NOTE : Do not access any of the internal electrical mechanisms of a dishwasher unless you are qualified . The purpose of this article is not to explain how to repair the internal electrical systems of a dishwasher , this is for the service engineer.
However if your dishwasher is not performing well there may be a number of things you can safely check to return it to form.
Since the late 80's we have owned two dishwashers. The first was a Bauknecht and lasted around 8 years. Our current model is a Bosch.
Dishwashers are complex machines, taking cold water from the main, heating it internally and pumping the hot water through various jets at the plates, pans and cutlery etc. stacked in racks.
They normally have a number of cycles which make up a program. For washing, detergent from tablets, liquid or powder, is mixed with the water. For the rinse cycle , hot water is mixed with a rinse aid liquid. This not only rinses the plates but increases their temperature and hence latent heat. When the cycle finishes the plates dry utilising this heat to drive off any remaining water. The rinse aid assists the process of preventing streaking from water running off. Salt is normally added to an internal reservoir , which adds it to the water to reduce the effects of calcium deposits in hard water areas.
Because cold water is heated to high temperature (normally between 55-75 degrees) dishwashers draw a lot of current from the mains electricity. That combined with the presence of water means they are potentially dangerous if they fail. I have a colleague who had a dishwasher catch fire, hence my warnings in the opening paragraph.
Most dishwashers have two baskets, upper and lower. Water is sprayed at the crockery from usually two rotors. These have water blasted through them at high pressure. Nozzles along the length of the spinner provide them with rotational energy, while other upward facing nozzles deal with the business of hosing down all your plates, cups etc.
The water is circulated around within each distinct cycle, and typically passes through a trap at the bottom. The trap attempts to filter out larger pieces of food, and to trap grease.
Ironically the best way to keep a dish washer working well is to keep it clean internally .
If plates are coming out with traces of food or detritus still on them then it's normally either the trap or the spinners that have a problem.
The trap can be removed, though how varies from model to model. This sits in the bottom of the washing area of the dishwasher, beneath the lower basket. You can normally remove this by either unscrewing it or some kind of unlocking action . These are designed to be customer removable. Once out, I wash the whole thing in hot soapy water to clear any grease and food items. It's worth doing this at least weekly .
The rotors are the part that actually do all the work. They can have 2 problems :
1) If items block their path and they are not able to rotate , which in turn means some plates will not get sprayed. Always check that no items block their rotational path . A fork hanging down or perhaps a large item on the lower rack blocks the spinner on the upper rack. When the washer is full, it's sometimes worth checking that they can rotate by giving them a push, they should turn easily.
2) Small items of food, especially hard things like small bones or pieces of shell etc. will pass through the filter and enter the recycling water. These will probably end up being pumped into the spinners and then clog the jet holes. You can remove the rotors, normally the top one is suspended beneath the top basket and can be unscrewed. On my washer the lower rotor just pulls out, but be careful. These components should be user serviceable , but you need to be gentle when removing, especially if you have never done this before. If in doubt, consult the instruction manual.
On inspection you may see small items poking out of the nozzles. You might be able to remove these with tweezers. However if not you basically have to shake the rotor and encourage the particles to leave it via the main inlet hole. Usually the centre of the rotor has a tube where it fixes to the water inlet. This is the only other way out for the detritus to exit the spinner. Shake vigorously up and down until the small pieces fly out. This is easier to do when the rotor is dry.
I have also known scale to accumulate inside the rotors if the salt has been allowed to run out for an extended period. Soaking in a hot water and distilled vinegar is good for removing scale or using a commercial dishwasher cleaner which you place inside and run the washer on the hottest wash.
Having removed the blockages from the rotors, it's worth washing them in soapy water , checking the part where water enters and the whole device pivots around. various detritus can accumulate here and impede the smooth running of this bearing. Coffee grounds seem to accumulate in this area on mine.
Lastly, run water from the tap through the spinner to see if any remaining fragments inside get pushed out and block the nozzles. Once all nozzles run clear , install back in the dishwasher.
A final area worth checking is along the rubber seal that runs the width of the machine at the base of the door. More "gunk" can accumulate here, and is worth wiping with a cloth. Mind your fingers as you may get folded steel joints here, which can be sharp. You can also check to see if the seal is starting to perish, which may happen over time. It's possible to get replacement seals.
Update December 2007
If technology is going to fail it seems more likely it will happen at the worst time i.e. during the busy Christmas season. The dishwasher stopped pumping out water. I could hear it trying but no water was pumped out through the waste hose.
I removed the filter and felt around in the dirty water for any obstructions. I then used a wet & dry vacuum cleaner to suck out the water so I could inspect the chamber that the filter sits in. I noticed a couple of hex bolts which held in place a small plastic cover. Removing this exposed the pumps impeller, which I felt around to check for obstructions. This was starting to look like a failing motor, but I decided that maybe if I left the impeller exposed and soaked the whole assembly in warm soapy water for an hour. I then put the cover back in pace, tightened the hex bolts , though not too tight. Hurray - it now pumps out and appears back to normal. I think maybe some debris got into the motor impeller chamber which was cleared when I removed the cover.
Update February 2008
Well the dishwasher limped through Christmas and new year and finally the pump gave out in early Feb and wouldn't respond to the previous treatment. I decided that the dishwasher had exceeded it's useful life of 8+ years, as replacing the pump was likely to be an expensive and difficult part to fit.
I purchased another BOSCH Exxcel model, which is basically the same as the one it replaced, though it has a lower electricity consumption on the economy wash, with lower water useage. What is gratifying to see is that Bosch have improved on some of the old design in a number of places, for example there is no longer a need to press the select button after closing the door each time to continue with the program. the baskets are better thought out, and the top rotor too has improvements where detritus used to collect.
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