Great news, it seems to be operating as designed.
The 24V indicator led is lit up by the 24V power supply from the compressors. This shows that the system is operating from a 24V supply. It can also operate from 12V, in which case the 12V LED will be illuminated.
The drop from 24V to 12V of the pump supply under load is the expected operation. There are various circuit incarnations which could be inside the E51385, but it is not unusual to see this behaviour in a 24-12V conversion circuit.
I'm planning to write up a detailed description of the intended operation of the system as originally delivered and post it in the files.
Good luck with the brushless motor experiment.
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