The Washing Machine Pressure Switch: How It Works (by Capitan Farloc)

Ever heard of the washing machine pressure switch? It’s a small but essential part that tells your machine how much water is inside the drum. In this guide, we’ll explain how it works, what it does, and what can go wrong if it fails. Perfect if you’re into DIY or just curious about how washing machines think!

What Is a Pressure Switch

A pressure switch is a sensor that detects when a predefined pressure level has been exceeded, triggering the opening or closing of an electrical contact. In simple terms, it functions as a switch that turns on or off as the pressure of air (or more generally, a fluid) inside it increases or decreases.
In a washing machine, this device is typically used to detect when the correct water level in the wash tub has been reached, thereby stopping the water from continuing to fill.

How the Pressure Switch "Senses" the Water Level

But how can a pressure switch, which is triggered by the pressure of a gas, detect the correct water level in the tub? The explanation is fairly simple:
From the bottom of the tub, a fairly wide rubber tube known in technical terms as an "air trap" extends upward. When the tub is empty, this tube is full of air.
As the tub fills, the "air trap" gradually fills from the bottom, and the weight of the water in the tub compresses the air in the upper part of the tube. The pressurised air is then channelled through a smaller tube to the pressure switch, which is calibrated to activate at the pressure corresponding to the optimal water level for washing.

In the next photo, you can see the air trap located in the upper right corner. It starts from the bottom of the tub (on the right in the photo) and is essentially a smooth, wide piece of tubing that ascends vertically. Towards the end, it narrows to connect with the smaller tube, which ultimately reaches the pressure switch.

In this photo, you can see the upper part of the washing machine, where the pressure switch is located. Connected to it is the smaller tube that originates from the air trap.
All these tubes are secured with hose clamps to prevent air leaks. Any loss of air pressure would lower the pressure within the system, interfering with the proper detection of the water level in the tub and ultimately halting the washing machine's operation.

Details of Pressure Switch Operation

When we take a look inside, the pressure switch is a relatively simple electromechanical device. Here's how it works in detail:
Air from the air trap enters the expansion chamber through the small tube (A) and compresses the rubber diaphragm (B). The diaphragm is counteracted by a spring, which is adjusted using the screw (C). This screw is used to set the pressure threshold at which the pressure switch should activate.
Additionally, the diaphragm controls a switch, with its contacts labelled (11), (12), and (13). Another adjustment screw (D) is used to modify the switch's hysteresis, which is the difference in pressure between the activation threshold and the pressure required for the switch to return to its resting position.

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Connections and Adjustments on the Washing Machine Pressure Switch

The pressure switch is factory calibrated, so typically it doesn't need adjustment. However, if it does need adjusting, it’s important to understand how to identify the adjustment screws.
Firstly, it should be noted that a pressure switch may contain multiple switches inside, each with its own contacts and independent adjustments. The one shown in the photo, for instance, has two switches, but there are also models with three.
These pressure switches with multiple contacts are used to manage different water levels in the drum: the first for washing (using less water to save detergent and energy for heating), the second for rinsing (using more water to remove detergent residues more effectively), and the third (if present) for half loads (to wash with even less water when there are fewer clothes).
The two adjustment screws (C) and (D) must be aligned. The screw for adjusting the water level in the tub (C) is always positioned more centrally compared to the other adjustment screw, which is for hysteresis (D). The hysteresis screw is located closer to the edge and generally near connector (12).

It is also worth mentioning that hysteresis or "Pressure Delta" adjustment is relatively less crucial. In fact, many pressure switches have a fixed hysteresis, and in such cases, there is no corresponding adjustment screw.
The electrical contacts marked on the pressure switch are identified as follows:

• (11): The central contact of the switch
• (12): The NC (Normally Closed) contact of the switch, which opens and interrupts the electrical connection when the water level is reached
• (13): The NO (Normally Open) contact of the switch, which closes and allows current to flow when the water level is reached
• (14): (if present) A second NO (Normally Open) contact, identical to the previous one

Connection Diagram

Often, on the back of the pressure switch, you can find the diagram of the electrical connections for the internal switches, which reflects the details explained in the previous section.

It’s important to note that if there are multiple switches, and the identification standard used is the same as the one shown in the photo, the contacts for subsequent switches will be numbered as follows: (21) - (22) - (23) - (24) and, if applicable, (31) - (32) - (33) - (34).

Faults, Symptoms, and Solutions

Faults caused by a malfunctioning pressure switch typically manifest as problems with water loading. An incorrect operation of this component will lead to issues in detecting the correct water level in the drum. This often results in the washing machine halting during the loading phase and displaying an error message.

Obstruction in the Air Path

The first check to perform is ensuring that the entire duct running from the tub to the pressure switch is free of obstructions. This issue often arises, especially when excessive amounts of detergent are used or when washing clothes with greasy dirt. Such conditions can lead to the accumulation of greasy foam in the air trap, blocking it and preventing
the passage of air to the pressure switch.
This can result in either an excessive amount of water being loaded (since air cannot reach the pressure switch to activate it and stop the solenoid valve) or no water being loaded at all (as the air cannot escape the pressure switch, which then remains stuck and incorrectly signals the presence of water in the tub, even when it's empty).
To resolve this, simply loosen the hose clamp securing the air trap, remove the trap, inspect it thoroughly, and clean it carefully if necessary.

Air Loss in the Circuit

Another cause of pressure switch malfunction is air loss in the circuit, which prevents the required pressure for activation from being reached. In such cases, either excess water will be loaded (since the air cannot enter the pressure switch to activate it), or after the water loading process finishes, it will restart again, consistently leading to a water level that is too high.
The air loss can result from a tear in the rubber diaphragm of the pressure switch, a crack in a connecting tube or the air trap, or the loosening of a hose clamp.
To check for leaks in the pressure switch diaphragm, attach a small tube and gently blow into it until the switch is triggered. Then, quickly seal the tube and wait for at least one minute. If the switch deactivates even when the tube is sealed, the diaphragm is damaged, and the pressure switch needs replacement. However, if the switch only deactivates after reopening the tube, the diaphragm is intact, and the problem must be sought elsewhere.
Rubber tubes should be visually inspected for cracks, especially around the areas secured by hose clamps. Additionally, hose clamps should be tightly fastened (if they are screw-type) and correctly positioned.

Lack of Electrical Contact in the Switch

Another possible cause of pressure switch malfunction could stem from one or more of the electrical contacts in the switches no longer functioning properly.
To verify their correct operation, you will need, in addition to a small tube to blow air into the pressure switch, a multimeter (or tester, either digital or analogue) set to measure electrical resistance.
Following the electrical connection diagram, you will need to check whether the contacts disengage and engage by blowing air into the pressure switch and releasing it. For instance:

• The contacts labelled (11) and (12) should show a resistance close to zero OHMs in the resting position, but when blowing air, the resistance should become infinite.
• Meanwhile, between contacts (11) and (13), the resistance will remain infinite until you blow air to trigger the switch, at which point it should become close to zero OHMs.

I Hope This Article Was Useful and Interesting
If you have any questions, suggestions, or just want to let us know whether you enjoyed the article, feel free to write in the dedicated thread that has been opened on the forum:<Come Funziona una Lavatrice - FORUM> (unfortunately the forum is only in Italian, but we try to answer also to messages written in English).
If you're instead interested in the general functioning of a washing machine, you can take a look at this article, which (hopefully) describes in a simple way all the phases of a washing process as seen from the perspective of the components that execute them step by step:
How a washing machine works (by Capitan Farloc)

Thank you
Luciano (Capitan Farloc)