Model # 15898790 displays '90' then goes immediately to '98'... no control button work. Can't get it do anything.

It sounds like the drain/pump circuit is clogged. You will need to remove it and clean it out. I don't have that exact service manual, but I have one that is close at: http://dl.dropbox.com/u/8010163/Whirlpool_Dishwasher_Service_manual.pdf
Just copy and paste into browser and download from Dropbox.com.