Hi
Please check this link and the related article to have an idea to resolve your problem…
http://forum.appliancepartspros.com/refrigerator-freezer-repair/12638-whirlpool-freezer-ice-build-up-bottom.html
Unblock the drain line behind the freezer panel underneath the silver evaporator coils. To remove the ice, use a hair dryer. It works well and prevents piercing a hole in the evaporator. The cooling element (evaporator) is located behind the rear panel of the freezer. Although frost-free, frost will develop and accumulate on the evaporator. To remove this, the evaporator has a heating element attached which is automatically switched on using set parameters within the control board. Once the ice has melted, the water produced should run down a drain tube (again, behind panel) into a tray located above the compressor where it is evaporated away by the heat from the compressor. The defrost circuit and heater appear to be working, but the drain tube is likely blocked, hence water in bottom of freezer (which will freeze and stick the lower basket to it!). The drain could be: A) Blocked with crud (dust/mould, grease etc.)
B) blocked with ice due to either just bad luck or failure of the drain heater located where you can't see it inside the freezer liner. An after-market drain heater and modified evaporator heater are available along with an additional wiring loom to allow fitting the self-adhesive drain heater.
Also check this link bellow if it helps…
http://www.repairclinic.com/Refrigerator-Troubleshooting#Thefreezercompartmentisicingup
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