Anyone fimilar with 9400i 870cm

[Chamberpains]

Waterloo, bodybuilder doesn't get in to pre Prostar year models. The 9000 model trucks schematics are are bit hard to find. But they are out there.

I didn't read much of the other guys responses but your alternator voltage is fine.

You more than likely have a wire broken or starting to corrode going to the ecm. More than likely from the battery box and around it. Check the fuses and wires in the battery box. Also check around the ECM harness's especially the 4 wire plug and wires going in to the ECM. That is the main power/ground plug.

Quickserve will have detailed pinout diagrams for you engine and ECM. You can always skip the dirty work of pulling all the wiring out of the frame rail by just running all new properly protected wires up to the ECM. just leave the old crap in place.