Last updated on October 11th, 2017
If you think you have a faulty door interlock they are usually pretty easy to replace, and not usually too expensive (buy door interlocks from 4Washerhelp). However, I see a lot of cases where people have fitted a new one – and it didn’t make any difference because they’d misdiagnosed the fault. In order to diagnose if something’s working properly or not you have to understand how they work. This article is part 2 to my main article explaining How does a washing machine door interlock work? so check out the first article – then return here for supplementary advice about variations of interlocks that may be very different from the most common type.
How can you tell if your door lock has a bi-metal device inside?
Most washers have a bi-metal door interlock – but not all. A simple bi-metal door interlock should have a live feed which activates the coil inside when the door is closed, a neutral return which allows the coil to work, and a common wire which has no power running through it until the interlock is activated. This is described in detail (with diagram) in my first article.
Normally once the door lock has been activated you have to wait a few minutes before the door will open. However, some machines can hide this process by not signalling the cycle has finished until the bi-metal has cooled down and therefore allowing “instant” door release. If it has a simple plastic device with 3 wires to it then it’s very likely to be a bi-metal interlock.
Some may use a relay instead
Some washing machines may use a different system. Miele for example often use a relay device, which lets you open the door immediately it’s finished. On mine there is a loud click when I press the door open button which is the relay operating. If you have to wait a minute or two before the door will open it’s probably a bi-metal device. Other brands might have a relay built into the PCB (see – Bosch WVG30461GB/01 won’t lock and start)
Bi-metal lock in conjunction with an air operated (pneumatic) lock
Some of these devices may have a pneumatic element to them which is an air operated device keeping the door locked whilst ever there is water inside the machine. If used there will be a thin plastic or rubber tube attached to the door interlock that runs to a small plastic pressure chamber bottle on the tub. When water enters the machine it also enters this bottle. Then as water levels rise, it creates air pressure in the tubing which eventually physically operates a locking device. When the water is pumped away, water should also drain from this bottle and release the air pressure.
Pneumatic door interlocks were very common but have largely fallen out of use due to a combination of cost-cutting and much lower water levels meaning doors can be opened without water pouring out.
If fitted, these devices can prevent the door from opening if there is still water inside the machine, or can malfunction if gunge in the pressure chamber bottle at the other end of the tube blocks the opening up and traps water in the bottle. This results in trapped air inside the tube keeping the door lock activated even though all the water has pumped away from the drum. (more details here – Faults on pressure system)
More complex door lock devices
Some Hotpoint door interlocks in the 90s had a relatively complex interlock which had a pneumatic device (discussed in last section) and cable with a “pecker” at the end that pecked down on the motor when the door catch was operated. If the drum was still, the pecker created resistance and allowed the door to open but if the drum was moving the pecker would slip on the revolving drive belt and disengage preventing the door from opening. The power was passed through when the door was closed using a a simple microswitch in the door interlock (as did some old Hoover machines).
The main thing that went wrong with the old Hotpoint door locks was that the wires overheated due to poor design and commonly burned out leaving an inch or more of rock hard baked wire. I must have repaired several hundred which needed the wires cutting back and re-tagging. The other common fault was the plastic pecker at the end of the cable fixed onto the motor above the belt used to get bent or knocked off completely so the door wouldn’t open any more.
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4Washerhelp spare parts Door interlocks section
How does a door interlock work?
Related: See Part one of this article – How does a washing machine door interlock work?
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