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A Beginner's Guide to Vehicle Wiring

Written by

CarParts.com

Staff Writers

Updated on May 23rd, 2022

Wires provide a path for electricity to flow from one point to another. Without a vehicle's wiring, many critical systems won't function.

In this article, we're going to talk about the different types of wires in a vehicle, their functions, and their importance to the parts of the circuit that they connect to one another. We'll also tackle the common symptoms of faulty wiring and identify the possible causes of wire damage.

Understanding Wires

There are several types of wires located throughout a vehicle. Some wires simply provide a pathway for power or ground to certain components, while other wires carry a variety of digital and analog signals to parts like the powertrain control module (PCM). The wire's intended use largely determines its characteristics.

Wire Properties

Depending on a wire's function, characteristics like length, composition, size, and how it's bundled can vary. These characteristics are important because a wire can only conduct a certain amount of electrical current. If too much current passes through a wire, it might overheat and melt.

Length

The overall electrical resistance of a wire is directly proportional to its length. This is why a longer length of wire will encounter more resistance and result in more energy lost.

Bundling

Typically, a wire is bundled in order to organize it. However, how a wire is bundled also impacts its ability to dissipate heat. If the wire is in a bundle with a bunch of other wires, it can carry significantly less current than if it were the sole wire in the bundle.

Material

Copper and aluminum are the two most common materials used in automotive wiring. Copper is more conductive, flexible, and less likely to corrode than aluminum. However, copper is heavier and more expensive. Aluminum, on the other hand, is lighter and less expensive, but it's less robust and is more likely to corrode or develop electrical resistance over time. Nevertheless, copper wiring is used in a majority of automotive applications.

Size

The resistance of a wire is also determined by its wire gauge or size. The larger the wire, the less resistance it will create. Wire sizes are determined by a wire's gauge. The larger the wire, the smaller its gauge. For example, a 16-gauge wire is larger than a 24-gauge wire. Since larger wires have more conductive material, they'll also be able to conduct electricity with less resistance. This is because the amount of electrical resistance in a wire is indirectly proportional to its cross-sectional area.

Wire Colors

Automotive wires are available in a wide range of colors. A wire is typically assigned a color depending on its function. This makes repairs easier because it helps the mechanic identify an individual wire's function from a group of wires in a wiring harness.

However, there's a common misconception that all wire colors are standardized and that each color corresponds to a specific function. In reality, the function of a wire color will vary depending on a vehicle's year, make, and model.

If you want to learn about the function of each wire color in your vehicle, then you have to read your vehicle's electrical diagram in its service manual. Service manuals are issued by the manufacturer and can be found reproduced in repair manuals and repair databases.

Types of Wires

There are several types of wires that are used in different applications in a vehicle. Identifying the right type of wire can be confusing. However, it's critical to make the right choice because using the incorrect wire type can result in failures and fires.

GPT or Primary Wires

This is the most common wire found in auto shops. GPT stands for General Purpose Thermoplastic, and it's the most common type of automotive wire. It has a multistrand core and flexible insulation, which allows it to pass through tight spaces. Unlike other wire types, GPT wires aren't designed to withstand prolonged exposure to outside elements. This is why GPT wires are primarily used for interior applications.

GPT wires can function in temperatures up to 176ºF although some are also rated at 221ºF. These types of wires are resistant to oil, chemicals, and acids.

Motor Wire

The main difference between GPT and motor wire is that motor wire has finer strands of copper wire inside its insulation. This enables it to withstand higher voltages without burning up. Motor wires are rated to withstand 600 volts and temperatures of up to 221ºF.

TXL, GXL, and SXL Wire

Thin-wall cross-linked (TXL) wires are stranded, single-conductor copper wires with cross-linked polyethylene insulation. SXL wire contains a bare copper conductor and cross-linked polyethylene (XLPE) insulation. Meanwhile, GXL wire is a thin-wall, single-conductor primary wire that also features XLPE insulation.

The three wires' cross-linked polyethylene jacket is more resistant to heat, abrasion, and aging than GPT wire. SXL wire is frequently the favored choice in high-stress applications such as racing or industrial vehicles. This is a good option if you need more durability.

TXL wires are the same as GXL and SXL, but with thinner insulation. A wire's insulation thickness can vary depending on where the wire will be installed. Thicker wires mean better durability. However, using the thin TXL wiring is great for racing vehicles because a vehicle can save around six pounds of weight if they use this type of wire. GXL wires have thicknesses that are in between SXL and TXL wires.

Battery Cable

A battery cable is mainly used to connect the battery to the vehicle's electrical system (typically to the starter) and ground. It's a heavier wire with a smaller gauge.

Speaker Wire

Speaker wires are used to carry the audio signal from the head unit or stereo receiver to the speakers. Speaker wires are intended for low voltage use. It's important to install the wire's positive and negative terminals at the appropriate locations. Otherwise, the speakers won't sound right.

Trailer Wire

Trailer wire is usually just GPT or primary wire that has been colored and designed for trailer use. Most trailers use a similar coloring system that includes green, yellow, brown, and white.

Signs That Your Vehicle Has Damaged Wiring

As previously mentioned, wires supply a path for electricity to travel between the various parts of a circuit. When wires develop issues, the vehicle will typically show symptoms that correspond to the connected part. These are the most common signs that there's a problem with your vehicle's electrical system:

Electrical Components Don't work as Designed

Electrical components that either don't work as designed or don't work at all are the most common sign of damaged wiring. Wiring issues can cause a variety of electrical problems, ranging from a radio that doesn't work to a car that won't start (more on that below).

Starting Issues

If your engine won't start, it could be due to a wiring issue, such as a damaged wire in the starting circuit or corroded PCM ground wire.

Fuses Keep Blowing Out

A fuse is designed to blow when there's excess current flowing through a circuit. Wiring that's shorted somewhere can cause excess current to travel through a circuit, resulting in a blown fuse.

Short Circuits

Wiring can become shorted to either power or ground. Depending on how and where the wiring is shorted, the components within the circuit might either operate erratically or not operate at all.

Open Circuits

An open circuit can occur when the wiring is severed, preventing current flow from one end of the circuit to the next.

High Resistance

High resistance can impede current flow, causing the electrical components within the circuit to either work poorly or not work at all. Issues, such as corroded or poorly connected wiring, can create high resistance.

As mentioned previously, a faulty wire can cause components like your radio to fail, but it can also prevent other important components like your tail lights or turn signal lights from functioning. Faulty wires can also prevent your starter motor from starting your vehicle. Wires are just threads of metal covered by an insulator. They're very basic components doing a relatively simple task, but vehicles depend on them a lot to function properly. Wires are key components of your vehicle, so make sure to repair any faulty wires as soon as possible.

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