The fridge and freezer work just fine for a couple of weeks, then the vent at the back towards the bottom of the freezes gets blocked by frost.
The evaporator will be completely covered in frost, turn off the unit for a couple of days to defrost and then restart and it works for another couple of weeks.
I am thinking i have a bad defrost heater or defrost timer?
Can i test the heater for continuity, is a certain resistance required? Is there a manual overide on the timer to test that way?
See page 30 in the manual for the defrost timer.
Mark the timer’s cam and a spot on the cover/frame.
Leave it for an hour or so if the cam does not move then odds are high that the timer is shot.
Rotate the timer cam till the unit’s fans go off. You are now in defrost mode, if the unit starts to defrost the timer is shot.
If it does not defrost check the following two items.
Note: not defrosting does not totally rule out the timer as it’s contacts may not be closing.
Other two possible causes are the:
Defrost heater should measure 30 ohms approximately.
Defrost thermostat should measure 0 ohms when frozen. Note it should be infinite ohms (open) when it is above freezing.