How Does A Washing Machine Door Interlock Work?

Understanding how the door lock, or interlock as it is known in the trade, works on a washing machine will help you in the event of it failing to operate correctly. You can’t tell if the door interlock is performing correctly if you don’t understand how it works.

This has often led to people fitting new interlocks on their appliance for no reason. The fault that was present before changing the lock, is still there after the lock has been replaced.

How Does A Washing Machine Door Lock Work?

When the washer door is closed, the door latch pushes inside the door lock and activates an electrical component that locks the door. This allows power to flow through the door lock and on to the rest of the machine.

This is a safety feature designed to prevent the washer from starting until the door is shut securely to stop flooding. The latching process is a simple spring-loaded mechanism and works by the latch simply sliding into the door catch which shuts the door. The actual locking process is a separate part of the door locking procedure.

How Does The Locking Procedure Work?

person looking at washing machine

The door needs to be locked securely to prevent it from opening accidentally during a wash cycle and flooding your home. To facilitate this, the door lock consists of a bi-metallic strip with an electrical coil wrapped around it.

Once the door is closed and a cycle selected, power runs through the electrical coil which heats up the bi-metallic strip causing it to bend. As the bi-metallic strip bends it operates the lock and makes another electrical connection which allows power to the rest of the appliance.

Once the cycle finishes and the power is turned off, the bi-metallic strip cools down after around 1 to 2 minutes. As it cools, it returns to its original straight shape which releases the door.

There are typically 3 connectors in the interlock which are Live, Neutral and Commons. Now the position of these 3 terminals can vary depending on the type currently used by various washing machine manufacturers.

You will need to test each terminal using a multimeter to work out which is which unless they are clearly marked. Make sure you’re confident of which terminal is which because connecting the wires incorrectly will damage the appliance and in some cases you could even blow the PCB (Printed Circuit Board).

Just to add more confusion, some interlocks have a 4th terminal which powers up an LED display notifying that the door is locked.

How Can You Detect A Faulty Interlock?

person in front of a stacked washing machine and dryer

In all honesty, I can’t say that there’s a simple way to tell if the interlock is faulty or not. With that said, you might be able to check the resistance reading between the live and neutral terminal which should read around 1,000 Ohms.

Other than that you’ll need to use your observational skills and check for any loose wires or signs of burning. You might also detect a lack of any power, an open circuit coil, a neutral return or of course, the obvious door lock not working properly.

If there are any burnt wires they need to be stripped back to find a good wire plus you might need to fit new connectors. If there are any signs of burnt wires etc, it’s best to replace the lock because the connections inside will most likely have overheated as well.

In some cases the only reason the door isn’t locking correctly is because the catch or locking mechanism is stuck. You can sometimes solve this by spraying it with WD40. If this doesn’t do the trick, you should think about replacing the lock.

Other Possible Faults To Be Aware Of

Pretty much every type of washing machine sold in the UK has always used this interlock system. But, recently, some manufacturers have changed the design and added a relay connected to the PCB that can cause the door lock to click but not lock.

Then there’s the fact that sometimes the door shuts and locks but another fault prevents the washer from starting. In some cases the common wire has a break, faulty connection or short which prevents power from reaching the rest of the appliance.

Just because the door locks, it doesn’t mean the internal connection allowing power to pass through is working.

Then there’s the problems that can be caused by a defective PCB, defective motor or faulty heating element. Any one of these problems could cause the door to lock and all the LED displays to work as normal but the machine won’t work at all.

Different Types Of Washing Machine Door Locks

Integrated Washing Machine

I’ve explained how the most common type of washing machine door lock works so far. Just to summerise, they work using a bi-metallic strip with a coil of electrical wire wrapped around it.

As power runs through the electrical wire, it heats up the bi-metallic strip which bends and activates the door lock. It consists of 3 wires: a Live, Neutral and Common, the live and neutral form the circuit that activates the bi-metallic strip and the common passes power to the rest of the appliance.

The common has no power at all until the bi-metallic strip locks the door. This system typically takes a couple of minutes to cool down after the cycle has completed which is why the washing machine door won’t open for around 2 minutes.

However, there are some interlocks that use a relay instead which will allow you to open the door as soon as the cycle finishes (this is the system used by Miele).

Pneumatic Door Locks

Others use a bi-metallic strip combined with a pneumatic (air operated) lock. This works by keeping the door locked whilst there is water detected inside the drum.

You can tell if your washer door has an air operated lock because there will be a thin tube attached to the door lock. This tube will connect to a chamber on the tub and as water enters the tub it fills the chamber.

This creates a change in the air pressure in the tube which is how the locking device is operated. After the cycle has completed, and the water is drained from the tub, the chamber empties, releasing the air pressure and the door unlocks.

This type of door lock can be problematic because the chamber can get blocked with detergent or other debris which can cause water to become trapped in the chamber. This means the door lock will not deactivate even once the actual tub has been emptied.

This system is still to be found on some washers but has largely been discontinued due to washers using less water which wouldn’t be enough to trigger the door lock.

Pneumatic Door Locks With A Cable

Back in the 1990s Hotpoint introduced a new system which had a pneumatic door lock like those above and a cable which connected to the motor when the door catch was activated.

If the drum was moving, the end of the cable would slip on the drive belt and wouldn’t allow the door to open. Once the drum stopped revolving, the end of the cable created resistance and allowed the door to open.

The problem with this system was the wires overheated regularly and burnt out. Plus the end of the cable was made from plastic which was fixed onto the motor and could also become damaged or dislodged which meant the door failed to open.

SEE ALSO: Washing Machine Door Won’t Open? (here’s how to unlock it)

Frequently Asked Questions

How does a washing machine interlock work?

A washing machine interlock consists of a bi-metallic strip which heats up as the door closes. As the bi-metallic strip heats up it bends and locks the door. It also allows power to pass through it to power the rest of the appliance.

Why is my washing machine interlock not working?

If your washing machine interlock isn’t working it could be because the appliance has overheated and burnt or short circuited the interlock mechanism. If you then try to force the handle, you could damage it and prevent it from opening.

Will the washer work if the door lock is broken?

In most cases, the washer will not work if the door lock is broken. This is because the control board will not allow the washer to start unless it detects that the door lock has been activated.

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