I have a previous article describing exactly how a washing machine controls water levels. This article is a follow up to it describing some of the faults that can occur with the pressure system on a washing machine or Washer dryer.
What is the pressure system?
The pressure system is made up of several parts. It has a simple purpose and that is to tell the washing machine when water inside the drum has reached a specific level, and when all the water has been successfully pumped away. If something goes wrong and the washing machine does not receive these signals then the washing machine can either overfill, or refuse to spin.
Various things can go wrong causing symptoms described below – but all these symptoms have other possible causes too. So it’s important not to make assumptions. In order to try and understand pressure system faults you should read the first article to understand how the pressure system works.
They are normally very reliable, but it’s possible for a pressure switch to be faulty.
The pressure switch
However, in my experience it is the last thing to suspect. Never fit a new one if you haven’t ruled out all other faults – especially a blockage in the pressure chamber bottle at the base of the outer drum (details later). A switch sticking on will make the washer think there’s water inside the drum when there isn’t. Conversely, sticking in the off position will tell the washer there isn’t any water in when there is.
The former would cause the washing machine to not do the fast spin. The latter would cause it to overfill. However, neither of these symptoms are exclusively related to a stuck pressure switch – there are more likely causes. Remember, a pressure switch is far more likely to be stuck because of a blockage.
Forbes rent appliances and specialise in renting Bosch appliances so they know them inside out. They also rent other brands and many other products – more details at Forbes
The modern pressure switch
The pressure switches shown and described above have now been replaced with a different type. Instead of having mechanical switches inside that are operated by air pressure they have small coils with magnets inside that give a different resistance reading depending on how much air pressure is applied.
The old style pressure switch has remained virtually unchanged for 40 years or more and has usually been one of the more reliable parts on a washing machine. It remains to be seen whether these (obviously cheaper) designs are reliable or not but in theory they ought to be very reliable still (if not more so) because of the reduced number of parts. Troubleshooting these modern pressure switches may be a bit more difficult without knowing the expected resistance readings.
The small pressure switch tubing between the pressure switch and pressure chamber bottle
Damage to the pipe, or air leaks at the connection points on the pressure switch or plastic pressure chamber bottle can cause overfilling because air pressure is lost. Without an airtight connection it’s impossible to create enough pressure to activate the pressure switch or at last to keep it switched on..
Pressure chamber bottle
If the pressure chamber at the bottom of the tube gets blocked with gunge inside, then when the washing machine empties the water, the water inside the pressure chamber (which is only really a small plastic bottle) gets trapped because it can’t get past the gunge. This means that the air pressure is held inside the plastic tube so the pressure switch is jammed ON.
A blocked pressure chamber bottle can also restrict or prevent water getting into the bottle in the first place causing the washer to take in too much water or to overfill.
This would need a fairly solid blockage though and is much more rare than trapping the air pressure as described in the last paragraph. This is because the sheer volume of water inside the tub can sometimes force its way past gunge into the bottle, but because hardly any water actually goes into the bottle this very small amount cannot force its way back out when it drains (if there is a blockage).
If a pneumatic door lock is fitted that prevents the door opening when full of water, a part of the small pressure tubing (at the top of the pressure chamber bottle) branches off and fits onto the door lock. The air pressure is therefore also used to operate a lock inside the door as described in this article different types of door locks.
This type of interlock is rarely used these days because of cost cutting, but it used to be common on washing machines from the 90s. The relevance here is that gunge inside the pressure chamber bottle can also trap air in the tube that leads to the door lock preventing it from opening even when water has been pumped away.
Fixing a blocked pressure system
This article looks specifically at blocked pressure systems and how to fix or diagnose one how to clear a blocked pressure system
Not taking any water in or refusing to do a fast spin are other possible symptoms of faults on the pressure system related to the machine thinking there is water inside when there isn’t – but as with most of the other symptoms, other unrelated faults can cause the exact same symptoms. General help for these two faults can be found here – not taking in water and not spinning.
Finally, a fault in the pressure system can cause the washer to abort the programme and give an error code – appliance Error codes – What You Need to Know
Get an engineer
Faults on the pressure system can be difficult to diagnose. They can also be very easily misdiagnosed when a fault is caused by something totally different. Don’t just start messing with or replacing parts in the pressure system because your washing machine exhibits symptoms that can be caused by the pressure system. Unless you fully understand how it all works you will be better advised to call an engineer.
Fixed-price repairs, Pay monthly, Repair & protect your whole appliance..
50% off annual membership
Subscribe to Which? today and save 30% on Digital Annual subscription for your first year. Offer ends 18.03.24. This offer is only available for new Which? annual subscriptions (excluding Gardening, Travel, Computing or Money packages).