Here's a typical aftermarket...
Here's a typical aftermarket harness-a 12-circuit universal kit from Painless Performance. These days we're lucky enough to have many excellent companies making high-quality wares that allow automotive wiring to fall within most of our do-it-yourself comfort zones.
Every harness manufacturer...
Every harness manufacturer has slightly different features and options. For instance, Highway Series kits from American Autowire use modular, or stand alone, fuse panels, which means you route wires to the fuse block, not from it. This gives you more panel placement options, and some argue it makes for a more attractive wiring job.
Wiring-the mere mention of the subject brings chills to the spines of most do-it-yourself rodders. For many, it's part of the sacred trio of chores left to hired guns-paint, upholstery, and wiring. But for those willing to do some research, practice a bit of trial and error, and spend the time and effort required, any of these chores can be (and often are) completed by those of us with at least a modicum of technical know-how.
We've got a good thing going for us when it comes to wiring our beloved vehicles these days. A few smart men and the companies they founded offer us access to complete prefabricated wiring harnesses and related components that eliminate much of the headache, guesswork, and time involved in rewiring an old car. That said, we're going to proceed under the assumption that very few will approach a complete rewiring job without taking advantage of what these companies have to offer, and will attempt the procedure armed with appropriate components and instructions.
Now comes our end of the deal. Instead of trying to rewrite the informative instructions you're bound to get with any quality wiring kit, we're going to provide some information that will help you choose the right harness for your particular application. In the process, we also hope to arm you with the confidence and motivation to get the job done.
Our first tip is extremely important, and it's one you've most likely heard more times than you can count-read the instructions! Yeah, we know it's not in our nature to do such a thing until we're hopelessly dazed and confused. But when it comes to a job like this, you just have to swallow your pride, go hide, and read 'em front to back. (Hey, you can always deny it later.) Now that you've agreed to that, here's some terminology you may want to become familiar with.
Is space at a premium under...
Is space at a premium under your dash? Compact fuse panels, like this one from Wire 1 Hot Rods, use OEM-style mini fuses and are designed to fit where standard fuse blocks won't. Several companies also offer harnesses designed to mount the fuse panel in the truck or in other out-of-the-way places.
Some wiring harnesses use...
Some wiring harnesses use bulkhead fittings to pass through the firewall; others are designed to use a simple hole and rubber grommet....
...You'll want to think about...
...You'll want to think about which style you prefer when selecting a wiring kit.
It can be intimidating to...
It can be intimidating to look at a wiring harness as a whole, but focus closer and you'll notice that most kits label wires every few inches, making it easy to keep track of what goes where.
Volts, Amps, And Ohms
In order to fully understand how the wiring in your car works, let's take a look at the element we're trying to harness-electricity. The three most basic units in electricity are voltage, current, and resistance. Voltage is measured in volts, current is measured in amps, and resistance is measured in ohms. A simple analogy to help understand these terms are plumbing pipes. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size.
Getting a little more specific, voltage is the force that pushes electrons-or units of electricity-through a wire. It is a function of the electrical source, which, in our cars, is a battery. While some late-'40s and early '50s cars may still have an original six-volt system in place (sometimes even a positive-ground arrangement), it's safe to say that if you're rewiring your custom project, it's likely getting a modern 12-volt, negative-ground system.
Current is the volume of electrons moving through a wire or circuit. Measured in amps, it will be dependent on voltage and resistance. The resistance in any given circuit is a physical characteristic of the electrical components involved. For example, the size and material of a light bulb's filament will determine how much resistance is in the bulb. All components will have a certain amount of resistance, although the resistance in wires, switches, and fuses is typically very minimal.
When discussing resistance, the natural segue is to define conductors and insulators. Basically a conductor is something that current can pass through; an insulator is something it can't. Consider these scenarios: If you ground a stereo to a plastic dash, it won't work because the plastic is an insulator; ground it to a clean metal surface and it will work. Note that we say clean metal because paint can be an insulator.
Not to confuse the issue, but there are situations when something can be either an insulator or a conductor. Your body is an example. If you touch both terminals of an automobile battery, nothing happens because you have sufficient resistance that 12-volts can't overcome. But if you've ever grabbed the spark plug wire on a running engine, it didn't take long to discover than 20,000 volts was more than enough.
Some companies offer wire...
Some companies offer wire in bulk for those who want to augment their harness or build their own. Quality automotive wire differs from general-purpose hardware store wire; it's higher grade and has thinner insulation that is more heat resistant. These 25- and 50ft lengths from Painless are available in a rainbow of colors.
This chart gives good gauge...
This chart gives good gauge guidelines to follow when planning circuit additions or rewiring from scratch. Of course, aftermarket harnesses eliminate the guesswork by selecting the wire and setting it up for you.
Relays are musts in modern...
Relays are musts in modern automotive wiring. High-load circuits like headlamps and starters can and will overpower the ratings of most switches, degrading them over time. Relays basically cushion these switches from those high loads, letting them operate with much less stress and heat.
A typical aftermarket wiring harness consists of about 47 1/2 miles of wire, give or take a dozen or two miles. Thankfully they're usually different colors and labeled to boot. (GM color-coding is most common, but other color systems-and even all-black harnesses-are available.) Also included in most kits are a fuse panel, fuses, terminals, connectors, and typically a variety of relays. The instructions will tell you where and how to hook everything up, but may not spell out what everything is and does. So let's take a look at the various components and learn their purpose.
Wires And Terminals
Wires and terminals come in a variety of sizes for a reason. Larger-diameter wires have lower gauge numbers; smaller-diameter wires carry higher numbers (in other words, a 10-gauge wire is thicker than a 12-gauge wire). Higher-amperage circuits require larger-diameter wire and terminals with higher ratings. The same is true if you are running an extra long circuit or bundling multiple circuit wires together in a loom. Just like switches, these parts need to be sized to carry the appropriate amperage and need to be able to ventilate to prevent heat buildup. Contrary to some people's beliefs, soldered connections are not a necessity; a good crimp joint is fine. In fact, if you're not careful, it's very possible to overheat a solder joint, causing a brittle connection that could break due to movement and vibration. The OEMs use crimp joints for nearly everything, including many main battery connections.
Every piece in a circuit has the total amperage running through it and needs to be sized accordingly. The things to consider when selecting a wire size and insulation are the ambient temperature, the current the wire will carry, the total length of the wire, and how much voltage loss is acceptable. (There is always some loss-the longer the wire, or the smaller the wire, the more the loss.) As a general rule, sensitive circuits and headlights can tolerate 3-percent loss, and most everything else can tolerate 10-percent loss. Headlights are sensitive because the light output varies as the square of the voltage or more. So if you want the best light output you can get, use a heavier gauge wire and use a relay. Also, EFI, ECU, and stereo systems have high peak-current requirements that demand a larger wire size.
You don't need an array of...
You don't need an array of specialty tools to install a wiring harness. Some wire strippers, a crimping tool, and maybe a test light are typically all you'll require beyond basic handtools and drills.
Most wiring kits come with...
Most wiring kits come with insulated ring terminals and butt connectors (top). We tend to prefer un-insulated versions (middle), which, when used with heat shrink tubing, make clean, finished connections (bottom). Soldering connections is another option.
Breakers And Fuses
A circuit breaker is a safety device that's designed to protect an electrical apparatus from damage caused by overload or short circuit. You're probably most familiar with circuit breakers from the electrical panel in your home. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.
A fuse performs the same task as a breaker but is much smaller and must be replaced after it trips. The conductor inside the fuse is made of a metal similar to solder. It has a lower melting point than the wire itself. The size of the conductor is calibrated very carefully so that when the rated current is reached, enough heat is generated to melt the conductor and break the circuit. A blown fuse must be replaced with a fuse of the same amperage. If you've got a fuse that keeps blowing, find the problem. Installing a higher-amp fuse and hoping for the best can cause significant damage or fire. Nearly all aftermarket harnesses use modern spade fuses instead of the old glass tube style.
Fusible links are used in some harnesses, as well. Much like a fuse, fusible links are short lengths of wire designed to melt at a certain current rating. They are commonly used instead of an inline fuse in main power leads to prevent current spikes from damaging the fuse block or harness.
To solder or not to solder:...
To solder or not to solder: That question continues to be debated among those who string wire. A properly soldered connection is strong, but usually slower than crimping, and excessive heat (a common problem with those who have little soldering experience) can make connections more brittle and prone to breaking.
Some harnesses come with switches...
Some harnesses come with switches and other accessories, while others don't. Most companies offer them separately if you need them. Ron Francis Wiring recently introduced new ignition switches that use OEM-style keys in several colors.
In addition to wiring kit...
In addition to wiring kit instructions, we always like having a factory repair manual for the car being wired. The factory wiring diagrams can be a good reference, especially if you're retaining original switches and electrical components.
There are several auto wiring...
There are several auto wiring books available if the pluses and minuses still puzzle you after reading this story and researching manufacturer websites. We'd suggest "Hot Rod Wiring," from Wolfgang Publications; "Tex Smith's How To Do Electrical Systems," from the Hot Rod Library; and "Basic Auto Electricity," from Watson's StreetWorks.
A relay is a component that allows low current to activate a much higher current circuit without overloading a switch. They are commonly found in circuits for headlights, air conditioning, power windows, electric fans, or other components that require a lot of amperage to start and/or run. Another way to describe a relay is as a "switched switch" or "triggered switch" capable of handling higher amperage than the switch it's replacing. A typical relay only needs about 120 milliamps (0.120 amps) to be triggered and held closed, but can supply 30, 40, or more amps (depending upon its rating) to the load being fed. By making the relay the main switch in the headlight circuit and using the headlight switch as the trigger for the relay, we shift the amperage load off the headlight switch and onto the relay, which is designed to do the job. It's basically cheap insurance that increases switch life and prevents circuit damage. You generally only want to run one component from a given relay, but you can use a single switch to activate multiple relays.
Selecting A Harness
Now that we've laid out the electric fundamentals and functions of primary components, let's help you use that knowledge to select the best harness for your custom project.
We're going to proceed under the assumption that you want more than just a stock replacement harness for your custom. Besides typically costing more than universal harnesses, OEM-style harnesses tend to use outdated technology and lack the circuits and capacity necessary for the accessories we add on. In some instances (Tri-Five Chevys, for example) you can get specific-fit aftermarket harnesses that fit in original locations and feature additional circuits, spade fuses, and other modern elements. Universal-style harnesses that can be tailored to the needs of the project car at hand, however, will better serve most modified '50s and '60s vehicles.
One of the first things you'll need to consider is how many circuits you'll need. Twelve circuits seem to be a standard baseline for aftermarket harnesses, but it's common to see kits with 15, 18, and even 20 or more circuits. How many circuits do you need? Twelve are plenty for basic custom and rod projects without a ton of accessories, but you may want to step up to 18 or more if your car is going to have power windows and locks, electric seats, or other creature comforts. It's generally easier to work with too many circuits than too few; extras can always be tied off for possible use in the future, or cut out completely.
Another key consideration has to do with harness construction and fuse block configuration. Some aftermarket harnesses come with all connections pre-terminated to the fuse block; others come with the various wiring harness bundles loose, leaving the installer to route them to the fuse block as he sees fit. While some may argue that the latter style is more labor intensive, others feel it offers more flexibility and allows for a cleaner installation. It's largely a matter of preference, but it's something to consider as you're shopping around and exploring your options.
Aftermarket fuse panels come in varying sizes and shapes, too. And while space is usually not as tight in '50s and '60s cars as it may be in earlier vehicles, you'll want to think about your mounting location and take some measurements before making a decision. The nice thing about most standard-sized fuse blocks is that they have provisions for mounting necessary flashers and relays. Compact fuse panels often require you to find remote mounting locations for such items. Nothing says your fuse panel must be under the dash, either. Several companies offer harnesses with extra-long wiring that allows the fuse panel to be located under the package tray or in the trunk.
Finally, it's always smart to find out what types of extras and accessories come with your wiring kit. Does it have all the necessary flashers, relays, and connectors you'll need? Are there any switches (ignition, headlight, etc.) supplied in the kit? Do you want or need them? You don't want to pay for more than you need. On the other hand, that "bargain" wiring kit may not be such a good buy if you have to purchase necessary components separately.
What You'll NeedAnyone who's going to rewire their car, or even perform routine electrical repairs, should invest in the proper tools. You don't need a toolbox full of sophisticated electrical equipment to install a typical aftermarket wiring harness or wire in a new stereo or other electrical accessories. Quality wire strippers, terminal crimpers, a good test light, and soldering gun are the few essentials, and they sure beat the heck out of stripping wire with a penknife (or your teeth), crimping terminals with a pliers, assuming a circuit is live, and soldering with a cigarette lighter.
It's always a good idea to have plenty of terminals and connectors-in a variety of sizes-on hand before starting a wiring job. Some kits supply an abundance, others only a few, but it's always easier to get them ahead of time instead of having to stop your work and run to the parts store. You'll also want to make sure you have a good supply of heat-shrink tubing, solder (if you so choose), and electrical tape. Oh, and you can never have enough zip ties. Trust us on that last one!
There's plenty of literature available on automotive electrical systems if you'd like to do more research. The Web sites and tech lines of the companies in our source box can provide a wealth of information, too. We'd also recommend investing in a repair manual-one with good wiring diagrams-for your particular vehicle. It's not only great for general reference, but can be vital if you're keeping any of your car's original electrical items (switches, ignition components, A/C, etc.).
All things considered, the aftermarket has taken much of the mystery out of re-wiring old cars. That's good, because doing your own wiring not only saves money and provides a sense of accomplishment, it also makes it easier to troubleshoot and make electrical repairs in the future. If you don't believe us, get out in the garage and give it a try. You may not be an electrical expert when you begin, but you may just feel like one by the time you're done.