Frezzer coils ice over causing fridge to get warm

I have already replaced the defrost heater and thermostat but i have the same problem, the only thing is the water line is’nt freezing up anymore. I got diagrams of the mother board but not quit the same as i have, and one more thing how do i test the defrost heater and thermostat. thank you for any help you can provide.

                                             my e-mail is [EMAIL="tm430@optonline.net"]tm430@optonline.net[/EMAIL]