The full factory wiring diagram is 20+ pages. If you drop your and ECU part number (starts with 8-971xxxxx-0) in the comments, I may have the exact pin assignment for your chassis.
If your 4HL1-1 has a Suction Control Valve (SCV) issue (common rail pressure code P0087 or P0093), back-probe the FRP signal (B-7). It should read roughly 1.2V at idle. isuzu 4hl1-1 ecu pinout
Cracking the Code: The Isuzu 4HL1-1 ECU Guide Ever stared at a 4.8L Isuzu 4HL1-1 engine and wondered what’s actually happening behind the scenes? This powerhouse is the backbone of many Isuzu Elf and NPR trucks. While the iron block is built like a tank, its brain—the Electronic Control Unit (ECU)—is where the real magic (and sometimes the real headache) happens. The full factory wiring diagram is 20+ pages