I have an old tq18 with the same problem. I have had many so called experts try to solve the problem over the years, with no success. I finally half solved it by doing the following.
I you hear the defrost timer snap into defrost mode ,the
compressor shuts down. I then take a hair dryer and direct the heat on the drain pipe that passes through the fridge compartment in upper right corner. This unplugs drain from icing , and defrost water flows to pan.
Crude but effective. If you don't catch it before it starts spilling into fridge, turn off freezer control, then use hair
dryer.It has been a good fridge otherwise.
Sid Parker
December 2012