You can turn your cold control on then off 3 times in 6 seconds to activate the defrost control. If you have a clamp on amp meter on the incoming power supply wire at the rear of the fridge you should read about 4 amps. If you do all is OK.
If the fridge shuts off and you read no amps,replace the defrost thermostat for sure,the calrod heater can be checked with your meter for continuity. If open,replace it too.
If it does not turn off,replace the ADC(defrost board).