Pressure System

Pressure systems vary depending on the model you’re working on, but they all serve the same purpose – the ensure the unit is not going to overfill. Depending on the model (generally older ones), some of the fill times are controlled by a pressure switch. Line or neutral routes through the pressure switch and then to the fill valve. When the water level rises to a certain level, contacts in the pressure switch open the path of electricity to the fill valve, stopping the flow of water until the timer advances to the wash cycle. 

On newer models, more often the pressure switch is there to prevent an overflow condition and will initiate an emergency drain if it detects the water level being too high.

Theory of Operations

Again, depending on the model, pressure sensors have a flexible membrane inside the sensor that reacts to changes in water pressure. When the tub fills with water, it builds pressure in the tube. That pressure is pushed up against the diaphragm. Inside, there will be various electrical contacts that will touch based on how far the diaphragm is depressed. The control board looks at those contacts to determine how full the washer is depending on the cycle selected, or the water level selection made (model depending).

On newer units with the pressure sensor built into the control board, there will be a pressure transducer on the control board. This electronically determines the pressure.

Diagnostics

Depending on the model and type of pressure system, often the main testing you will do is by disconnecting the pressure tube from the pressure sensor. Blowing through the pressure hose should indicate whether you’ve got an obstruction or “pressure” that might be telling the control system that there is too much water, and that it needs to be drained out.

If you leave the pressure tube off the control system and you are still experiencing draining, then you’ve likely got a failed pressure sensor.