Here's a shot of a typical...
Here's a shot of a typical aftermarket harness assembly. These days we're lucky enough to have excellent companies manufacturing complete high-quality wares that allow automotive wiring to fall within most of our DYI comfort zones.
Wiring. The mere mention of the subject brings chills to the spine of most performance car do-it-yourselfers. For most, it's part of the sacred trio of chores usually left to hired guns: bodywork/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 are often) completed by those of us with at least a modicum of technical know-how.
We've got one good thing going for us when it comes to wiring our beloved vehicles these days-a few smart men and the companies they represent that offer us access to complete prefabricated harnesses and related components which eliminate much of the heartache and labor involved in the wiring process. That said, I'm going to proceed under the assumption that very few will attempt a complete rewiring job without taking advantage of what these companies have to offer and will attempt the procedure armed with the components and accompanying instructions provided with said components.
Instead of trying to rewrite my version of the instructions you got with your wiring kit, I'm going to try and provide some usable information, tips, and tricks that will hopefully arm you with the confidence and motivation to get the job done.
My first tip is extremely important and one you've most likely heard more times than you can count: read the instructions! I know, it's not in our nature to do such a thing, that is, 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 from front to back. Hey, you can always deny you read 'em when the job is done. Now that you've agreed to read the instructions before you start, here's the additional information.
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 neat 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.
The Components
For the most part, the harness consists of about 47 1/2 miles of wire, give or take a mile or two. Thankfully, they're different colors and labeled to boot. There's also a fuse box and normally a bunch of relays (little black cubes with spade terminals sticking out) as well. The instructions will tell you what circuit they're for and what wires to hook up to them; but what are they for and how do they work?
Relays are a must in automotive...
Relays are a must in 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.
Relays
A relay is a gizmo that allows low current to activate a much more powerful current without overloading a switch. Circuits, like those for headlights, air conditioning, or power windows for example, require a lot of amperage to start and/or run. Another way to describe it 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 then 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.
Circuit breakers are good...
Circuit breakers are good insurance. They operate much like a fuse (protecting systems from voltage spikes and shorts) by breaking a circuit before massive damage occurs.
Breakers
A circuit breaker is a safety that's designed to protect an electrical apparatus from damage caused by overload or short circuit. 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.
Here's a shot of three different...
Here's a shot of three different types of circuit protection: the age-old glass fuse, the modern spade-type fuse, and an inline breaker.
Fuses
A fuse performs the same chore 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 so 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 don't just install a larger amperage fuse or you'll be sorry.
This chart gives us a great...
This chart gives us a great rule-of-thumb to follow when planning circuit additions or rewiring from scratch. The manufacturers of aftermarket harnesses of course construct their harnesses to exacting standards.
Wire And Terminals
Wires and terminals come in a variety of sizes for a reason. Higher amperage circuits require larger diameter wire and terminals of higher ratings. The same is true if you are running an extra-long circuit or bundling many circuit wires together in a loom. Just like the switches, these parts need to be sized to carry the amperage and need to be able to ventilate to prevent heat buildup. Contrary to some folk's belief, soldered connections are not a necessity (though, I do prefer them), 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 solder connection that could break due to movement and/or vibration. The OEM's, as a rule, use crimp joints for everything including many main battery connections without a problem.
Again, every single 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 be carrying, 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 a high peak current requirement that demands a larger wire size.