[MrE]

Bill, according to the Wilipedia link you supplied;
"Originally, Controller Area Network (CAN) was not mentioned in J1939, which covered cars and tractor-trailer rigs, and with some dual and triple use 8-bit addresses assigned by the SAE J1939 board. CAN was not originally free, but its instruction set did fit in the custom instruction format of J1939. This was true as of 2000. Since then, CAN has been included, the chipset for J1939 has been clocked faster[clarification needed], and 16-bit addresses (PGN) have replaced 8-bit addresses.

J1939, ISO 11783 and NMEA 2000 all share the same high level protocol. SAE J1939 can be considered the replacement for the older SAE J1708 and SAE J1587 specifications."

My 2000 Orion transit donor bus came with J1939.

Simon, I would use a wire tracer to make sure that your CAN high and CAN low wires are actually connected to your replacement ECM.



Check your DDEC connector, verify that J connects to the ECM CAN high and K connects to the ECM CAN low.