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Which Wiring Harness Do I Need?
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You might be wondering “which wiring harness should I buy” when shopping for a harness to go into your classic, your off-road buggie, your project car, etc. The answer comes down to two things:

  1. Circuits – how many do you need?
  2. Quality – how do you determine if the harness you’re looking to buy is well built?

Which is good news – it makes the choice of the exact kit you need pretty simple when you break it down. Here’s what you need to know.

Red hot rod truck

What’s a Wiring Harness, and What Does it Do?

An automotive wiring harness is a collection of wires that connects all of the electrical devices and components in the automobile to the main power source. Generally, harnesses include a fuse block and – at a minimum – 8 to 12 circuits.

What’s a circuit you ask? Basically nothing more than a designated wire for a specific purpose, like a set of wires for the headlights (2 circuits), a wire for the dome or glove box light (1 circuit), a wire for the starter, etc. The number of circuits you need is determined by the number of electrical components you have, with very simple vehicles (like an off-road only buggie) needing only a small number of circuits. The main difference between a 12, 18, and 24 circuit harness is mostly just compatibility with your vehicle.

Put another way, if you have a very basic vehicle (say, an off-road buggy that doesn’t have any brake lights, dome lights, etc.) a simple 12 circuit harness is A-OK. If you’re replacing the wiring on a classic car with a lot of luxury items, you might need a 24 circuit harness.

Determining Wiring Harness Quality

If a wiring harness is just a bunch of wires with connectors, how can you tell the difference between a good quality harness and a bad one? Here are a few key factors the differentiate good quality from bad quality:

See how the connectors are connected. Wiring harness manufacturers can attach wiring and connectors or fuse blocks in two ways:

  1. Crimping
  2. Crimping and soldering

The problem with option #1 – crimping only – is that crimped connections don’t always include every strand of wire. A crimped connection that only grabs half the strands of wire, for example, conducts far less electricity than a crimped connection that grabs every strand. Less conduction = more resistance, which means voltage loss. Many components won’t operate if the voltage is too low, or will work but will wear out prematurely.

By crimping and soldering connections, the conductivity of the connection is no longer a concern, even if some of the strands aren’t “grabbed” in the connector. Additionally, the combination of crimping and soldering is inherently more rugged than crimping only, and important consideration for vehicles used in racing or driven off-road.

Generally speaking, soldered and crimped connections are the only way to go. This is the standard used by all new vehicle manufacturers.

Find out if the harness is expandable. You want a wiring harness that’s easily expandable, even if you don’t plan to expand the number of circuits. Here’s why:

  1. In order for a harness to be considered ‘expandable,’ the harness has to be built to accommodate more amperage than the included number of circuits.
  2. The best way to make sure a harness can accommodate more amperage is to use heavier-than-needed gauge wiring.
  3. So-called “oversize” wiring is generally a good thing, as it ensures minimal voltage drop across each circuit, and it also ensures the harness will perform as expected in all situations (even in very hot temperatures).

Basically, expandable harnesses are “over built,” and that’s what you want if you’re buying a harness.

Ask technical support a difficult wiring question. Good tech support is important when you buy a harness, mostly because wiring a vehicle isn’t always a simple, straight-forward thing…especially if you’re doing some sort of custom build.

The specific questions you ask really don’t matter; what matters is the quality and confidence of the answer. Ask the retailer if they know how many circuits you need for your specific year, make, and model. Ask them if the harness is expandable, and if so, what the circuit limit is. Ask them if their connectors are aluminum (the cheapest, lowest quality option) or copper and brass (the preferred option). Ask them if the harness is compatible with a 100amp alternator (many are only rated for 60-80amps). Etc.

Silver hot rod car

Your Wiring Harness Shopping Checklist

As you browse for a new wiring harness for your car, keep in mind that “application” has very little to do with what you’re going to end up getting. Most importantly, you need to decide by circuits and quality, and if you’re not sure what you need, make sure the manufacturer can help you decide. In short:

  • Count the number of circuits you have in your vehicle/project car
  • Decide how important quality is to you
  • Check out each manufacturer’s tech support by calling and asking a wiring question

If everything checks out, you’ve found a great harness for your vehicle!

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