Don't tell me you only replaced one little ol' thermistor? You are having issues with prechill and I think the freezer thermistor is involved in the prechill process. I would change all thermistors in your fridge. I have no support for this but I have seen with my own peepers an apparently good thermistor interfere with board communications. Everyone else thinks I must be on crack. Anyhow, there must be something causing your boards to react the way they do. It could be a remaining thermistor, shorted wire or your temp controller/encoder board. Do the easy/cheap thing first.. ie change thermistorS <----meaning all.
You might be right... yes, I only changed the evap thermistor. That was based on an exerpiment I did earlier in the week. During a defrost cycle, I monitored the evap thermistor voltage at the control board. It consistently rose to ~3.4V or so, flattened off for a while (just like the temperature), and then spiked. Once it rose above ~3.47V the system shut off the defrost. Then, during a subsequent defrost cycle, I put a 12k resistor in parallel with the evap thermistor. If 3.47V was the threshold, then the defrost should terminate at about 40F (evap):
70F thermistor = ~6k
40F thermistor = ~12k
40F thermistor || 12k = ~6k
Sure enough, the defrost terminated early once the evap temperature rose above ~40F. This confirmed to me that the freezer thermistor played no role in terminating the defrost cycle - just the temperature at the evap thermistor was sufficient. Plus, I had seen no reference to anything else in any of the GE technical literature.
So, I have simply been trying to address why the defrost was staying on so long. However, you have a good point that there still is no sign of pre-chill. I am going to let the new control board run for the day and see what happens. I may have been premature last night in dismissing it because I did force a defrost soon after installing the new board, not giving it any chance to run a full cycle.
btw you could also replace your defrost limit with one the opens at 55°. Yours open at 140 but there are some GE's that uses your same board that requires the limit to act as a defrost terminator despite the presence of a thermistor. I have not seen this but one of GE's manuals cautioned about one particular model that has this configuration. I don't think it's your model but what the heck, give it a try. At least it will turn off the heater. But don't change too many things at once. Btw pt 2....did you check your board installation instructions carfefully to determine if you need to cut a thermistor wire?
Interesting idea. Might be a fallback plan. Yes, I did read the instructions carefully. My model is not one that needs a thermistor wire cut, at least according to the instruction sheet I received.