How to Build a Wiring Harness

This chapter covers the last major topic—harness basics. In addition, I’m going to present a number of examples that tie together what you’ve learned in the first eight chapters of the book. I chose examples that I feel most readers of this book will be likely to attempt.

 


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Harness Basics

This is one of my absolute favorite parts of wiring—assembling harnesses. Over the years, I’ve made thousands of harnesses for projects.

You get the benefit of that experience here, as I assure you that I’ve made all the mistakes! There are only two considerations: the accessory came with a harness and you need to “prep” it for installing it in the vehicle, or the accessory did not come with a harness and you need to build one from scratch and then install it in the vehicle.

 

This is the complete kit from Pioneer. As you can see, I’ve already taken care of the physical mounting of the radio as it is mounted in the custom 3/8-inch billet aluminum panel that is shaped identical to the stock radio. Below the radio is a Dakota Digital Volt Meter that allows me to keep tabs on the voltage at my amplifiers directly.

This is the complete kit from Pioneer. As you can see, I’ve already taken care of the physical mounting of the radio as it is mounted in the custom 3/8-inch billet aluminum panel that is shaped identical to the stock radio. Below the radio is a Dakota Digital Volt Meter that allows me to keep tabs on the voltage at my amplifiers directly.

 

Prepping a Harness

Again, this is a case where your new accessory came with a wiring harness of some kind. Take a moment to inspect it to be sure that it is truly ready to install, rather than get in the vehicle and start wiring it up only to find out that:

  • Some or all of the wires are not long enough.
  • It isn’t complete—more wiring is needed to complete the install than was provided.

Having installed thousands of aftermarket accessories over the years, let me assure you that maybe 1 percent of the included harnesses are truly plug-and-play. And even those will benefit from a little prepping on the bench prior to their installation. This example starts with a new piece of gear for the Mustang—a Pioneer Premier AM/FM/CD player.

Car stereos have come a long way. This unit has a number of features that only a few years ago would have been impossible to imagine in a single DIN head unit, the following being the main ones: iPod interface via USB, Bluetooth capability, Audio from Bluetooth-equipped storage devices, cell phone syncing for your Bluetooth cell phone.

This deck has a really slick inter-face for iPods and accessories.

As such, this radio has a number of guzintas and goes-outtas as well as a USB cable and microphone that need to be installed to use the iPod and hands free features so it makes for a really good example.

Looking at the included harness, I notice right away that the ACCY lead and the remote lead are much shorter than the ones on the radio I’m replacing with this one. This needs to be addressed to ensure that my installation goes smoothly—better to do it at the bench.

 

Step-by-Step

Step 1:

Secure the harness so that the wiring is facing you.

Step 2:

2Group and tape the power, ACCY, and ground wiring together. (Note: I extended the accessory wire slightly by soldering and heat shrinking another 8 inches of 18-gauge Red wire to the one Pioneer included)

 
 
 
 
 
 

Step 3:

Group and tape the speaker leads together.

Step 4:

Group and tape the remote out and mute input leads together.

Step 5:

3As my Mustang has two amplifiers, I do not require the speaker outputs of the radio so I insulate them with tape to prevent them from accidentally shorting to the chassis; same goes for the mute input lead.

Now that the harness is prepped, I can install it in the vehicle in a much neater fashion than would have been possible if I had done this in the car. A couple of cable ties, and this radio is ready to install.

 
 
 
 

Step 6:

Once in the vehicle, I connected the power, ACCY, and ground wiring. As the audio system is fairly elaborate, I decided to make these connections to the main power and ground distribution in the rear of the vehicle. (If your audio system is not so elaborate, you should install a female plug, so you can easily connect to the factory radio harness without cutting any wiring. This plug is readily available at your local car stereo retailer.)

Step 7:

4I connected the remote output terminal to the trigger lead for the processor and amplifiers.

 
 
 
 

Step 8:

5Route the USB cable under the center console and into the storage compartment at the rear of the console—this is where I locate my iPod.

 
 
 
 
 

Step 9:

6Route the microphone cable up the driver-side A-pillar, securing it in place with duct tape (the good stuff please!) and mount it to the headliner with the included mounting bracket.

 
 
 
 
 
 

Now that all the cables are coming out of the radio opening, note that I’ve left plenty of length so that the radio can easily be installed and removed and they are neatly grouped together.

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Finally, note the number of connections to the radio itself:

  • Main harness.
  • USB cable—to the iPod.
  • 1/8-inch audio jack—to the microphone.
  • Antenna.
  • RCA cables for the amplifiers.

Admittedly, this was a simple harness prep. Something like a keyless entry or auto security system is a more involved prep. When do you know it’s time to make your own harness? When the harness you have won’t support the equipment you’re adding.

 

Building a Harness from Scratch

This requires a bit more planning and “stuff” on your end than simply using, or even modifying slightly, a harness that came in the box with your new accessory. Let’s first discuss the planning.

Planning the Harness

Before you can actually make the harness, you need to determine a few things:

  • Define the objective.
  • Outline the considerations.
  • List the parts required.
  • Draw a diagram.

Define the Objective: Skip this part, and you could spend days or even weeks trying to sort it all out after the fact. This isn’t terribly difficult to do, but you need to have a firm understanding of what the harness needs to accommodate. For example, let’s revisit the example I gave you in troubleshooting the electric fan circuit in Chapter 7.

 

MuscleCarB

 

I built this circuit and harness from scratch. These were the objectives of this circuit.

  • Turn on a pair of cooling fans automatically when:
  1. The engine was running
  2. The engine temperature had reached 180 degrees
  • Turn on the cooling fans manually via a dash mounted switch that could override the automatic feature when:
  1. The key was in the IGN /RUN position
  2. The ACCY switch on my panel was ON

 

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Outline the Considerations: We can only determine what these considerations are after we’ve defined the circuit’s objective. As we have, let’s first consider the basic parts the harness will connect together and its routing:

  • Pair of 16-inch Spal puller fans mounted to the radiator; each require 22 amps of current according to the manufacturer.
  • Thermostatically controlled switch tapped in the intake manifold near the water neck.
  • Dash-mounted override switch—already in the Painless Panel as it were.
  • Passage through the firewall safely.
  • Source of power—in this case, we can use the accessory fuse panel mounted to the firewall on the passenger side.
  • Proper safety via the correct fusing.

 

Having some primary wire on hand certainly makes your wiring tasks easier. Yeah, you’ve probably got a box under a workbench somewhere full of odds and ends, but you’re going to need a good selection of wire when you start building harnesses.

Having some primary wire on hand certainly makes your wiring tasks easier. Yeah, you’ve probably got a box under a workbench somewhere full of odds and ends, but you’re going to need a good selection of wire when you start building harnesses.

 

List the Parts Required: We know that we will need a pair of 30-amp relays to operate the fans, as the switch can only handle 10 amps or so. Before we go any further, we need to decide where these relays can be located so that we can determine the gauge of wiring to use. I chose a spot on the firewall above the transmission hump to mount the relays. As this is less than 15 feet away from the fans, we know that 10-gauge is the way to go. Let’s list the parts needed to complete the build:

  • Three 30-amp S.P.S.T. relays (sure, you can use S.P.D.T. relays like I did as either will accomplish the same job here).
  • 50 feet or so of 10 AWG wiring.
  • 180 degree thermostatically controlled S.P.S.T. switch.
  • Insulated female push-on terminals (blue and yellow in this case) for connection to the relays.
  • Quick disconnects between the thermostatically controlled switch will provide an easy way to disconnect the harness from the manifold if it has to come off for any reason.
  • Other miscellaneous crimp connectors to connect the harness to the plugs on the end of the fans, power to the relays to the accessory fuse box, ground connections, etc.
  • Split loom tubing to cover the wiring under the hood.
  • Cable ties to anchor the wiring.

As the vehicle already had a snap busing in the firewall large enough to accommodate the wiring, we will utilize it. Of course this job also calls for all the regular wiring tools.

Draw a Diagram: Pull out a sheet of paper and sketch it out. This saves you the trouble of forgetting something simple.

Now, it’s time to consider the things that are handy to have on hand when embarking on such projects. What will you need?

Required Stuff

When it comes to assembling your own harnesses, you really only need a few things:

  • Wiring of the correct gauge and color.
  • Plenty of it to choose from.
  • Proper connectors.
  • 3M Super 33+ tape—Use it to group a harness (rather than ties that can get hung up in jute or around other body work).

 

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As you can see, I believe in having plenty of connectors on hand to choose from. Pictured is about half of what I keep on hand.

As you can see, I believe in having plenty of connectors on hand to choose from. Pictured is about half of what I keep on hand.

 

Let’s say that you only had red and black wire on hand in 10-, 12-, 16-, and 18-gauge. That’s eight rolls of wire, a bunch, but only two colors. Although you could make a harness from this, it’d be next to impossible to tell what was what in a harness of any size. For that reason, I recommend that you have at least the following wire at your disposal:

You can use half as many colors of 16 and 18 AWG, but there really is no way that you can have too many. As your project increases in complexity, the amount of wiring choices you need increases exponentially. You can find suppliers that sell such wiring in bulk. You always pay less per foot for a big roll than for a small roll, and this pays off in the long run. As I do a lot of wiring projects, I also keep on hand 8 AWG, 4 AWG, and 1/0 AWG in at least Red and Black.

In addition, you need crimp connectors of all types and sizes. This can also be a major investment, but again shop for them in bulk. Although it sounds like a lot, a pack of 100 connectors won’t last as long as you think it will—especially now that you’re comfortable with wiring in general.

Building Your First Harness

Before I show you how to build a harness from scratch, let’s talk about some fundamental harness-building basics. Here they are:

  • Plan your work as you just learned.
  • Build your harness on the bench, not in the vehicle.
  • Group wiring together that will run in a common direction.
  • Use 3M Super 33+ tape to keep your work together.

 

Harness Construction Project

Example: Installing aftermarket door lock actuators

Vehicle: 1972 Olds Cutlass

I covered this circuit in Chapter 6 in my discussion of relays.

Before we do the actual building, let’s follow the guidelines I provided previously, starting with planning.

Define the Operational Objective: Add the convenience of power door locks to a vehicle without them. Specifically, we want to be able to:

  • Operate them with a single console mounted switch
  • Allow for a future addition of a keyless entry system

Outline the Considerations: Right away, our objectives tell us about the harness we’re going to have to build as we know:

  • Harness must route into both doors.
  • Relays need to be located under the dash, centrally is preferable.
  • Switch needs to be located centrally in the dash so that both driver and passenger can reach it.
  • Power will be sourced from the accessory fuse box mounted to the firewall on the passenger side.

Since I’m building this from scratch, I’ll elect to trigger the relays via negative pulse from the switch. In addition, I’ll install quenching diodes across the coils of the relays. These steps make it a snap to add a keyless entry at some point down the road.

 

The parts required for this project. Anytime I embark on a project like this, I lay all the parts out to be sure nothing is missing.

The parts required for this project. Anytime I embark on a project like this, I lay all the parts out to be sure nothing is missing.

 

List the Parts Required:

  • Two aftermarket door lock actuator kits
  • An S.P.D.T. Center OFF momentary switch (SPAL Part # 3700050)
  • Two S.P.D.T. relays
  • Two diodes
  • Associated crimp connectors
  • Super 33+ tape
  • Wiring
  • 12 AWG—For the main power and ground
  • 16 AWG—Between the relays and the actuators as they require less than 5 amps of current each and the wiring run to them is about 10 feet each (20 foot total run as we have to go to and back)
  • 18 AWG—For the low-current connections to the switch
  • 15 amp ATC fuse

Lay Out the Harness: This is actually a pretty simple harness to build, as we can assemble nearly all of it on the bench. This installation will actually have three separate harnesses going in three different directions:

  • From the relays to the driver-side of the vehicle, through the jamb, and into the drivers-door to connect to the actuator
  • From the relays to the passenger side with two sub harnesses:
  1. To the source of power at the accessory fuse panel
  2. Through the jamb and into the passenger door to connect to the actuator
  • From the relays to the dash mounted switch

 

Step-by-Step

Step 1:

15Secure the relays upside down in a bench vise so you can easily wire them.

 
 
 
 

Step 2:

Wire the relays as shown in diagram 7-2 on page 106.Note the quenching diodes across the coils, as well as a second pair of trigger leads, in anticipation of adding a keyless entry system to this at a later time.

Note the four sub-harnesses in addition to the power and ground: left actuator harness, right actuator harness, switch harness, and keyless entry harness (with the red push-on connectors).

Note the four sub-harnesses in addition to the power and ground: left actuator harness, right actuator harness, switch harness, and keyless entry harness (with the red push-on connectors).

Step 3:

Note that I’ve left myself plenty of length for the harnesses that go into the doors—16-gauge wire is relatively inexpensive and we don’t want to have to extend this because it wasn’t long enough. (I’ve always used blue and green wires for aftermarket door lock actuators. This appears to be a standard.) Note how nicely things are grouped together based on which way they are routed in the vehicle. I always secure my harnesses high in the dash and never under the carpet for a job like this.

Look closely at the quenching diodes. One is installed across the coil of each relay.

Look closely at the quenching diodes. One is installed across the coil of each relay.

 

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Step 4:

Mount the relays.

Step 5:

Ground the wiring connected to terminals 87a to a suitable

Step 6:

19Choose a location for the switch and drill a hole if necessary (as an entire interior re-work is in my Olds future, this switch will find its way into a custom center console).

 

Step 7:

Route the switch harness from the relays to the switch location, properly anchoring this along the way.

 

MuscleCarB

 

Step 8:

Route the wiring toward the kick panel areas of the vehicle, being sure to anchor it up high as you go.

Note the new Green and Blue wiring routed neatly along an existing harness going up and over the steering column on its way to the driver-side kick panel.

Note the new Green and Blue wiring routed neatly along an existing harness going up and over the steering column on its way to the driver-side kick panel.

 

Note the Green and Blue wiring tied neatly to an existing harness as it makes its way to the passenger-side kick panel.

Note the Green and Blue wiring tied neatly to an existing harness as it makes its way to the passenger-side kick panel.

Step 9:

Since this vehicle does not have a boot between the body and the doors, we must drill holes and install snap bushings—remember to drill the holes in the body higher than the ones in the door to keep water from getting into the body by traveling down the harness.

Installing a snap busing in the door itself can be a tricky matter. In this case, I was able to sneak my entire drill into the interior of the door and drill it from the inside out. Of course, a right-angle drill would certainly make this job easier.

Installing a snap busing in the door itself can be a tricky matter. In this case, I was able to sneak my entire drill into the interior of the door and drill it from the inside out. Of course, a right-angle drill would certainly make this job easier.

Step 10:

Run the harnesses into the doors and cover with split loom tubing.

 Notice that I secured the split loom tubing to the wiring in the jamb via the electrical tape method I shared with you in Chapter 6. Not only does this look neat, it keeps wiring itself hidden as the door is opened and closed over the years.

Notice that I secured the split loom tubing to the wiring in the jamb via the electrical tape method I shared with you in Chapter 6. Not only does this look neat, it keeps wiring itself hidden as the door is opened and closed over the years.

Step 11:

Secure the wiring away from moving parts, such as the window regulator and door lock mechanism.

This is an interior shot of the passenger door. Note that the wiring comes through the snap bushing, and it is anchored to the door itself.

This is an interior shot of the passenger door. Note that the wiring comes through the snap bushing, and it is anchored to the door itself.

Step 12:

Connect the wiring to the actuators themselves.

I elected to cut off the wimpy bullet connectors that were on this actuator in favor of some fully insulated push-on connectors. As they’re in the interior of the door itself, I filled them with white lithium grease. Look closely at the two silver screws to the left—these secure the actuator itself to the back of the metal.

I elected to cut off the wimpy bullet connectors that were on this actuator in favor of some fully insulated push-on connectors. As they’re in the interior of the door itself, I filled them with white lithium grease. Look closely at the two silver screws to the left—these secure the actuator itself to the back of the metal.

Step 13:

Connect and install the switch.

As my center console in the Olds isn’t finished yet, I just laid the switch on the transmission hump so that I can verify operation. The system works great!

As my center console in the Olds isn’t finished yet, I just laid the switch on the transmission hump so that I can verify operation. The system works great!

Step 14:

Connect the power lead to the fuse box and insert the 20-amp fuse.

Step 15:

Try the system.

Everything should work correctly. In the event that the motors operate backward from what you intended, then you can either reverse the wiring at the motors directly or at the relays—whichever is easier. This is not a big deal so don’t fret. As I used Green for lock and Blue for unlock throughout, this was not an issue this go.

Although I didn’t cover the mechanical part of the actuator install, it’s really quite simple. After you’ve connected them to the door unlock/lock rods within the door, you need to verify that you can still easily manually unlock and lock the doors with both the key and interior plunger. If not, you may have to play with the alignment to get all to work smoothly. Better to determine this while the door panel is off!

The hardest part of the job was drilling the holes in the door and body to get the wiring from the body and into the doors. The second most difficult part of this install was lying on my back and routing cable over-head with my feet around the head-rests of the seats.

Installing a Multi-Function Harness

This is no more difficult than installing a single-function harness. Let’s say that you wanted to add power windows and power door locks. Most aftermarket power window kit manufacturers sell pre-assembled wiring harnesses that vary slightly based on location of the power window switches. The switches can be located in the doors or centrally, such as in a center console. If I was installing both power windows and power door locks, I would install the wiring harnesses to them simultaneously—this greatly cuts down on the amount of work required as both require sub harnesses run into each door and common switch locations.

Let’s say that you had such a vehicle with a center console and wanted the power door lock and power window switches located next to each other in the console. Then, you need the pre-made power window harness that was designed for console-mounted switches. At this point, you need to either build your own harness for the power windows or buy a pre-made harness.

When it comes to installing these two separate accessories, proceed as above to Step 7. Then, begin the installation of the power window wiring harness by laying it loosely in the vehicle based on the direction the sub-harnesses need to go. From this point on, treat the installation of these harnesses as if it were a single installation and follow the remaining steps with both harnesses.

As you’re routing, it’s easier to treat multiple harnesses and sub harnesses as one when anchoring them—up high and out of the way, of course. Compare this to what you typically see drooping out from under the dash of even the nicest of custom-built cars at the shows. It’s the behind-the-scenes stuff that counts in such wiring jobs.

When you’re building and installing harnesses, the time you spend on the details makes the difference. Here are a couple of common pitfalls to avoid.

 

The connector for the valet switch for my security system is on the right and it is pre-terminated with a small plug—how handy that it came terminated this way. Notice how the Radio Shack six-pin connector makes the pair of switches and LED a snap to disconnect should this panel need be removed for any reason.

The connector for the valet switch for my security system is on the right and it is pre-terminated with a small plug—how handy that it came terminated this way. Notice how the Radio Shack six-pin connector makes the pair of switches and LED a snap to disconnect should this panel need be removed for any reason.

 

Pitfall #1—Short Wiring: Short wiring is the act of using just enough wire to get to a specific component or controller. Keeping serviceability in mind, there may be times when something needs to be removed, such as the center console in the example I just gave. In this case, you would need to use enough wiring to allow the switches to be easily unplugged from their respective harnesses in the event the center console needed to be removed for whatever reason. As the door lock relays, door lock actuators, and window motors are permanently mounted, this is not nearly as important for them. Yet, consider if a relay were to fail and needed to be replaced. Your wiring must allow for this.

Pitfall #2—Ease of Removal: Consider the use of Molex plugs to be able to quickly remove something that the harness passes through. You learned how to assemble these in Chapter 3. Consider again all the plugs in the kick panel of my Mustang, pictured in Chapter 7. This allows the entire door to be easily removed by a body shop in the event that it needed to be replaced.

There’s no telling what kind of vehicle you might be working on or what kind of a project you build. Keep these kinds of plugs in mind to make disconnecting harnesses quickly a snap. When disassembling consoles and dashboards in vehicles, take notice how often the OEMs use these types of plugs for this very reason.

Thinking about serviceability as you’re doing an installation saves yourself time in the long run. The most common thing that I run across are switches, lights, and indicators installed in removable dash panels with no thought given to easily disconnecting them. A great example of this is the dash panel surrounding the radio in my Mustang. Note the switches and LED that I added. I installed a Molex type plug to allow for easy removal. Yup, I got it at Radio Shack.

If you’re installing a switch, light, LED, or any other controller or indicator in a removable panel, it will be difficult to remove the panel with all the different wires going to the back of it, especially if they’re short wired. Now imagine a mechanic at the dealership faced with removing the same panel. They will probably just start cutting wires. If nothing else, use quick disconnects for ease of disconnecting.

Pitfall #3—Mummifying: While it is true that the OEMs typically enshroud their harnesses entirely with tape, this isn’t really necessary or desirable to do when you’re adding a harness. The OEMs typically use a special harness tape for this, which can be several inches wide and with a radically different adhesive than electrical tape. Using electrical tape to mummify a harness becomes expensive in a hurry.

As a result, many choose to use the “cheap” tape for this duty. Don’t fall into that trap because it creates a gooey mess over time. Trust me when I tell you that there are few things messier than troubleshooting a problem in such a harness in the heat of the summer where you have to cut the cheap tape off to get to the wiring inside.

 

Mounted in the trim panel around the instrument cluster in my Mustang are a pair of LEDs for monitoring operation and low fluid level of the after-market water/methanol-injection system. A couple quick disconnect push-on connectors allow this panel to be easily and quickly removed.

Mounted in the trim panel around the instrument cluster in my Mustang are a pair of LEDs for monitoring operation and low fluid level of the after-market water/methanol-injection system. A couple quick disconnect push-on connectors allow this panel to be easily and quickly removed.

 

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Over the years, I’ve typically mummified only security system harnesses in order to make them to be indistinguishable from the factory harnesses. This is one of the ways professional auto security installers ensure that your security system isn’t vulnerable to the thief in a hurry. If you do elect to mummify a harness for any reason, use the good tape—yup, 3M Super 33+!

Documentation: it is always a good idea to Document your work. Whether you’re building a simple harness or multi-function harness, taking a few minutes to write down the color of wires you used for what will save you a bunch of time should you have to troubleshoot this some time down the road. In addition, should the circuit consist of multiple components, you may also write down their location.

This won’t take you much time if you take a minute and document the info down as you go. Should you decide to sell the vehicle down the road, this will make the future owner’s day.

Well, this kind of brings me back full circle.  I began the book telling you what inspired me to write it to begin with—the wiring malady in my Olds. Funny how that worked out, but it was certainly fitting. I hope that you’ve enjoyed what’s between the covers.  With any luck, this book will find its way into your garage and soon be covered in greasy hand prints—nothing would make me happier than to know that I helped you get your wiring job done easily!

Next time a buddy tells you how much they dread wiring, just smile…as it’s now second nature to you!

 

Written by Tony Candela and Posted with Permission of CarTechBooks

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