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The main harness carries all sensor inputs to and control outputs from the Holley Engine Control Unit (ECU). It does not carry power and ground for the ECU (see ECU Power Harness). It is meant to carry the signals from the location of the ECU (commonly in the driver compartment) to the engine compartment. The Main Harness connectors inside the engine compartment do not typically connect directly to the components there, but connect to a sub-harness for that component. This allows a single Main Harness to be customized to work with a huge combination of different injectors, ignition components, and sensors. (See Injector Harnesses and Ignition Harnesses.) The Main Harnesses included in kits are customized for that kit, and can be purchased separately for replacement. The Dominator ECU does not include a Main Harness, so it must be purchased separately. All Main Harnesses will fit all Holley ECU's.