On at least the 15 through 20 series, KA claimed the filters to be self-cleaning "under normal circumstances". The inlet to the drain impeller on all of them is below the fine screen. On the 15-17 models, the any soil on the screen theoretically gets washed down with the downward movement of the draining water. On my KDS17A, I find the screen needs only occasional cleaning (every 6 mos or so), and at that point the screen is by no means clogged. On the 18-20 series, there was a single small jet on the underside of the lower wash arm that would "hose down" the screen and flush particles down toward the drain inlet. On this pump, the drain impeller was also a soft waste disposer. I've never cleaned the filter on my KDS-18 (though I continue to check it). The pump on the 21-22 series (which actually first appeared on the 20A series) had a reversing motor. While in wash mode, the drain impeller (which is also a hard food disposer) becomes part of a "bypass filter" system--it pumps water up from the drain inlet through a microfine filter at the top of the pump housing--water actually comes OUT of the microfilter in the pump top. The wash arm has a large hub with 12 or so needle jets which continuously backwash that micro filter, while a small jet out on one of the arms "hoses down" the main filter.
The micro filter on the 22 series is much much finer than that on the 20A-21 series, and can be retrofitted (its the only part they sell for all 20A-22 series).
Of course cycle sequences vary depending on the model, but after most wash segments finish draining (all wash segments, in the case of gregm's imperial), the machine fills with a small amount of water and briefly runs in "wash" mode to purge the micro filter's "soil collection chamber", then drains. Sometimes you can hear the drain impeller grinding something during this process.
Because of this micro filter, all cycles have only one rinse after the main wash, and that seems to be fine.
Many diesel engines filter their oil on the same bypass principle--diverting a portion of the oil through a much finer filter and back to the oil pan (to filter out soot) while the oil going to lubrication points goes through a regular oil filter at maximum pressure.
I think the WP/KM/KA dishwashers today (except the BOL WPs with the "sewer grate" filtration and "hit-or-miss" wash action) use a similar type of system, with a microfilter in the top of the pump and a large wash arm hub with backwash jets.
WHew, sorry this was so chatty.
T.
PS: the link is to the microfilter pump top of the 20A-22 machines. Gregm--lift off the bottom wash arm and check yours. I've come across many 21-series models with torn microfilters. It would be a great excuse to upgrade to the finer filter.
http://www.repairclinic.com/0081.asp?RccPartID=2033
