That all makes sense and since I've fully charged the start battery, the ONAN cranks every time.
I had been through ALL the connections including the hidden from view ones (you don't really want to get in there !) disassembled, cleaned and treated with CorrosionX.
I even put the negative lead directly to the crankcase, still no good.
But it was Mark's thread about starting the main engine that led me to it.
My little maintenance charger had died and the start battery was down a bit - I should have checked that first, so I'm not that competent !
I agree with you, there must be quite a volt drop along those long cables that feed the ONAN starting system, as you say, undersized.
BUT with a fully charged start battery, it's not an issue.
We learn things every day.
However this is not the end of the story. While the ONAN cranks it only runs for 2 seconds and shuts down with error code 12, overvoltage.
I had the Cummins man here today and he's visually checked the diodes and condition of the windings - all looks OK, and he will come back with the tester to test the rotor and stator windings to be sure they are within spec, and if they are, he says its a fault on the control board...which means replacing the board...hmm