about a slow fill is which side of the valve the problem is on.

One trap: a hum from the back at the start of a fill tells you the coil got power. It doesn't tell you water flowed. A valve can hum while its plunger is stuck, or while something upstream holds the water back.