How a washing machine door lock or interlock works
This video will explain how a washing machine lock or interlock works and how to test them.
How can you tell if a door interlock is faulty?
Washing machine door locks really only get 3 major faults
- The points between live and common fail in some way.
- The NTC heater or bi-metal strip fail.
- The plastic gets to hot and distorts inside the lock causing it to jam.
Understanding how a door interlock works helps you decide if it is faulty or not.
Washing machines must have some way in which you can lock the door closing mechanism when the machine is started up and then unlock them with a certain delay (normally two minutes) after the current has switched off via the program or on/off switch, in order to ensure that the door cannot be opened while some of the components are still rotating initially (in particular the motor and the drum of the spin-dryer).
When you shut the door, the latch on the door pushes inside the door lock onto a sliding bar which moves over and activates an electrical part inside which locks the door shut. At the same time, power is passed through the interlock on to the rest of the machine. The actual latching shut of the door is a simple spring-loaded mechanism where the latch just slips into the door catch and the door stays closed. Locking the door is a separate process.
The locking of the door.
Washing machines have a bi-metal strip inside the door lock which is heated by PTC Heater (resister) when live and neutral are activated on to the pcts it heats up and bends the bi-metal strip which then moves the arm to activate the common terminal and push a pin into the closed door to lock it in place. Once this has happened (usually a second or so see video) the power then can flow through to the common wire, and therefore on to the rest of the machine allowing it to start.
The only test you can do without a test rig is to check the PTC Heater on the live and neutral you might expect to get a resistance reading of something like 1000 Ohm but this can vary depending on manufacture of the interlock and the condition of the PTC,
Faults to look out for
- It is common for the wires to sometimes be burnt on the terminals due to lose connections and overheat and burn the wiring block. The points that activate the live to common can carbonise and burn. If wires are burned and have gone discoloured or hard they may need stripping back and new connectors will need to be fitted.
- PTC Heater can get bad connections and become open circuit.
- The door latch may not properly activate the electrical part of the interlock.
- The plastic parts inside may have over heated and distorted, stopping the lock from working correctly.
- If a metal slide bar is fitted these can corrode and become rusty.
Please note not all interlocks are the same and wiring will differ from manufacture to manufacture.
Do not assume anything about the wire connections on your machine unless you have the wiring diagram. 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 and possibly blow the PCB or programmer. It may be possible for a fourth wire on some lock to carry power to an LED which may indicate that the door is closed and the interlock activated.