Defrost Timer in my fridge?

I seem to have a defrost problem. There is frost on the back panel of the freezer compartment and the upper section stays warm. Earlier in the week the entire freezer compartment was frosted (I thought it had been left open), and almost all of that frost went away. But a couple of days after that the upper section was warm and will not warm up. I know the first step in checking this out is to turn the timer, but is there a defrost timer in this model? How else can I check the defrost system?

Go to web site %url% and do the manual defrost test for a Whirlpool fridg.You have a adaptive defrost control that initiates defrost.
If no defrost, you might have a bad defrost tirminator clipped to the top of the evaporator coils. Short the wires to it, if you get defrost, replace the defrost terminator clipped to the top of the evaporator coils. If no defrost, replace the control board.