I agree Roger. Thanks to the wealth of knowledge and experience of COPAns such as yourself, along with Cirrus Tech Pubs and Tech Support, I troubleshot and repaired my MCU120 that was having the same issue under the supervision and signoff of my A&P/IA.
My experience with my MCU issue tells me that even though the troubleshooting guide is good, the culprit (if not the alternator itself or the field control module power connect to the alternator) is difficult to isolate. There is little to no way to determine if the culprit is the Field Control Module, the Logic Unit, or the current sensor. One suggestion is to swap FCMs and swap current sensors to troubleshoot, then order the appropriate failed part.
If the J111 power pin connector to the PCB is fried, obviously that is a clue to high current draw in the field current circuit due to something (corrosion, bad MCU component, or ALT2 switch on at low RPM on the ground), but even after changing the J111 harness and cleaning up the pin on the PCB, replacing the current sensor and logic unit as a matched set (since both parts have rolled their dash numbers a few times in recent years telling me their design has been problematic) is what finally solved the problem. I had replaced the Field Control Module first to no avail.
I know various members have different levels of interest or free time to undergo such a tedious trial and error process because it requires potentially a few iterations of disassembly, component replacement, reassembly, and functional checkout, but MCU repair is very doable by the average mechanically inclined owner/pilot. I do it because it keeps ownership costs lower, and I enjoy it, but the downside is the airplane was down for almost two months while I troubleshot, waited for parts, did the repairs, and tested the fixes.