How do I wire the ECM-side power for an LS swap correctly so the fuel pump and relays work?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

Other wiring

• Holley Terminator X; GM LS: Holley Terminator X provides a fuel pump relay output on J1A connector pin A2 that supplies +12 volts at up to 10 amps maximum. (Holley (via CARiD mirror))
• Sources differ: This +12V high-side output is Holley-specific; GM P01/0411 factory pinouts list the fuel pump relay control as a low-side (ground-switched) driver, so polarity must not be assumed across controllers.
• P01/P59; Gen III LS; 1999-2007: PSI standalone harnesses include a fuel pump relay whose signal (coil) side is prewired; the installer supplies battery 12V feed and the output wire to the pump, and must size the wire and fuse to the pump's capacity with a correct ground and fuse on the circuit. Blade-style terminals for the relay socket are included. (PSI Conversion / Performance Systems Integration (via manuals.plus))
• P01/P59; Gen III LS; 1999-2007: PSI standalone harnesses require an ignition feed that has 12 volts both with the key on and while cranking; a feed that drops during cranking is a documented no-start/stall cause. (PSI Conversion / Performance Systems Integration (via manuals.plus))

Harness and power

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• E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+: Pink ignition-switch feed powers the E38 ECM ignition input. (Chevrolet Performance)
• E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+: Minimum 8-gauge wire for the fuse-block battery stud. (Chevrolet Performance)
• E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+: Three harness ground eyelets on the block plus a 3/4-inch braided chassis strap. (Chevrolet Performance)
• E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+: Bulkhead fused-power and ground pins K, L, M. (Chevrolet Performance)
• E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+: ECM-controlled fuel-pump output is fused and relayed. (Chevrolet Performance)
• E67 ECM; Chevrolet Performance LSA E-Rod control system 19299067; LSA: LSA fuse/relay center has 50-amp fused accessory studs. (Chevrolet Performance)

Confirm against the instructions for your exact parts before wiring. More: Other wiring in our tuning guide.