Hello,
This unit has no defrost timer as it is electronically controlled. As for the getting stuck in the defrost cycle, this can be one of several things. The main control board has relays on it that can and do stick. There is also a defrost thermostat that should turn off the defrost heater to prevent thawing the units' load during defrost. There are also temperature sensors that tell the control what temperature the different areas are running.
Now to go over these individually, the cheapest are the temperature sensors. There is one behind a louvered cover on one of the fresh food walls. There is one at the very bottom left in the freezer compartment. And there is one at the top of the evaporator coils. While the one at the evaporator coils is considered a defrost part, it still plays an important part of the units' operation. While they are the cheapest parts we have covered to replace, they really don't fail often.
I would start by replacing the defrost thermostat because there isn't a test you can easily perform on it to prove it is opening at a specific temperature. If this doesn't correct it, the board is the most likely cause and would be the next candidate for replacement. If this doesn't take care of it, the sensors are the last. Other technicians might argue with me on the order, but I tend to lean on past experiences.

Jerry,

Thanks for the info. I have noticed that the electronic temperature display for the freezer section does vary 5-10 degrees from the actual temperature as measured by a digital thermometer. But I reasoned that the temp sensor for the display must be the one located at the bottom of the freezer compartment. I think I may replace the temp sensor located next to the thermostat because it probably palys a role in the defrost cycle and also the thermostat itself as long as I'm in there, the parts are reasonably cheap. I hope it is not the main board. We'll see. Thanks again.