Everything You Need To Know About Wiring Harnesses For Your Classic Truck

American Autowire’s Worry-Free Wiring Harnesses

By Ron Ceridono – Photography by the Author

For many classic truck fans, there is nothing more mysterious than an electrical system. After all, how can you understand something you’ve never seen—that is unless you’ve walked across carpeting in your socks, grabbed a doorknob, and saw a spark. Of course, you don’t have to see electricity to know it exists—if you’ve been zapped by the ignition system of a running engine you know it’s real.

- Advertisement -
02 We choose the Highway 22 Wiring harness kit from American Autowire for a custom installations
American Autowire’s Highway 22 harness is ideal for custom installations that incorporate a long list of electrical accessories. It provides 22 separate circuits and includes electric fan and horn relays along with an LED flasher for four-way and turn signals.

Because of the theoretical nature of electricity, it can be hard to know where to start when replacing a dilapidated old wiring harness or wiring a truck from scratch. At one time the common approach to solve either issue involved some sort of diagram, spools of wire (often all the same color, which made troubleshooting impossible), a variety of butt connectors, and lots of electrical tapes. The results were seldom satisfactory, often trouble-plagued, and sometimes even dangerous.

03 Classic update harness is perfect if you have more Electrical accessories
The Classic Update harness is a modular design with wires attached to the fuse block. Provisions are made for additional electrical accessories when compared to an OEM system.

Read More: Classic Car Restoration: 1931 Model A Ford

A better, safer, and much easier to install alternative is a complete wiring system like those available from American Autowire. But before we get into selecting a wiring harness there are a few electrical terms you should be familiar with:

- Advertisement -

Voltage: Electrical pressure, or the push that sends electrons through a wire. Measured in volts, 12V has greater electrical pressure than 6.

Current: The volume of electrons flowing in a circuit. It’s measured in amperes or amps. If voltage is increased more electrons flow.

Resistance: The opposition to current flow. Resistance is like electrical friction; it’s measured in ohms and is represented by the symbol omega (Ω).

Conductor: Something that current can flow through. Some conductors are better than others. Copper has less resistance than aluminum, so copper is a better conductor of electricity.

- Advertisement -

Insulator: Something current can’t flow through, like the porcelain on a spark plug or the insulation wrapped around a wire.

04 They offer a factory fit series _60 Chevy truck from American Autowire
For those looking for originality, American Autowire offers the Factory Fit series. This example is for a ’60s Chevy truck and uses glass fuses. Most American vehicles went away from glass fuses in the early ’80s.

Types of Circuits

Circuits are the paths the electricity follows. In an automotive application, electricity travels from the source (the battery) via conductors through a switch to the load (the device that uses electricity) to ground (the metal body or frame) that’s used as a return path to the battery.

Closed Circuit: The preceding is an example of a complete circuit—there is a return path to the source, as a result, electricity flows through the circuit and operates the load.

Open: An open circuit is one that is not complete and lacks a return path to the source—a switch opens and closes a circuit and shuts off the load. A broken wire or a burned-out bulb also opens a circuit.

Short: A short is a complete circuit that goes to the ground before the load. The danger is the resistance in the circuit may be drastically reduced. As a result, current flow may increase, creating enough heat to melt the wire, or in the worst-case scenario start a fire.

05 Classic update series wiring harness kits like this one from 1957-1960 Ford kit
Some of the Classic Update Series wiring harnesses, such as the 1957-1960 Ford wiring harness kit, come with a compact ATO-style fuse panel that is fully encased in a closed box. The fuse coverlid is labeled on the inside, identifying all circuits and their fuse ratings.

Read More: What is a Neutral Safety Switch and Why it is Important

There are a number of considerations when selecting a wiring harness. Certainly one of the most important is the number of circuits required. Obviously, that will depend on the electrical accessories the truck is equipped with. However, keep future additions to the electrical system in mind and plan ahead for them. Not all the circuits have to be used until they are needed—as an example, adding air conditioning, later on, will be easier if there is a circuit available to power it.

06 These are the factory open panel with glass fuses but American Autowires offer replacement panel
Speaking of fuses, this type of open panel with glass fuses was common on very early trucks and often found as a replacement panel as well. It’s something that should be replaced.

An additional issue to think about is the ease of installation. A direct replacement wiring harness, like American Autowire’s Factory Fit series for GM trucks, will be the simplest to install. Based on original factory blueprints, installing these kits is basically a remove-and-replace operation. Few, if any, wires have to be terminated and factory-style plugs and light sockets make these truly plug-and-play harnesses. Like the factory wiring harness, there are subcircuits available (engine compartment, dashboard, rear body) that allow portions of the original wiring harness to be replaced if needed. Of course, complete kits are also available and there are options available to include minor modifications, such as internally regulated alternators and HEI distributors.

07 For ease of installation these are color coded and labeled for GM vehicles
For ease of installation Classic Update and Highway, series kits have GXL/XLPE wire with GM color-coding and labeling every 3-4 inches.

For those building a mildly modified truck, American Autowire offers their Classic Update Kits. These kits provide more circuits than a factory wiring harness replacement to power options such as air conditioning, power windows, electric fuel pumps, and even EFI systems. But while these kits provide additional circuitry they are designed to incorporate original switches, heater controls, taillights, parking lights, and factory gauges. The American Autowire Classic Update kit comes with a modern, ATO-style fuse box with the majority of the wires already connected to the fuse panel (terminals are then attached to the wires at the loads). Extra-long wire lengths in the engine compartment and rear body sections allow for custom routing of the wires.

08 The highway 22 plus kit is perfect for classic cars with extra electrical accessories
The Highway 22 Plus kit provides enough circuits for the most accessory-laden electrical system. The fuse block includes a 20-amp relay that can be easily changed to function for either keyed 12V or ground-triggered operation.

If a custom wiring harness is what you’re after, American Autowire offers a variety of options with their Highway Series Universal kits. For trucks with large electrical demands, the Highway 22 Plus, Power Plus 20, and Builder 19 kits are available (the numbers indicate the fused circuits available). For trucks needing fewer circuits, the Highway 15 or Power Plus 13 kits are more than adequate to power all the must-have items, with a little extra capacity for adding accessories later on. For a system that just provides the basics, there is the Route 9 wiring kit. (Recently introduced is the Highway 15 Nostalgia kit that includes ’50s-style braid and lacquer coated wire with GXL wiring underneath).

09 Modern fuse panel for classic cars for wiring harness kit
Highway 22 Plus and Highway 15 fuse panels are not prewired. The wires are cut to length, the ends are then stripped and attached to the numbered and labeled screw terminals in the panel. This style of harness offers the most flexibility in terms of fuse block location.

Read More: Installing a Lokar Floor Mount Shifter & E-brake

As well as the number of circuits, there are also differences in the way the various Highway Series harnesses are configured. The wires in the 22 Plus and the Highway 15 kits are cut to length and then attached to the fuse blocks with screw terminals. This design provides the utmost flexibility to mount the fuse block and route the wires (Highway 15 uses a smaller fuse block). The other kits in the series have wires attached to the fuse block.

10 American autowire offers professional wire crimpers for the perfect harness set up
American Autowire offers two professional wire crimpers—a single crimper for 14- to 20-gauge wire and a double crimper for 10- to 18-gauge. Avoid using cheap, flimsy crimpers, and never use pliers to make a crimp.

Installing any wiring harness takes time and patience. American Autowire emphasizes all connections must be clean and tight to avoid corrosion, resistance, and arcing (the most important places to check for poor connections are the battery, alternator, and starter). They also point out the main cause of most automotive electrical problems is caused by bad ground. Never rely on just the negative battery cable alone as the only ground; always incorporate a separate ground wire or cable from the engine block to the chassis. Choose the proper American Autowire kit for your application, invest in a good crimping tool, follow the instructions, and your classic truck’s wiring will be worry-free.

American Autowire
(800) 482-9473

- Advertisement -

Related Articles

Search Our Site

More Classic Truck Performance

Daniel Stewart’s 1967 C10 Lookin’ Like a Million Bucks

By Fuelish Media You don’t see a killer truck with...

LMC Bed Box Assembly for an F100 Flareside

By Ryan Manson  -  Photography by the Author Sourcing the...

Todd & Natasha Collins’ Kruzin Kustoms–Built 1971 C10

By the CTP Staff   -   Photography by Aaron Mai Bought...

2023 C10 Nationals Goes Full Throttle in Music City

By Christopher Reynolds   -   Photography by the Author Setting the...

Block Sanding Tips for First-Round Primer-Surfacer

By “Rotten” Rodney Bauman   -   Photography by the Author Final-primed...
More Classic Truck Performance

Put a Lid on It!

Old Anvil Redesigns an International Harvester Roof By Eric Geisert  ...

I Am Gert

Kenzie Moss’ Goolsby-Built ’68 C10 By Kenzie “Gert” Moss &...


Part 2: The Harrison’s Rod & Custom ’71 GMC By...


Jose Galvan’s Second Chance ’88 OBS Comes to Life By...

Evacuate a Big V-8 in Style

Tips and Tricks for Building a Granatelli Motor Sports...


Rob Brooks’ Patina-Skinned ’72 F-100 Lives its Life Doin’...