I finally got some free time to work on the Mendel yesterday. I removed the extruder and replaced the thermistor with a spare one that I had laying around. It involved carefully cutting away the Kapton tape and removing the thermistor leads from the crimped fitting that Botmill uses to connect it. Not an extremely complicated procedure and it only took me about 20 minutes to replace it and re-crimp the connectors and tape the new thermistor in place.
I thought that I would do some testing prior to bolting the x-carriage back on the Mendel and it's a good thing that I did. The temp showed as 875C so the heater never cut in. When I checked the resistance of the thermistor I found it to be about 115K Ohms rather than the 80K that it should be. Should have checked it before I installed it. Either it is a bad thermistor or I picked up one with the wrong values at some point. Luckily I have another one and it tests at the correct 80K so I will switch it out again.
Since I was dismantling things anyway I decided to switch out the build surface for the aluminum one that I bought to prepare for adding heating to it. I removed the old one and then found that the aluminum plate blocked two of the holes in the plywood frame piece. I will drill them out tonight.
I also made a cable with two female connectors so I could add a ground cable between the main and extruder boards. I don't know if my problems with the extruder motor are due to grounding issues but this certainly can't hurt.