How To Wire A Harness: An Ultimate Guide on Automotive Harness Wiring

How To Wire A Harness?

When you get a factory harness for your vehicle, you will get all the wiring essential for all the standard electrical components such as headlights, horns, windshield wipers, etc.

However, you sometimes want to add more electrical components to your vehicles, such as racing electronics or high-end audio.

If you want your systems to work at their full potential, you need to learn a few tips related to making the wiring of a harness easier and more effective.

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Round up the right tools

The wiring job becomes much more manageable if you pick the right tools. 

First of all, take good quality wire strippers. Mostly, wire strippers have several holes that can fit different wire gauges.

The wire strippers help strip the insulation off the wires properly without damaging the conductors inside. 

The second tool that you may require is a wire crimper. You can install solderless connectors with a wire crimper.

If you choose a quality crimper, it will have two-three sections on its nose to fit into various lug sizes.

Additionally, there are crimpers with wire cutting and wire stripping sections. 

Some connectors are difficult to crimp; thus, you need a soldering iron to splice and secure such oversized connectors.

Several solder types are designed specifically for electronic products and electric wiring.

They have active resin core flux, which cleans the wires on heating.

As a result, it removes any extra oxidation or corrosion on the wires.

When cleaned, you can solder the wires with lesser heat, making the connections strong and having less electrical resistance. 

Other than these basic tools, you should also have these items in your electrical toolbox:

  • A wire brush so that you can remove grease, oil, or heavy corrosion before soldering.
  • A voltmeter and an ohmmeter measure the circuits’ voltage and resistance levels.
  • A test light with which you can check the power in the circuit.
  • Shrink tubing materials for insulating connections from outer elements. You can insert this tube over the connector or the solder joint, which shrinks permanently on heating. 
soldering iron

soldering iron

Gauge your wiring needs

While choosing wires for electrical systems, you must consider three essential factors.

Size: You measure the wire size with its gauge. As the gauge becomes heavier, the wire becomes smaller, and vice versa.

Now, how will you decide on the wire gauge? You can choose it based on the current drawn in the accessory and the length of the wire between the power source and the accessory.

If the current in the accessory is high, you must take heavier gauge wires.

Caption: Wire Gauge to Wire Diameter Comparison Chart

However, when the wire is of longer lengths, you may face the issue of voltage drop.

But you can offset this drop by increasing its size. As a thumb rule, you should try to get less than a .5% of voltage drop to ensure the highest performance.

Wire materials: Generally, the wires are made from aluminum or copper. If you choose wires for automotive applications, you must take copper wire for higher flexibility and conductivity. 

Wire color: It becomes essential when you have to find out about any faulty wire.

Thus, it is always better to color-code all the wires based on the accessory.

Such marking will help you identify wires during installation and troubleshooting. 

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Make the connection

You can take either of the two connectors: soldered and solderless. 

You will need soldered connectors for oversized wiring or splicing the wires together. 

On the other hand, all other wires use solderless or crimp connectors. Solderless connectors are very easy to use and give a powerful connection.

These connectors come with color-coded insulators so that you can know which one to use for which gauge wire.

You can get various configurations in solderless connectors as follows:

  • Butt connectors: These cylindrical connectors are ideal for joining the ends of two wires together. You can insert each wire end into each end of the connector. After inserting, crimp the connection to secure it tightly.
  • Spade connectors: You must use this type of connector for all those components requiring removal or servicing quite often. Here, you connect the male connector at one end of the wire and the female at another. This male and female connector will fit into each other to form the connection. When you want any repairs, you need to pull the wires apart.
  • Ring connectors: You can fix the wires to screw-type terminals with these connectors and secure them with a terminal screw.

Whether you are using soldered or solderless connectors, it is always advisable to use heat shrink tubing to make the connection more secure and safe.

Heat shrink tubing is easy to install and protects against short circuits and other outside components.

Insulated female electrical wire crimps terminal connector.

Insulated female electrical wire crimps terminal connector.

Choose the correct wiring harness components.

When wiring a harness, you should take some overload protection to save your expensive electronics from getting damaged.

There are three main types.

How To Wire A Harness: Fuses

You can connect a fuse to a circuit.

Whenever more power is in a circuit than required, the fuse will blow, thus protecting the circuit from shorts. The fuses come in amperage ratings; the most common ratings are 3,4,5,6,7.5, 10, 12.5, 15, 20, 25, and 30 amperes.

It is advisable to use a fuse slightly higher than the rating of the accessory.

For example, if an electric fan has a rating of 19.5 amps, use a fuse of 25 amps.

How To Wire A Harness: Fusible links

Fusible links are another way of protecting the circuits.

These alloy-made wires have a melting point lower than regular copper wires.

You can splice the link in line with the accessory’s power line.

When there is electrical overloading, this link will melt and thus prevent the electrical power from reaching the accessory.

How To Wire A Harness: Circuit Breakers

The circuit breakers switch off the power to the electronic accessory whenever there is an electrical overload.

Once you restore the correct power, you can reset the circuit breaker.

There are circuit breakers with manual as well as automatic reset options.

They all come in different amperage ratings as fuses.

How To Wire A Harness: Relays

You can also use relays in the wiring of the electrical accessory if they draw higher currents than standard power rates.

As most switches have designs to work with limited currents, you need relays whenever you buy a new electrical accessory from the market.

Relays become more critical when using electrical accessories with high amperes, such as fuel pumps, electric fans, HID headlights, etc.

These accessories activate with an electrical coil and get controlled by a switch.

No power flows to the accessory if the relay is in a close position.

When you turn on the accessory switch, the relay opens and sends power to the concerned accessory.

Apart from these, several other components make your wiring easier, such as 

  • switch panels, 
  • push-button, 
  • toggle, 
  • remote-mount switches, 
  • multi-circuit main and auxiliary fuse blocks, 
  • and wiring harnesses.
How a relay works

How a relay works

Map out a plan

It would help if you always mapped out a plan before you started your wiring project. 

First, lay out the wiring and harnesses to understand the basic wiring scheme and whether you have enough wires to complete the task.

Put the fuse box in an easy-to-reach location, such as in the glove compartment or the center console. 

Now, put all the required relays, circuit breakers, fuses, and fusible links at the wiring connections between your electrical accessory and the power source. 

See if the wiring is labeled.

If not, label all the wires and harnesses according to the name of the electrical components they are routing to.

If any wire goes through the firewall, use a grommet so that sheet metal does not cut the wires.

Secure the connectors only when the wiring has been done through the firewall.

Pick a spot near or on the firewall for the cable harness’s ground point and another point for the chassis ground on the vehicle’s negative side.

With this, you get one path to the negative side of the vehicle and provide a more effective ground point.

Always use a gauge wire of 10 or more to connect the chassis ground to the common ground. 

Keep calm, stay organized, and take sufficient time to understand and manage things to complete harness wiring.

If wiring is done correctly, the installation time of the wire harness assembly process decreases rapidly.

Lay out the wiring harness

Lay out the wiring harness


Complex wiring is like a nervous system for your vehicle.

If not done correctly, one or more electrical accessories may stop working overtime. If you need any support, Cloom technicians are here to help you.

Our knowledgeable and expert team of engineers and technicians can guide you through the entire process.

Also, we manufacture high-quality custom wiring harnesses and cable assemblies for various application requirements. 

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Explore our custom services now. Email us at [email protected] for more details.

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