Automotive Wire Harness Systems: what to consider when purchasing a harness kit

Like the human body’s blood vessels and nerves, automotive wire harness systems deliver electricity and signals through the car, enabling various functions. Read on to learn more about how they work, their electronic features, and what to consider when purchasing a harness kit. 

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How Does Automotive Wiring Harness Work in the Car Electrical System?

Electric vehicles have a closed circuit system with an independent power source (battery). The system typically operates on a minimal fraction of the power of an ordinary circuit.

Besides the primary circuits that are the ignition, starting, and charging, other auxiliary circuits power electric motors, lights, heating elements, radio, doors, sensors, and gauges electronic appliances. Also, there’s a relay (A remote switch operating under electromagnets) or a switch operating each of these circuits.

Current flows through a specific cable to the appliance and back to the power source via the vehicle’s metal body. Current flows from the positive terminal to the functional component during the operation. Next is earthing the vehicle’s body to the component and then earthing it to the negative battery terminal. Connect the car’s body to the battery via a thick cable for safety and functional purposes.

You measure current strength in amps, while the pressure that moves it round the circuit in volts. Most modern vehicles feature a 12-volt battery whose capacity you measure in amps per hour. For example, a 56-amp per hour battery can supply a current of 1 amp per hour or 4 amps for 14 hours. However, if the voltage drops, resulting in lower current flow, it could cause some advanced features to fail.

Ensure you complete every circuit with cables and wires. The automotive wire system is an organized set of wires, connectors, and terminals running through the car, relaying electric power and information. To summarize, the auto wire harness system plays a vital role in the car’s overall performance and reliability.


Caption: Alternator

Standard automotive wire harness systems

Often, a typical automotive harness comprises around 12 circuits, each with wires of different lengths, to perform specific functions. Every one of the circuits is assigned a fuse holder in the fuse block. 

The 12 circuits include the horn, wipers, distributor, radio, gauges, parking lights, turn signals, tail lights, dash lights, hazard flasher, stop lights, and heat and AC. These circuits serve as a specific accessory in your vehicle. 

However, some electric vehicles have 18 circuits. The additional six accessories could include; power locks, an electric fan, power windows, high-level stop light, radio B+ memory, and electric fuel pumps. These serve any additional electrical components beyond the primary.

There’s also the 24-circuit option that offers additional support for accessories such as the clock, trunk light, under hood light, lighter, dome light, and glove box light.

Therefore, the number of circuits your vehicle model requires depends entirely on the number of electrical components available in your car. 

Car Distributor

Caption: Car Distributor

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Classification of an Automotive Wiring Harness System

An automotive wiring system typically connects the engine (start, ignition, power generator, and EFI), lighting, AC, instrumentation, and advanced auxiliary features to the power source.

The primary wiring harness in the vehicle considers the instrument panel as the central part, extending before and after it. It comprises the front wiring harness (engine, instrumentation, battery, AC, and front light assembly), rear wiring harness (license plate light, luggage compartment light, and tail light assembly), and roof wiring harness (audio speaker, roof light, and door). 

Features of an Automotive Wiring Harness System

When designing automotive wiring harnesses, manufacturers must consider the following features.

Current, Voltage, and Resistance

Resistance is the extent to which a wire resists the flow of current. It’s measured in ohms.

Thicker electric wires conduct more electricity than thinner wires because there’s more room to facilitate the movement of electrons.

The extra energy required to force current flow through resistance gets converted to heat. This heat proves helpful in the thin filament of a bulb glowing white hot.

However, it’s essential to ensure that you don’t connect a component demanding a higher current with wires that are too thin as they will likely overheat and blow a fuse or burn out. Generally, the amount of power consumed is determined by multiplying the volts with the amps.

Positive and Negative Polarity

From the battery, electricity flows only in one direction, and various components work only if the current flows in the correct direction. This flow of electricity in one direction is polarity. 

Often, vehicle models have the negative battery terminal earthed, while the positive terminal feeds the electrical system. Fitting equipment with incorrect polarity damages the set-up.

Short Circuits and Fuses

Using the wrong wire size or a disconnected or broken wire can cause a short circuit to pass through the resistance of the electrical appliance. If the current passing through the wire reaches dangerously high levels, it can melt the wire, causing a fire.

It’s common to find the fuse box located beneath several components. And the most common type of fuse is the short and thin wire wrapped with a heat-proof casing often made of glass. The fuse wire is usually the thinnest wire size that can facilitate normal current flow in the circuit without overheating.

A short circuit causes the fuse wire to blow, melt, or break the circuit in case of a sudden high current rush. When that happens, it indicates a disconnection or short circuit that’ll need you to install a new fuse of the correct amp rating.

The system has several fuses, each responsible for a small group of components, to prevent the entire system from collapsing because of a single blown fuse. The fuses are then grouped in the fuse box, although there might be line fuses in the wiring.

Series and Parallel Circuits

Most circuits have more than one component, such as the bulbs, in the lighting circuits. It matters how they’re connected; in parallel, side by side, or in a series of one after the other.

For instance, manufacturers ensure headlamp bulbs have a degree of resistance. It usually enables the bulbs to consume a set amount of current to glow. There are two headlamps in the circuit, which, if connected in a series, the electricity would have to flow through one headlamp to get to its partner. However, if you connect the bulbs parallelly, the current would only pass once through each bulb. Therefore, the current would encounter double resistance, cutting the current into two, causing the bulbs to glow feebly.

Ancillary Circuits

The starting motor has a heavy cable running from the battery directly. The ignition circuit provides the high-tension impulses to the sparkplugs and the charging system that includes the generator to recharge the battery. 

All the additional circuits are referred to as ancillary circuits. You wire these to the ignition to function when it is turned on. That prevents unknowingly leaving any component on, which would drain the battery. However, you can leave the tail and side lights on when the car is parked since they’re wired to the ignition switch independently.

Always wire any additional harness accessories that consume heavy power through the ignition switch. Some subsidiary accessories can operate without having to turn on the ignition switch. For example, you can wire a radio through a switch to allow it to function even when the engine is off.

Wires and Printed Circuits

Manufacturers categorize cable systems and wire sizes according to the max amp they can safely carry. With the automotive wire harness network having numerous complex wires running through, a color code is essential to avoid confusion.

The service manuals and car handbooks include a wiring diagram that could prove difficult to follow. However, with the help of the color code, you can easily trace a wire. Wires running close to each other are likely to bundle up. You can prevent bundling by using a fabric or plastic sheath to keep them in place.

Modern cars need more room to fit countless wires in small spaces. Today the automotive industry uses printed circuits as a choice of wire over bundles of wire, especially at the back end of the instrument panel. The printed circuits are plastic sheets printed with copper tracks to facilitate the direct plugging of components.

7 Things To Look Out For When Purchasing a Wiring Harness Kit

Before purchasing a wiring harness kit, look for a few things.

Soldered or Crimped Connections?

Wire harness manufacturers only connect wires to the switch and fuse block by either soldering or crimping

Crimping is more method because grabbing every strand of wire is challenging. Also, crimped connections overheat easily and could quickly move out of place. However, with soldered connections, you won’t face such challenges.

Is it compatible with Modern alternators?

A universal wire harness system is often rated as a 60 to 75-amp alternator. That’s where the problem lies since most alternators are 100 amps. Therefore, connecting a 60 to 75 amps harness to a 100 amps alternator will likely overheat and fail.

Is it compatible With Modern Lights?

Manufacturers design most harnesses in a way that they facilitate a standard sealed beam incandescent light. However, older models have halogen-sealed beam replacements. The difference in wattage between the two is rather significant (twice as much). If you use an 18 or 20-gauge wire, it will likely overheat and result in dim headlights. A 14 gauge wire is considered the best choice for such instances.

Automotive Wire Harness Systems: Can I Expand the Harness?

As previously mentioned, harness systems facilitate a set number of circuits. Therefore, to add a circuit, the first step is replacing the fuse block and then upgrading the wiring. In some cases, you might have to replace the entire wiring harness system as it’s impossible to expand the circuit.  

It’s advisable to get a harness capable of handling an expansion of a few more circuits if the need arises.

Do I Go For Copper Wiring or Other Alternatives?

Instead of using 100% OEM quality copper wire, most wire harness manufacturers use a copper alloy. Granted, you’ll spend less with copper alloy wires, but the voltage will drop over the system is a common occurrence. 

It’s advisable to have your terminals made of brass or copper for better quality instead of using cheaper alternatives like copper alloys, tin, or aluminum.

Automotive Wire Harness Systems: Is the Wire Enough?

Reducing the length of the wire will help save on costs. However, it’s much simpler to shorten a wire than to add additional cables. Therefore, ensure that your choice of wires is long enough for your entire project.

Automotive Wire Harness Systems: Will Hot Starts Pose an Issue?

When the wire energizes, the starter cannot carry the sufficient current needed by your starter, the temperatures increase to over 1200C, the starter cannot draw enough power, and the car fails to turn over. 

It’s best to use a 12-gauge wire to stimulate the starter. However, most manufacturers go wrong on this by using a 16-gauge wire.


A car’s wire harness system is integral to how a car performs and how reliable it is. Be sure to consider the factors mentioned above when selecting a harness kit. We at Cloom Tech are experts in automotive wiring harnesses. So if you need help with cable assemblies, feel free to contact us

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