How a washing machine controls water levels

Full of water All washing machines control water levels using the exact same system, which works on air pressure. It’s normally reliable, but when it goes wrong it can cause overfilling or flooding. Or problems related to filling and spinning. These days though it’s most likely to cause the wash cycle to abort with an error code.

Here’s how a washing machine controls the water levels.

Pressure-switch The washing machine needs to know when water is in the drum, what level it is at, and when it has drained away. To troubleshoot filling faults you need to understand exactly how the pressure system works. There is a small device with several wires on it called a pressure switch. This part should be at the top of the machine under the lid. The pressure switch featured here is an old type. Modern ones are much smaller.


However, any pressure switch will always have a thin rubber or plastic tube connected to it that runs down to a plastic bottle at the base of the outer drum so that air pressure can switch it on and off. The word pressure chamber sounds grand, but it’s simply a plastic bottle.

This is what a more modern pressure switch looks like modern pressure switch

How the pressure system works

Inside-pressure-switch As water enters the main drum, some of it also enters the pressure chamber bottle attached to the outer drum. As the level of water increases inside the bottle it forces air up the small tube.


Conversely, when the water gets pumped away, water also drains from the bottle. This releases the air pressure in the small tubing. Once this happens the spring-loaded switch inside the pressure switch releases and turns it off. This tells the washer that the water has gone, and it’s OK to go into fast spin.

The photo above shows inside an old pressure switch. It’s an old triple level one but the principles are the same. A rubber diaphragm lays behind spring loaded switches, each switch needing stronger pressure to operate than the previous one.

Air is forced up the small tube and pushes against the rubber and activates the switches. Some pressure switches only have one level, others have two (wash and rinse) and others may have three (wash, rinse and overfill). If you know how many levels it should have you can gently blow up the switch and check the number of clicks. The more wires connected to it the more levels it should have.


New style pressure switches

Modern washing machines now have a different pressure switch fitted that works with a coil and magnet but the principles are exactly the same in that they are operated by air pressure created by the rising water levels.

More help on pressure system faults

These articles look at several of the things that can go wrong with this system, Faults on pressure system | Overfilling washing machine

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