holley Holley EFI 558-506 EFI Harness Kit Posted on December 24, 2023 by admin This wiring harness interfaces a Holley EFI ECU to a 4V Ford Modular engine. It is meant to be used in conjunction with an injector harness, a coil harness, and ignition modules. Most stock sensors are used with this harness. 558-506 harness holley