Based on this theory I just did a couple of experiments to try and narrow down the problem area:
- With the controller board removed from the charrger and a charge cable plugged into the car (but not the mains) I briefly grounded CHGP (CN101 pin 3) via a low value resistor - replicating what would happen if TR701 were turned on. Sure enough, the car sprang to life - the usual charging lights, motors, clunks etc and then after a few seconds it shut down again. This makes me thing that the EV-ECU side of things is fine - it powers the car up when asked and the issue is only that the BMU is not asking when the mains is applied. Without the power cable plugged into the car nothing happens when CHGP is grounded.
- I refitted the charge controller board but connected only CN101 - not the other cable which heads down to the power boards - and repeated the brief CHGP grounding experiment. The car behaved as before, and additionally, the processor board woke up for a few seconds. The decimal point on the 7-seg display started blinking at about 1Hz and the PFCPWM LED came on also. This leads me to believe that the processor on the control card is alive and doing its thing - which is a big relief.
- I disconnected the charrger board again, and tried to keep the car active longer by holding the grounding, and in a separate try by making regular short groundings on CHGP. The car did not stay active any longer in either case, but a diagnostic scan afterwards showed a new U111D code (onboard charrger CAN timeout), so this tells us a little about what is expected at power up.