I might have this wrong a bit but here goes, near as I can remember description, during start PCM energizes fuel pump relay on OBDI truck but all power is still fed through OPS at any time. OPS contacts are not really strong enough for sustained 5amp ??? draw of pump and go bad even though pressure is indicated through a separate path on dash giving false impression switch is good.
A mod used by many had the OPS contacts energize a relay with only <500ma??? draw and power pump from the relay on dedicated circuit.
OBDII truck PCMs control this during start and thru OPS once running under separate circuit feed controlled by either once GM figured out what was happening
The lift pump relay should be in the under hood fuse box. On my truck after she starts the power switches through the oil pressure switch. While cranking the power comes from a 10amp fuse. My ECM does come into play through the ECM B fuse under the hood.
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