How does a washing machine door interlock work?
How do they lock the door?
Locking the door so it can’t be opened whilst the washer is running is usually achieved using a bi-metal part inside the interlock. I’ve been repairing washing machines for over 30 years and seen many different types of interlock some of which do not use a bi-metal device at all. Some (e.g. some old Hotpoint washing machines) had a more sophisticated system that only locked the door when the motor was running and/or there was water inside. It achieved this using a pneumatic device (discussed in part 2). However, the majority of washing machines just use a bi-metal strip inside the door lock, which is cheap and effective.
How does a bi-metal device work?
Two different types of metal with different properties are combined on top of each other. When the door is closed, a connection inside the lock powers an electrical coil wrapped round the bi metal part. This heats up the metal, which then bends. This bending operates the lock and then makes another electrical connection passing power to the rest of the machine. Once power is turned off, the metal strip cools down after a minute or two and straightens back up again releasing the door.
How the interlock passes power on
Note: Do not assume anything about the wire connections. They may be in totally different positions on different machines. Unless the connections are marked Live Neutral and Common you would need to work it all out with a continuity test meter. If you get the wires the wrong way round you can blow the device. It may be possible for a fourth wire on some variants to carry power to an LED which may indicate that the door is closed and the interlock activated.
How can you tell if a door interlock is faulty?
There isn’t a simple test, although you may be able to get a resistance reading between the live and neutral which should measure the small coil. You might expect to get a resistance reading of something like 1000 Ohm. As with most fault diagnosis it’s mostly a matter of the process of elimination, observation and previous experience. But as my opening
paragraph states, if you understand exactly how something works you are much more likely to be able to diagnose why it’s not working.
Faults to look out for
- First, the obvious. Are there any connection faults on it? Wires can come loose, or overheat and burn. A lack of power, neutral return, an open circuit coil, or even the door latch not properly activating an otherwise perfectly functioning interlock can all cause faults.
- The coil can go open circuit or it can develop mechanical faults preventing its proper operation.
- Poor connections or burned wires are common. If wires are burned and have gone discoloured or hard they need stripping back to good wire and new connectors fitting. Also replace the interlock because it will have overheated connections inside too.
- In order to operate, the door interlock needs the door catch to click firmly in place. If the door catch has broken, come loose, or the door isn’t closing properly it won’t operate the interlock. It’s always worth spraying the inside of the interlock and door catch with WD40, it costs nothing and you never know.
Anomalies to be aware of
After the door interlock has been activated and the door locks shut, other faults could stop the washing machine from doing anything. For example, (This is a possibility but much more rare) if the common wire at the back of the door interlock has a break in it somewhere or faulty connection where it connects to other parts of the machine so the power passing through the interlock cannot reach its intended destination. The door may well lock, but this only shows that the lock is working. It doesn’t prove that the connection inside the lock is passing power on, nor does it prove that the wire used to pass power on is connected properly.
A fault inside the main PCB, or even an open circuit motor or heating element on some washing machines can result (bizarrely) in the door lock activating and locking with displays and lights coming on but washer will not do anything at all. Some Hotpoint and Indesit machines have this anomaly.
Different types of door interlock