How To Build a Custom Wiring Harness

Let's wire your baby up!

Time Needed: 2-10 hours, Difficulty: Advanced, Cost: $50-$500 (depending on your project)

The time has finally come. You’ve been putting it off, but you need to make a wiring harness for your vehicle, and the excuses have run out. Whether you’re installing a new radio, rewiring the engine bay, or reconfiguring the entire car, your current setup isn’t cutting it, and the car demands a change. 

The idea of dealing with a crow’s nest of wiring is enough to send most people running, but you’re one of the brave ones. You’ve opted to jump right into the madness and teach yourself a new skill, or you’re at least considering it, and we’re here to guide you through the process.

The reality of the matter is that you’ll have plenty of planning, preparation, and homework to do on your own. Even if you buy an updated fuse block that’s a direct fit for your application, there’s a chance that it’s just a universal system with an appropriate number of circuits for your application. The nitty-gritty of each harness is subject to variation, and depending on the year of your vehicle, it could have multiple types of electronics that old vehicles do not have. That means you still have a fair share of splicing or customization ahead of you.

We can’t jump through your screen to point out what goes where, but the Car Bibles team of wiring wizards can talk about the major points you should be aware of and offer some tips and tricks as well. Let’s get after it.

The Safety Brief

Nothing will leave more scars on your knuckles than running wiring through your car. Even if you’re just hooking up a new radio, you’re bound to snag your skin on some sharp edge and wind up with cuts. And if you’re hanging under the dash, your eyes are likely to collect some debris. We understand that safety glasses and protective gloves can get annoying in this line of work, but they’re worth putting to use. 

Furthermore, electrical shorts are no joke. They will happen, and they will start a fire if you’re not careful, so make sure you have an up-to-date fire extinguisher. It’s essential that you never supply power to the wiring harness while you’re working because of this and the risk of electric shock. You also need to be mindful of where you run the wire. Moving parts, even with dull edges, must be avoided. The friction will chew through the casing, and you’ll wind up in dire straits before you know it.

The Tools & Parts You Need

Making electrical connections is an essential part of this job. Whether you like it or not, you need to do a good bit of terminating and splicing. Luckily, there’s no shortage of tools to help you get the job done.

First and foremost, locate or stock up on as much tape and zip ties as possible. Building your own harness is heavily reliant on the organization of the wiring. Not only that, a layer of tape helps to shield the wire from abrasion, which is necessary for preventing electrical fires. 

As for slicing wires, there’s no replacement for soldering. It is more time-consuming and skill-reliant than using crimp connectors, which turns many off to the idea. However, the result is a much stronger, more professional-looking joint that you know won’t come apart. You will need a soldering station or soldering iron, some solder, heat-shrink tubing, and a heat gun for this process. We also strongly recommend picking up a helping-hand station as the clamps can hold your wires together while you work.

Even if you’re splicing all of your electrical connections from your old harness to your new one, you will still need to rely on crimp connections from time to time. A basic set of wire cutters and crimpers will get you by. But if you really want to beef up your arsenal, you will want to consider ratcheting crimpers and automatic wire strippers. They really speed up the process and ensure that you’ll achieve perfect crimps.

No matter the method you use to make connections, you’ll need a workbench with ample room to extend the harness or a wide-open bit of floor space. And don’t forget the importance of using dielectric grease when the time comes to install that new harness.

How To Build a Custom Wiring Harness
If possible, taping your factory harness down on your workbench is the best option. This allows you to build your new harness to the exact length your application calls for. Photo by Hank O’Hop

The Job: How To Build a Wiring Harness at Home

1. Study Your Schematics

Throughout the entire process, you will need to refer to wiring diagrams and schematics for many reasons. One is to ensure that you’re sending power to the right place as you work. Another is to prevent tapping into the wrong wires in an attempt to power auxiliary devices. It helps to study your diagrams before you jump in to reduce the amount of time you spend reading when you should be splicing.

2. Lay Out Your Old Harness

If you’re lucky enough to have the old harness that you’re replacing, it will serve as a great point of reference. It helps to lay it out to take notes of the general routing and wire lengths. You may not use the factory routing for some components, so keep that in mind.

3. Decide Where To Mount Your Fuse Block

If you’re replacing the old harness under your dash, you need to decide where to mount the fuse block before you get to work. The chances are that it won’t mount in place of the old one, which will throw off the measurements for certain wires compared to the factory harness.

4. Take Note of Routing in the Car

Whether you’re installing a fuse block or not, you want to know exactly where you’ll run the wires before you start making connections. At various points in the process, you can mock up your harness to ensure that you have proper lengths, but knowing where it’ll run before you start cutting and splicing will help eliminate potential mistakes.

5. Decide Which Wires Go Where

This is where the headaches start. Whether you’re making a short harness to connect your taillights or rewiring the car from front to back, you’ll spend a good amount of time deciding which wires go where. Factory wiring diagrams help with this, but you will also want to create your own schematic to reference down the line.

Some universal-style harnesses do have prelabeled wires to make things easy. However, you should still take notes so that you’re not left trying to read those labels upside down and under the dash during future repairs.

How To Build a Custom Wiring Harness
Again, we opted to build our own harness with the old one right next to it. This ensures we have the exact lengths of wire necessary to follow the factory routing. Photo by Hank O’Hop

6. Replicate Your Routing on Your Workstation

If possible, it helps to make a replica of the wiring route in the vehicle on your workstation. This way, you can get very close, if not dead-on accurate results, without having to run back to the car to mock things up a thousand times. Simply tracing the dimensions with tape and laying the wire over them as you work should suffice.

How To Build a Custom Wiring Harness
Tape, wire looms, and zip ties are your best friends when wiring. Photo by Hank O’Hop

7. Separate, Protect, Secure

It’s important to separate all of your wires before you begin splicing so you won’t create a crow’s nest to hide under the dash or carpet. Take the time to add any wire looms, sleeves, or tape to help ensure the wires don’t get jumbled as you work. Finally, if you’re up for it, take the harness for one last mockup in the car after this step.

How To Build a Custom Wiring Harness
As you can see, we’ve removed the majority of the connectors from our factory engine harness. If you’re running any aftermarket components or auxiliary systems, you may need to replace the factory connectors altogether. Photo by Hank O’Hop

8. Source Old Connectors/Begin Making Your Own

Now you’re ready to start transferring those old connectors from the factory harness if you choose to do so. It’s tempting to cut them all now to speed up the process of replacing a large harness, but you might confuse things if you do. Instead, it’s best to work through one connection at a time and take notes along the way to keep yourself organized. A notebook is good, pictures are excellent, and labels make everything clear. Alternatively, you can just use crimp connectors in place of sockets, but the finish is a lot less clean, and you will need to sort through those connections once more as you install the harness in the car.

The Car Bibles Glossary of Electrical Terms

Welcome to bible school!

AWG

An abbreviation for American wire gauge. This is a measurement of the diameter of the wire. It’s essential to familiarize yourself with this form of measurement as the gauge of the wire paired with its length tells us how much current the wire can safely carry. If your wire gauge is too small, the wire will likely melt and cause an electrical fire. That doesn’t mean larger is always better. If the wire is too big, it will result in a loss of energy.

Terminate

Contrary to what common sense tells us, terminating a wire does not mean eliminating it. It simply means that you are connecting the wire to a device. For example, you terminate a wire when you connect it to a switch.

Splice

To splice is to join two or more wires together with some form of mechanical bond that allows the current to flow through the connection. Splicing is a term that’s thrown around freely because you can use it any time you extend a wire or tap into an existing circuit.

Crimp Connection

A crimp connection is a form of connector used to terminate wires. Rather than using solder to create a bond, it is crimped directly to the wire. Crimp connectors come in various shapes and sizes and are primarily used to connect wires to switches and parts that rely on electrical connections.

Schematic

It’s easy to assume that a schematic and a wiring diagram are the same things, but there’s a significant difference between the two. A wiring diagram shows the general layout of the wiring harness and all of the parts contained. A schematic, on the other hand, strictly shows the plan and function of a circuit. In other words, a diagram is about the big picture, while a schematic focuses on the fine details.

The Car Bibles Questionnaire

Car Bibles answers all your burning questions!

Q: How much does it cost to install a wiring harness?

A: Replacing a factory wiring harness can cost anywhere from less than $100 to about $1,000. It depends on the vehicle, which harness you are replacing, how complicated it is, and the cost to manufacture it. Making your own or buying a universal unit is generally considerably less expensive, but it does take more time to install.

Q: How long does it take to install a wiring harness?

A: That depends on the application and the harness you are replacing. An engine harness can take under an hour to replace, while replacing the harness under your dash can take several hours. Even if it’s a direct replacement, tearing down the dash and locating all of your connections can take a considerable amount of time.

Q: Can a wiring harness go bad?

A: Yes. The most obvious reason to replace a wiring harness is due to severe wear in the wire casings on the wires within it. Over the years, wear on the insulative covers will eventually expose the wires beneath, leading to shorts in the system. Wiring is also subject to corrosion, which will also lead to the eventual death of the harness.

The Video Tutorial on Building a Wiring Harness

We’ll admit that making a harness isn’t as simple as it might sound. There’s a lot of research and time that goes into the process. Thankfully, videos such as the one below in combination with our tips can give you a pretty good idea of how to approach this job.

Best Places To Buy Tools and Parts To Build Your Own Wiring Harness?

As much as we’d love to sit down and talk to each of you about your personal tool collections, we can’t. We can, however, talk about some of the tools we think you should have for this job. Making crimp connections requires little more than tools such as the Klein Tools 11055 Wire Stripper and Cutter and Klein Tools J1005 Journeyman Crimping/Cutting Tool. However, the Klein Tools 11061 Self-Adjusting Wire Stripper/Cutter and Haisstronica Ratchet Crimping Tool Set will make quick work of things. If you’re soldering, we recommend you check out the Weller WLC100 40-Watt Soldering Station, Wagner Furno 300 Heat Tool, 1200W, and Eccomum LED Magnifying Magnifier Glass with Light on Stand Clamp Arm Hands. These can all be bought on Amazon or at Walmart.

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