The last generation did more for aftermarket wiring than any that preceded it. While even the most serious do-it-yourselfer once clutched at the prospect of installing something as simple as power windows a mere decade ago, just about all of us can wire an entire car nowadays due to well-designed kits, comprehensive instructions, and precious technical assistance.
While the way we install our wires has improved by leaps and bounds, the same can't be said of the way we end them. All too often an enthusiast will tediously route and bundle a whole harness and simply kludge the terminals with poor-quality tools or the wrong tools altogether. Making matters worse, a poorly installed terminal, much like many a poorly welded joint, doesn't necessarily look shabby.
That's a potentially fatal pitfall since most electrical problems start at wire tips well hidden by the terminal itself. Above and beyond connecting the terminal to the wire and conducting electricity, the terminal installation must effectively seal the end of the wire. If it doesn't, oxidizing elements like the air around us will wick along the wire strands inside the terminal and corrode the wire well under the insulation. In the best-case scenario, a poorly installed terminal will eventually work intermittently or stop working altogether. On the other hand, since corrosion increases electrical resistance, resistance increases heat, and heat induces more resistance, a damaged wire can slip into a destructive cycle and fail with catastrophic (and blazing) results.
Automotive manufacturers over the past 50-odd years have consistently created effective and airtight terminal installations by crimping or deforming terminals' barrels to capture the wire strands. If there's a detractor to this process, it's that OEM-grade tools are incredibly expensive and require frequent calibration. Furthermore, OEMs use a type of terminal variety that is often pretty hard to find. While non-OEM-grade tools and consumer-grade terminals have been available to enthusiasts for some time, a lot of them-particularly the cheap ones-don't do a very effective job. Crimping has earned a pretty bad reputation as a result of failures due to inadequate tooling, chintzy terminals, or poor education. Making matters worse, just about all poor-quality tools and terminals are still available at even reputable auto-parts stores, perennially leading would-be electricians to failure.
Luckily for us, more and more specialty tool manufacturers and vendors like Molex, Klein Tools, Wire 1, and Snap-On offer high-quality electrical tools that bridge the gap between expensive OEM-grade and dime store-quality tools. While these tools cannot be considered OEM-grade since they don't feature the approved calibration mechanisms for mass production, Molex's Bob Grenke assured us that such exacting controls are of little consequence since we enthusiasts and professionals can take the extra time to verify a crimp's quality. He adds that most of these tools are fully capable of creating thousands of effective crimps over their lifespan if cared for properly.
To find the proper way to use these tools, we asked a few wiring specialists to tell us their own definitive way to fasten a common closed-barrel terminal to the end of a wire. Since we have a collection of professional opinions, this is the point where things potentially get sketchy. Naturally there are multiple schools of thought, and each one has its merit; however, some philosophies seem to contradict others.
The terminal's barrel is the section that actually meets the conductor material and is the area within the sleeves on insulated terminals. There are three basic barrel dimensions based on wire gauge, each with its own corresponding color. From left to right, they're 10- to 12-gauge (yellow); 14- to 16-gauge (blue); and 18- to 22-gauge (red). According to every technician we talked to, always choose a terminal barrel size range to match the wire gauge; an improperly sized barrel will prevent the essential wire seal and will likely fail. | 
All terminals feature a seam (shown here), a result of the manufacturing process. If given the choice, Ken Whitney said to pay the modest premium for brazed-seam terminals since they maintain their integrity under crimping pressure. Furthermore, personal experience proves that thicker barrel walls retain their crimp better. Here's a tip: While some owner/operator parts stores may or may not offer premium-quality terminals, assure yourself that franchise stores and discount tool outlets most likely do not (based on more personal experience). If at all possible, buy from a dedicated electronics vendor. | 
Strip enough insulation so the exposed conductor material matches the barrel's length and the insulation abuts the barrel. While most terminals feature a tang at the end of the barrel that prevents the conductor from interfering with the business end of the terminal, some don't. In such cases, trim the exposed conductor flush with the barrel's end or let it exceed the barrel by about the conductor's diameter. |