I’ve just written an article describing exactly how a washing machine controls water levels and this article is a follow up to it describing some of the faults that can occur with the pressure system. 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.
The pressure switch
They are normally very reliable but it’s possible for a switch inside to stick either on or off (usually on). Connection faults on the switch can also prevent messages getting through to the controller resulting in similar faults. A switch sticking on will make the washer think there’s water inside the drum when there isn’t, and sticking off will tell the washer there isn’t any water in when there is.
This would cause the washer to either not do the fast spin, or overfill. However, neither of these symptoms are exclusively related to a stuck pressure switch – there are more likely causes.
If you blow up the pressure switch it should click on, and then off when the air escapes out. If it doesn’t click it’s faulty. However, it may have just 1 switch or more levels (up to 3 switches) so unless you know how many switches are inside you won’t know if it’s got one jammed or not. On average I would expect 2 levels, low level for wash and high level for rinse but it’s possible to have just one, or a third one for overfilling detection. The more wires, the more levels. If you know what you are doing you should be able to work out how many switches are inside with a continuity test meter or by looking at the wires.
(For more pictures and information about pressure switches see my previous article how a washing machine controls water levels)
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).
Finally, if a pneumatic door interlock 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 interlock. The air pressure is therefore also used to operate a lock inside the door as described in this article different types of door interlocks. 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