Sharp Carousel II MW, 16 years old, keeps on heating unless door is open. When opening the door it shuts off like it should, light comes on, turntable stops- upon closing door I get a buzzing sound (magnetron?) but no light or turntable rotation, and it boils water as if it was on highest setting. Put in time and a reduced power setting, close door and it seems to operate normally, but it ignores the reduced power setting and heats at full power.
Short version- the magnetron is on full blast any time the door is closed.
Sounds like it needs a new electronic board or PCB. Although I would consider a new microwave as your microwave is getting on in age and can be a danger to health or dangerous if something blows up.