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First, DON'T PANIC! It is entirely unlikely that you will require a fully unterminated, build-it-yourself harness like the one shown in the image on the left. Holley does offer those but we only recommend those in extremely rare cases. More often than not there is a harness available that will allow you to install your Holley EFI system with a minimum of crimping and soldering. So let's get started.
Excellent question! In the Holley EFI world, the main harness is a bundle of wires that runs between the 34- and 26-pin connectors on the side of every Holley ECU. These connectors are referred to as J1A and J1B. (Okay, so it's not an especially meaningful name but it works.) It generally provides about 5 feet of length between the ECU connectors and the bulk of the engine compartment connectors so it easily has enough length to mount the ECU in the passenger cabin (though it is noteworthy that Holley ECU's are durable enough to be installed in the engine compartment--with some common-sense constraints.) Once in the engine compartment, there are connectors for various sensor and other connectors that are designed for sub-harnesses like the injector and ignition sub-harnesses. By using this sub-harness configuration Holley allows you to select and pair a more generic main harness with a vehicle-specific injector or, for the most popular engines, provide a connector that simply click-connects to the stock injector harness.
The image above is a diagram of Holley’s universal multi-point fuel injection harness but gives a good idea of the components you can expect on all Holley Main Harnesses. P1A / P1B are the connectors to the ECU. The relay, fuse, and CAN bus connectors are close, as is a loose lead used to connect to the ignition-switched power. A connector for user-programmable inputs and outputs is a bit further up the harness, close enough for easy access from either the cabin or the engine compartment.
Moving to the right, we get into the essential engine compartment connectors. All of these are for sensors but two: Ignition and Injectors. These are normally connected to sub-harnesses specific to your application. We will discuss selecting the correct injector and ignition sub-harness a bit later.
Getting a harness that is specifically built for your make and model vehicle is a huge benefit. Not only does it mean that your installation is essentially a plug-and-play affair, it usually means that you will be spending less. This is because the vehicle-specific harnesses typically use original equipment subharnesses and/or sensors.
But don’t worry if you don’t see yours below. Using one of the universal multi-point harnesses like above still puts you in a position where very few connection have to be terminated by the installer.
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