DIY Or Buy: A Look At The LS Standalone Harness Dilemma

At shows and various events across the U.S., we have witnessed some of the most atrocious and appalling wiring displays imaginable. We know you’ve have seen them, too. It’s the engine bay with wires running all over the place that have been spliced together with electrical tape and wire nuts. While we’ve also seen some very nice displays that look better than the factory, it’s pretty obvious that wiring isn’t for everyone.

If you’ve been on YouTube lately, you will notice that there are a ton of do-it-yourself (DIY) videos dealing with LS wiring and swaps. So the question is: should you buy a new engine swap harness, or build your own? While this is not an easy question to answer, we’re going to take a look at the pros and cons of each method for an LS swap with the help of Howell EFI Conversion Experts.

Here is an example of a wiring job that we spotted at an event that needed some serious attention. And while it looked pretty bad, the car made passes the entire weekend without any problems.

Know Your Limits

If you’ve never wired anything in your life, then trying to create your own custom LS harness probably isn’t the best idea for a first project. While there are a lot of excellent how-to videos on the web, precision is critical, and having a basic understanding of schematics, the tools used, and wiring processes are all invaluable. There’s nothing hard about the process, but the problems start when you get in a hurry and don’t pay attention.

The DIY Process

When ordering or creating a standalone harness, you will need to identify your project needs. Questions like what generation engine are you using, automatic or manual transmission, drive-by-wire, or drive-by-cable, will need to be answered. You also need to determine where to mount the ECU and have an idea for wire routing. After you figure out all of the details, you will be able to start the process, which is not a short one. 

When someone decides to create a DIY standalone harness, in most cases, they will start with an existing factory unit. In our experience, these harnesses are old and dirty. After all, the LS engines have been on the market for over 20 years now. As wiring harnesses age, some common problems that can develop are broken wires and melted connectors, and some are nasty. 

We purchased a harness and ECU for $200 off Facebook’s Marketplace. While this unit was uncut, it was filthy. 

Our first step was to remove the old factory wire loom and electrical tape. While this sounds easy enough, we spent over two hours wrestling with the harness trying to remove every last piece. It’s a disgusting process that will leave the table you’re working on dirty and your hands sticky and tar-like. Dirt and debris fell out the entire time we worked with the factory harness. After we had stripped down everything, some problems were unveiled.

The first image is the factory harness stripped down and cleaned up. The second image is all of the conduit and tape that was removed.

The wires going to the transmission had been pinched, and the copper was exposed from the insulation. Since we’re not too keen on soldering wires back together, we crimped the wire together with a butt connector. We attached the wire ends together and then applied heat shrink with adhesive over the connector. The heat shrink will keep the moisture out and prevent corrosion. After we fixed this problem, we noticed that someone else had previously done the same thing we just did with two other wires. Instead of using a non-insulated connector with adhesive heat shrink, they used the standard type, which will allow for moisture, causing problems down the road. We then noticed that some of the OE connectors were broken on the harness and would need to be replaced.

As you can tell from the above images, this harness was a wreck and extremely​ dirty.

Our next step was to remove the unwanted wires and connectors. It’s pretty straightforward, but you will want to double-check your work while you remove the pins and strip the wire. Oh, and if you’ve never worked on a harness before, get ready to spend a good bit of time watching YouTube videos and performing Google searches. is a big help with this process — he site has diagrams, pinouts, and a plethora of information. If you’re planning on building a standalone unit, be sure and check this site out first. 

We disassembled the red and blue connectors, carefully removing each pin that was no longer required. The plug housings are numbered, so it’s easy to identify the wires and remove them. One problem that we found is that after you remove the unused wires from the harness, it leaves an opening straight to the ECU pins. This makes the ECU susceptible to moisture and debris. We would highly recommend plugging these holes to keep these elements out of the pin area of the ECU. Another predicament is, when you get ready to mount the ECU you’re at the mercy of the factory harness wire lengths. There is no easy or reliable way to fix this problem. 

Pulling the wires that we didn't need was not that time-consuming. Just make sure you are removing the correct wire and don't get in a hurry.

From here you will have some work to do with the old fuse box, unless you plan on using it. Some of the wires need to be removed as they will tie back to relays, and others will need to be fused. After all of the above is done, you can then wrap your wires in tape, conduit, or leave them bare, even though we do not recommend the last option. We opted for split and plastic loom and used electrical tape to secure the ends.

The top left picture is everything we stripped out of the harness and didn't use, including the factory fuse box. The top right image shows you what wires will need to be hooked up to a fuse box or relay.

Our efforts resulted in a harness that looked much better than when we started. However, even though the loom cleaned it up nicely, everything underneath was junk. It was so rough in fact, that for our purpose, this harness is a lost cause and will not be used. With so many broken wires and melted connectors, it would be hard to rely on it for any of our projects. The last thing that we want to deal with is wiring problems on the side of the road somewhere.

Here is the harness after the conduit and loom were added. The unit still needs a fuse block and relays to function. Unfortunatly the harness was in such bad shape it would never make it onto one of our projects.

Howell EFI’s Harness

One of the nice things about calling a company like Howell EFI is that an experienced technician will walk you through the process to make sure you get exactly what you need even if you aren’t sure. When we called, they asked us about the vehicle, transmission, throttle body, and where we wanted to mount the ECU. If you’re going to build a DIY harness, you’re on your own to figure it out. While there’s plenty of information on the internet, it will take time to weed through it all, but it can be done. People do it all of the time.

At a glance, the Howell LS swap harness is already superior in the looks department when compared to our DIY factory harness. It’s clean, each wire is new, labeled, and wrapped with a black flex loom. The company also uses a high-temperature shielding for wires that route around the exhaust. Our unit also included Bosch relays for the fuel pump and dual-fan control, pre-terminated weather pack connectors, fuse block, and an OBDII port with a built-in check engine light. As Billy Mays would say, “But wait, there’s more!” Unlike our factory rewire harness, we were able to add an additional 3-feet of wire. The extra length gives us the freedom to mount the ECU in the driver’s compartment or more flexibility under the hood. With our DIY factory job, we will need to mount the ECU where the harness allowed us, which limits us to under the hood. The Howell harness was also much more flexible than our DIY harness since the wires are new and haven’t been exposed to under-hood temperatures over the years. 

Our package included the harness, ECU, new O2 sensors, weather-pack connector ends, instructions, and decals.

For our 5.3-liter LS swap, we also ordered an ECU as part of the package. The beauty of this part is two-fold.

First, we know that this ECU is in good working order because Howell tests them. Secondly, Howell turns off everything on the ECU that we are not going to use. Things like downstream oxygen sensors and emissions components have already been removed, and we don’t need to worry about them on startup now. If you’re planning on running a stock engine, you don’t have to worry about buying a computer, software, or hiring a tuner — it’s ready to go right out of the box. The ECUs are not locked, so if you want to make adjustments to the engine down the road, just take it to your favorite tuner. 

The DIY Delima

If you decide to modify a factory harness, you can save yourself some money. We only spent $300 on ours, which included buying a used harness and ECU. If you do choose to count your time as labor, things get more interesting. For instance, if we add $20 per hour as a rate, which would be extremely inexpensive for any work on a vehicle, our total would now be $510. Howell’s brand new harnesses start at $450 depending on what you need.

DIY Standalone Harness Cost:

2004 Used Harness – $200

Fuse box and relays – $40

Loom – $30

Connectors – $10

Zip ties, tools, tape, heat shrink, etc. – $20

Labor 8 hours – “Free”

A Little More Money, Zero Problems

When purchasing a new unit from Howell, you will talk with one of its experts. They will take all of the guesswork out of the process and eliminate any problems, giving you precisely what you need. Everything is new, including the connectors, wires, pins, and these units are built in the USA. If you want to mount the ECU in the driver’s compartment, they can make the harness longer. The only downside to the new unit is that it will cost you more money. But, you will get a new unit that has been tested and is 100-percent ready to install straight out of the box. 

Howell Standalone Harness Cost:

5.3-liter V8 Vortec SFI Wiring Harness with a 4L60E, O2 Sensors, with 3-foot of additional wire – $650

2000 Pre-programmed ECU w/o core – $350

Time Is Money

As hot rodders, it’s very seldom that we consider our time and labor when working on a personal project. We will pour our hearts and souls into our vehicles and count every penny spent to finish the task. But usually, the hours spent grinding away in the wee hours of the morning go unaccounted for. Maybe we do this because we enjoy wrenching so much, or perhaps we just think it’s saving us money in the long run. Either way, your time is worth something, and we doubt you like giving it away.

We spent about 8 hours making our harness, and that was stopping a little short with the process. We still need to add a fuse box and relays for it to be functional, which would add another hour or two of labor. If you don’t consider your labor, you could save money instead of buying a new harness. If you did keep track of your hours and put a dollar amount with them, you could save a little but probably not as much as you think. 

Here is a picture of the Howell swap harness next to the 20-year-old unit on the right. The Howell unit is brand new, terminated, and all of the wires are labeled out of the box. Which one would you trust on a cross-country road trip?

In our opinion, A DIY harness is not worth the effort. Think about it: there are a ton of things that could be done in an 8 hour day on a vehicle. This is plenty of time to install an engine and transmission, fit the interior, weld up an exhaust system, and a lot of other things. Plus, the savings for us just weren’t justified, especially after seeing the final product compared to the new harness. Howell’s unit was nice and new, urs was not. It was stiff and old like most of Joe Biden’s jokes. While it would be an improvement over some of the harnesses we’ve seen at shows, it’s not up to our standards.

We know that some of you out there are pros and have figured out how to make a standalone harness in a matter of hours. For those of you that can, then a DIY harness makes sense, if you start with a nice harness. For the average guy, we would argue it’s a different story. We would much rather spend the additional money and get a new harness and use our time on other areas of a project. The words, “You get what you pay for” could not be any more true when dealing with purchasing a standalone swap harness in our opinion.

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About the author

Brian Havins

A gearhead for life, Brian is obsessed with all things fast. Banging gears, turning wrenches, and praying while spraying are just a few of his favorite things.
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