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Practical Automotive Wiring Tips & Tricks

Posted in How To: Electrical on December 26, 2018
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Photographers: Trenton McGee

Just like any other major system on a vehicle, wiring can make or break your build. Do it right and the entire electrical system is something that disappears in a vehicle, never to be thought about again. Do it wrong and you’ll be chasing gremlins, nonfunctioning gear, popped fuses, and even fires.

Some of the things wiring does is not easily understood, which may be why there are a lot of people out there who would rather get stabbed in the eye than mess with wiring. But the truth is that wiring really isn’t all that scary once you understand a few basic principles. And even if you don’t take the time to learn them, you can still be a competent wiring person by following a few basic rules.

We are by no means professionally trained electricians, but we’ve spent a fair amount of time with wire strippers and crimpers. We learn something new just about every time we do anything, from small projects to a full bumper-to-bumper rewire job. We have had the good fortune to spend a little time around some pros and have learned a lot from them, but a lot of what we’ve learned has come from trial and error along with good old-fashioned experience.

We covered a lot of the basics and principles of wiring in other articles, so this story is really more about making your wiring projects easier, cleaner, and more reliable. Some of this stuff may be familiar to you, but we’re willing to bet most of you will be able to pick up a trick or two to help make your own wiring projects better.

Most wiring problems are the result of a poor connection, whether it’s within a plug, a connector, or splice. And don’t forget to check your grounds. If you’re lucky, a bad connection will just break the flow of electricity and whatever it’s feeding will stop working. Worse are the bad connections that restrict electrical flow but don’t stop it. Restrictions mean resistance, and resistance equals heat. Enough heat equals fire, even on a circuit that’s protected by a fuse. If you understand nothing else about electricity, understand that good connections are critically important to a safe and reliable circuit.
There are two schools of thought when it comes to electrical connections. Soldered connections offer excellent conductivity, so there’s no resistance to electrical flow. However, soldered connections can work harden and break when subjected to extended periods of vibration, and they are more labor-intensive. This is why some wiring guys stay away from them. Crimp connections involve mechanically locking wires in a metal barrel. A properly crimped connection is more or less impervious to vibration but offers less conductivity. Therefore, a crimped connection introduces more opportunity for resistance and voltage drop.
Why not have the best of both connection types? We learned this method many years ago from Mark at MAD Enterprises. Start with an uninsulated terminal, which is available at most hardware stores but not often at auto part stores. Strip and crimp the wires like you normally would, and then apply solder to both ends of the barrel. Slide some adhesive-lined shrink tubing over the connection, and apply heat to shrink it. This method supplies the strength of a crimped connection with the superior conductivity of a soldered connection. You can do most other types of terminals in a similar manner.
If you’re wiring something with multiple connections in the same area, such as a pigtail for a relay or a headlight, whenever possible stagger the connections. If you put all of the connectors in the same place you end up with an unsightly bulge in the harness and a greater likelihood for shorts. Staggering the connections makes it easier to wrap the harness and just looks better. This is also why we like using uninsulated connectors and shrink tubing over the hard plastic insulated connectors—you end up with a smaller overall diameter connection.
It’s a pretty safe bet that a 4x4 is going to see mud and water, and an open rig can count of seeing mud and water everywhere. Water is bad for wiring for many reasons; therefore it’s a really good idea to use weatherproof connectors whenever possible. GM Weather Pack connectors are a great choice and can be found in a variety of different configurations, and shrink tubing is available with adhesive inside that flows out to the wire insulation when heated, sealing the connection from the elements. When in doubt, it’s hard to go wrong with anything marine grade. Marine and weatherproof stuff is more expensive, but it’s worth it in the long run.
It’s not always feasible on late-model vehicles, but whenever possible we like to use these military-style battery terminals rather than the regular parts-store terminals that you commonly see. These have several advantages. You can cleanly attach multiple large leads to the 3/8-inch through-bolt, and all of the cable ends themselves only need to be a conventional ring lugs. There’s a much more positive, solid connection with multiple cables than other methods, and should the terminal get corroded, replacing it will be easy. You can find them at parts stores, although it usually takes a good counterman and a book to find the right part number. Don’t mistake these for the cheap terminals that use butterfly bolts; we’ve melted plenty of those.
Speaking of battery cables, it’s not a good idea to run large cables through sheetmetal. The potential for cuts and abrasion is very high, and a compromised battery cable is a dead short as well as a guaranteed fire. A much better method is to use bulkhead connectors like these, which are studs molded into an insulator that provide a clean, safe way to pass through sheetmetal. We’ve used these to go through floorboards, trunks, and firewalls. The single-stud versions are handy when you have multiple leads that must be connected to the battery but have run out of space on the terminal itself.
Nothing screams “Hack!” like bundles of exposed wires going everywhere. Wrapping harnesses in wire loom helps protect the wires and looks much cleaner. But did you know not all wire loom is the same? Most of the stuff you get at home improvement stores is not rated for vehicle fluids and typical underhood temperatures. It gets brittle and melts easily. The short lengths they sell at auto parts stores is usually rated properly but is also ridiculously expensive. Most parts stores also sell different sizes in bulk that is much more reasonably priced, so ask the counterman. Corrugated split loom looks OK, but we really like the PowerBraid and Classic Braid kits from Painless Wiring. The kits come with a variety of sizes, and one of them is more than enough to do an entire vehicle.
Laying out the routing for a wire harness is tedious, but taking your time in the routing phase will yield a much nicer finished product. Break out the harness into sections, such as the dash, tail, engine compartment, front lights, and so on. Routing is rarely done by itself, but instead is done in conjunction with making connections. We use lots of zip ties at known anchor points when routing, even if the loom will later be secured by another method. This ensures that we’ll have adequate length to reach our anchor points when complete. We usually leave the sections of harness unwrapped until we’ve tested the circuits within them.
Invest in a good pair of wire strippers and crimpers. We like the ones that Klein makes, which also happen to be made in the USA. Good crimpers will have different sections for insulated and uninsulated terminals. Another tip: If the connector has a seam in it, be sure the seam is positioned in the concave portion of the crimper (middle). This reduces deforming the connector (bottom) and leads to a better crimp. This is important for both insulated and non-insulated connectors.
Make sure you use the appropriate wire gauge to handle the load for the circuit you are building. Using undersize wire will result in melted wires and other problems, while oversize wire adds unnecessary expense. When choosing wire gauge, think about both the amperage of the circuit and its overall length. The longer the run, that larger the gauge needed. There are gauge charts available online to guide you, and it helps to remember that the lower the number, the bigger the wire. Generally speaking in 12-volt automotive applications, anything under 10 amps can be wired with 18-gauge, up to 20 amps with 14-gauge, up to 30 amps with 12-gauge, 40 amps with 10-gauge, and up to 60 amps with 8-gauge. But again, this will vary depending on length and whether the load is continuous or momentary.
Late-model vehicles can make wiring accessories a challenge. Tapping into the factory wiring can cause warranty issues, and CAN-BUS systems add another level of complication. If you are planning on adding more than one or two accessories, investing in a stand-alone auxiliary fuse block is a good idea. There are solid-state versions such as this one that enable you to keep everything under the hood but which have a neat little digital switch panel that gets mounted on the dash. There are also more ‘analog’ auxiliary fuse panels available from several companies.
Securing a wiring harness is as important as wrapping it. Zip ties are better than nothing, but a cleaner, more permanent solution is using rubber coated clamps. Not only do they keep the harness out of harm’s way but they’re also stronger than zip ties and don’t get brittle over time.
Having a bunch of in-line fuses or individual fuse holders can get cumbersome and disorganized. If you have multiple high-powered circuits that need to be fused individually, consider combining them into one power distribution center. These modular fuse holders are manufactured by Eaton and can be used individually, or combined to hold up to 6 MIDI fuses on a common buss bar. MIDI fuses are a great choice when you need high-amperage circuit protection but don’t have a lot of room.
When wiring a car from scratch we like to make it easy to add more stuff later. These two buss bars are wired so that one is constant and the other is switched, so adding future accessories is easy and doesn’t require tapping into our finished harness. The aftermarket chassis harness we were using had power window and door lock circuits that the vehicle didn’t need, so we repurposed the circuits to power the buss bars. We’ve also used junction blocks with a single stud for the same purpose when space is tight.
When starting any major wiring project, gather up the manuals for all of the components you will be using and read them several times before you even make your first connection. In this case we were using a GM crate engine with a stand-alone harness, an aftermarket universal chassis harness, an aftermarket digital gauge package, and a dual battery management system. All of these individual components needed to be interconnected to some degree, and having a thorough understanding of what everything needs before you start can save a lot of confusion later.
When you are doing a full chassis rewire you will inevitably have to reuse some original switches that require original connectors. Some of the better aftermarket harnesses will include new original connectors like this GM headlight switch, but some cheaper ones tell you to reuse the original connector or, worse, to substitute individual spade connectors. We’ve grudgingly reused connectors when they’re in good shape, but doing so partially defeats the purpose of a rewire. If you’re going to be reusing some factory switches, take the time to track down the proper connectors if they aren’t supplied in your harness.
Passing all of the necessary wiring through the firewall is a pain, even when you’re starting from scratch. It can be hard to get a gauge on how big a hole you need, and fishing everything through it without ending up with a tangled mess is a challenge. We use factory holes whenever possible, and the fewer the penetrations, the better. Always use a grommet to protect the wiring from sharp edges, and always wrap everything in wire loom. We like to run all the wires unwrapped and leave some extra slack until we’re sure we’ve run all the wiring we need through the hole. Then we pull the extra slack, wrap them all in loom, and then push the wrapped harness back through the grommet. This way you avoid pulling everything back out just to wrap it when you’re done.
When mounting computers and other modules, it’s not a good idea to mount them horizontally as shown. The thought process is that even when the computer uses sealed connectors like this one, water that reaches the connectors can just sit there and eventually work its way inside and cause corrosion. By mounting the computer vertically, gravity will prevent water from pooling around the connector sealing surfaces and reduce the chances of contamination.


Mad Enterprises
Springville, CA
Painless Wiring

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