The light blue wire—the crank signal wire—had corroded right at the splice. The ECU was blind. It didn't know the engine was turning over, so it didn't fire the injectors.
In the typical pinout link or diagram, you will view the connectors from the wire harness side (back-probing), not the ECU side. isuzu 4hl1 ecu pinout link
Related search suggestions (These are suggested search terms to continue research.) The light blue wire—the crank signal wire—had corroded
The Isuzu 4HL1 ECU pinout diagram is shown below: through the firewall
Following the pinout diagram, he traced the wire from the ECU connector, through the firewall, and found a corroded splice in the main harness behind the glove box. He cleaned the connection, wrapped it in new tape, and reconnected the battery.