S 10 truck starts but shuts off

Putting a 1998 Chevy S10 truck engine
in a home made portable saw mill.
Saved all wiring harnesses under the
hood . I have everything hooked up
with the original parts, fuse-relay box,
ECM-PCM, cut all of the wires from the
fire wall to everything. Took a big red
wire from the fuse box to my key
switch as power , the big pink wire as
the ignition it gave power to fuses and
relays. Found the wire that made the
starter relay click from looking at the
wiring diagrams in a repair Manuel.
The motor will start but shuts down
immediately . Not loosing spark, and
has good fuel pressure , replaced the
oil pressure sensor and the MAP
sensor. Motor ran before I put it in.
Something must not be hooked up and
it probably is something that came
from inside the truck to the fuse box .
Need input to figure it out .
James Gooding