OK, my "final" update on the Flojet pump and Frigoboat E51385 interface. I cannot explain it, but the interface is definitely out-putting 24 volts to the pump at all times.
Maybe my interface is malfunctioning, so I installed a solid-state relay and a DC converter to step the voltage down to 12 volts. I used a 24-volt relay in place of the E51385 pump connection and then feed the converter and pump via the live 24-volt terminal box in the engine room. The relay simply engages the pump when the compressors call for it.
A bit more complicated than I would like but the pump runs perfectly now and obviously I no longer get the pump overload signals at the interface. The higher capacity pump may draw more power, but it really cools down the 3 refrigerator units fast.
My next task is to get the brushless motor working to replace the Flojet motor. I will drive it directly via the 24-volt terminal block and eliminate the DC converter.
Until then I am "over and out" regarding this perplexing issue with the E51385. I really appreciate all the suggestions and assistance, but will report once I have the brushless motor controller operational. That will be the gold standard.
Phantom, A54 #44