Not every engine swap project needs a high-dollar aftermarket EFI system. Sometimes, a simple OEM ECU with basic wiring will get the job done. You could just reuse the OEM wiring harness, but that harness might have its own issues you can’t see with the naked eye. That’s why using a professionally made harness for your project might be the smart move.

Let’s face it, OEM parts you grab from the local junkyard or Facebook Marketplace could be compromised. You don’t know what kind of life those parts lived, or if they were removed with care. OEM wiring harnesses can have age-related issues, and that will make your life difficult at some point. A new aftermarket wiring harness is going to eliminate those problems, plus eliminate all the unneeded sections of the wiring harness.

Planning And Prep For A New Harness

If you want to use an OEM harness that’s fine, but you better be ready to do a lot of work to it before you install it. There will be lots of areas of the harness that you need to remove to work with your build, and there could be repairs that need to be made. However, an aftermarket wiring harness that’s professionally made will eliminate those hassles right from the start.

You don’t want to jump right into throwing a new harness into your vehicle without some planning. Bill Hillock’s company, BP Automotive, has been making aftermarket and custom LS wiring harnesses for many years. Hillock’s insight into how to prepare for a harness installation is valuable.

You’ll want to take the time to lay out your engine harness before installing it. That way you can identify any modifications that might need to be made.

“The first thing you’ll want to do is lay the harness out and familiarize yourself with the layout and where everything is. This will help you plan for the final harness installation. It will ensure that the harness matches your application before moving to the installation phase.”

Once you’ve taken the time to study the harness and how it’s laid out, you need to decide how you want to route the harness in your vehicle. There’s a good chance your project vehicle didn’t come with an LS engine or even EFI, so there might not be an obvious route for the harness. You’ll want to avoid any areas where there’s excessive heat, pinch points, sharp edges or abrasive surfaces. These are all things that will damage a harness in some way.

“Another thing you’ll want to keep in mind is where the ECU is going to go. Most aftermarket harnesses are designed to mount the ECU and the fuse block in the passenger compartment. When mounting the control module and the fuse block, try to plan a location that does not require vehicle disassembly to gain access to the fuse block or control module,” Hillock states.

You might discover after laying the harness out that some changes need to be made. It’s a good idea to have some basic supplies and tools on hand to make any modifications needed to the harness or the vehicle itself. Remember, you’ll want to make sure the harness is not only going to be safe, but also secured in place.

You will want to have some basic tools and supplies on hand. The harness might need to be adjusted slightly to fit like it was made for your engine bay.

Chuck Stefanski from Big 3 Racing took care of the wiring for our Project Swedish Meatball, which BP Automotive supplied the LS wiring harness and other parts. There were some changes that needed to be made to the harness, and the vehicle needed some massage work. Here is what Stefanski suggests you have on hand before you start a harness installation.

“You want to make sure you have basic wiring tools like strippers and cutters, along with a heat gun. It’s also a good idea to have a soldering iron on hand as well. You’ll need a hole saw to make a hole for the harness to pass through the firewall, and a firewall grommet to protect the harness. A nice assortment of zip ties is a must-have to secure the harness as well. You’ll also want to have a multimeter handy in case you need to do any troubleshooting.”

Professional Harness vs A Homemade Harness

We talked earlier about the pitfalls of just using an OEM harness for a swap. While a professionally made harness is going to cost more, the benefits it provides are going to be worth it in the end. You can’t put a price on not having to chase electrical issues.

A big advantage of a professionally made harness will be the materials used in its construction. You’re not going to find questionable wire sourced from Amazon in these harnesses. They will be made from the right materials, and that makes a big difference.

“The harness should be made from TXL (Cross Linked) wire. This type of wire has superior abrasion, temperature, and chemical resistance. A harness should also have circuit protection, such as fusible links and a properly laid out fuse block. No one likes an engine fire,” Hillock explains.

A good aftermarket harness will have everything labeled. That’s going to save you the time of having to figure out or guess what a section of wire or connector is for.

It’s one thing to use the correct materials as the backbone of a harness, but you also need to make sure the harness has been assembled correctly. There are specific tools that should be used to build a harness to OEM standards.

“A professional harness will have been built with the proper production tools. You don’t want to buy a harness that was made using repair tools. We only use production tools that meet the most stringent standards set forth by the terminal manufacturer. A professional harness will have been quality tested to ensure that your harness is built correctly,” Hillock says.

Another advantage that Hillock mentioned is the fact that a professionally built harness is going to come with tech support. That means if you run into any kind of issues, you’ll get to work with someone who understands how these harnesses are supposed to function.

Our Swap Setup 

The BP Automotive harness fit Project Swedish Meatball perfectly.

Project Swedish Meatball is going to be a multi-use vehicle. It could be going to get groceries in the morning, and then making rips down the drag strip in the afternoon. This mission objective put reliability high on our list, so using a well-built harness and high-quality ECU was important.

After talking with Hillock, we decided the GEN IV Engine Controller kit from BP Automotive would fit our needs perfectly. This LS wiring harness kit is made for engines with the 58x reluctor wheel and comes in different configurations based on what type of transmission you’re using. You do have to let BP Automotive know what type of alternator and MAP sensor connector the harness will need, and if the engine uses VVT. The kit comes with the harness, ECU, Drive By Wire gas pedal, and O2 sensors.

Since we plan on turning up the boost at some point, a Holley 4-bar MAP sensor was used with our setup. We didn’t need to worry about the VVT connection since that’s been deleted from our LS engine. As for the alternator, we elected to use an OEM GM alternator. The reason for that choice is simple: if we ever have an alternator issue while on the road, a replacement will be easy to find.

Having such a complete LS wiring harness kit makes installation simple. You know what you’re getting right from the start and that makes a big difference in how well everything goes.

“As a shop, we like being able to use a well-built harness that comes pretty much ready to go. It saves us time since we don’t have to fix anything, worse case, we have to make a small adjustment to the harness based on the application. Even then, kit harnesses like this are easy to work on since they use good materials. The fact that this came with everything we needed to get the engine and ECU wired was very nice,” Stefanski explains.

An engine wiring harness isn’t the best place to try to save a few bucks on a big project. Old OEM harnesses could be riddled with damaged wires that will leave you chasing random electrical issues. A professionally made engine wiring harness might cost a bit more, but the time it saves and problems it prevents are well worth the cost.