Where is the defrost timer located on hss251fmb ww

I am pretty sure the model number you are referring to are HSS25IFMCC / HSS25IFMWW This model has no defrost timer like the typical defrost timer. The defrost time is determined by the Motherboard. If you are having defrost problems with your model it is most likely the defrost heater causing the problems. The defrost heater is part=> WR51X10055 Defrost Heater

Most times after manually defrosting the defrost heater on your model can be seen burned out
by just looking at the outer glass of the heater itself. This takes unplugging the refrigerator and removal of the rear panel inside and back wall of the freezer

Most times after manually defrosting the defrost heater on your model can be seen burned out Testing the defrost heater can be found here=> WR51X10055 Defrost Heater Assembly Once inside the freezer it is always a good idea to change the defrost thermostat, Click Here=> WR50X10054 Defrost Heater
I will be here should you have questions, Thanks Sea Breeze
Click Here=> HSS25IFMAWW Freezer Components parts 230 is the Defrost Heater and part 230 is the Defrost Thermostat

There is no defrost timer on this model. Defrost is controled by the main board. There are four things that will cause a defrost problem with this refrigerator and they are the main board, the defrost thermostat, the defrost heater and the evaporator thermistor. This page will help http://appliance-repair-it.com/GE-refrigerator-repair.html