Simple works, especially when it comes to wiring harnesses. A long time ago, I took a hard look at the stock wiring harness assemblies in one of my project cars. It was big, bulky, and ugly, and there were a bunch of circuits I just didn’t need. What I wanted to do was simplify the harnesses, keeping them sort of stock looking yet reliable and easy to work on.

In the process, I learned a few things about working on wiring harnesses—things I will now pass on to you so you can do a harness right-sizing.

Removing Circuits

Wiring harness assemblies are typically color- and color stripe-coded. If you refer to the factory shop manual, you’ll be able to trace wires back to the component (circuit) you’re not using.

For example, one of my cars didn’t need circuits for the Transmission Control Spark device, idle stop solenoid, oil pressure gauge light, and the heater blower motor. I traced all of them back from the connector into the harness and cross referenced them to the shop manual wiring diagram.

Next, I opened up the harness by removing the cover tape. Some Chevrolet vehicles have a special “harness tape” that does not have an adhesive. The end of the tape can sometimes be tied in a knot. It’s easy enough to untie the knot and unravel the harness tape. 

With the tape removed, you can sort through the harness and separate the wires for the unneeded circuits. Then you can “de-pin” the circuit. I regularly use a small screwdriver and needle nose pliers to pull the connector from the block. Summit Racing carries several types of pin and terminal extractor tools.

You can re-wrap the harness with the original tape if it’s not damaged. If it is, Summit Racing offers non-adhesive harness tape.  Once wrapped, you’ll be pleasantly surprised at how small a harness can become when the unnecessary wires are removed.

Harness Routing

Something else I do to eliminate excessive amounts of visible harness is to reroute it. A good example is a first generation Camaro. The forward lighting harness runs from the firewall bulkhead connector and over the top of the driver’s side inner fender to the radiator support. It runs alongside the hose for the windshield washer bottle. I’ve found that it’s possible to completely hide the harness by rerouting it between the inner and outer fenders. There’s sufficient harness length to make the connection. You can re-route the washer hose in the same manner if you choose.

Abrasion resistance is also important. You don’t want to damage the wire insulation by rubbing against sharp edges or rough surfaces. Harness tape provides some protection, but there are more durable options:

Spi-wrap is a plastic loom that looks like a coil spring with gaps between each coil. You thread the harness through the spiral wrap, and it’s easy to add or remove wires as needed. You can also control the coil gaps depending on how tight you wind the loom. Wide gaps between the coils allow you to make tight bends. Small gaps between the coils make the harness more rigid and better protected. Summit Racing carries Pico clear Spi-Wrap in 1/4-, 3/8-, and 1/2-inch diameters in various lengths.

Another good option is wire loom and wrap. It’s available in styles ranging from convoluted tubing to braided and woven sleeving made from various materials. Some are self-wrapping, closing over the wires with no need for tape or zip ties.

What if you encounter a high heat source? For example, the starter wires on my projects are usually very close to the headers. My fix is to use a woven thermal sleeve around the harness that runs from the firewall to the starter alongside the headers, but behind the cylinder head. Heatshield’s Hot Rod Sleeve is a good option. It can withstand temperatures up to 2,000 degrees F and has a weave that allows it to expand and contract over fittings and terminals.

One more thing to make the harness simple and easy to use. If I add a circuit to the car that might need servicing down the road, I usually add an inline connector like a Weatherpack. This allows the component in question to be easily removed for service or replacement without tearing up the harness.

It’s not that difficult to slim down, hide, cover and detail wiring harness assemblies. It takes a bit of time to get it right, but when it comes to electrical systems, simple really is good.

Wiring diagram
The best place to start slimming down a wiring harness is by consulting the factory wiring diagram for your vehicle. This is the engine and forward lamp harness diagram for a Nova. Note each wire is identified by color or color and stripe code. (Image/Wayne Scraba)
Transmission Control Spark harness
This is the Transmission Control Spark harness, which is part of the engine harness for the Nova. It can be identified by the connector along with the wire color code. This is the exact point where I began to trace the wire back through the harness. (Image/Wayne Scraba)
Removing wiring harness wrap
Removing the wiring harness wrap is easy. OEM wrap is typically non-adhesive—just carefully cut open an end and begin unraveling it. If it’s in a knot, undo it carefully so you don’t damage the wire. (Image/Wayne Scraba)
Depinning a wiring connector
De-pinning the harness is easy if you have a connector like this. There are tools available for this but a tiny screwdriver, paperclip, or even a sewing needle can work. Sometimes small needle nose pliers will do the job. (Image/Wayne Scraba)
Harness tape
On older vehicles, it’s often necessary to replace frayed or loose tape with new stuff. Summit Racing sells harness tape specifically for this job. (Image/Wayne Scraba)
Rerouting wiring harness
Another way to reduce wiring harness bulk is to hide it. Case in point is the forward lighting harness in a first gen Camaro or 1968-74 Nova. From the factory, the harness runs over the inner front fender alongside the washer fluid hose. There’s sufficient slack in the harness to route it between the inner and outer fenders. That really cleans up the look. (Image/Wayne Scraba)
Spi-wrapping a wiring harness
I use spi-wrap to cover harness assemblies in places where they encounter abrasion. It winds around the harness to cover the wires. More details in the story. (Image/Wayne Scraba)
woven wiring harness sleeving
Another option for wrapping harness assemblies is Techflex woven sleeving. It wraps easily around wiring and has extra overlap to enclose them. The woven construction provides superior elastic flexibility and increased abrasion resistance. (Image/Summit Racing)
Heat Shield Products Hot Rod Sleeve
When it comes to routing harness assemblies in high-heat locations, it’s hard to beat Heat Shield Products’ Hot Rod Sleeve. It can handle 1,100 degrees F for sustained periods of time and is non-flammable. (Image/ Summit Racing)
Weatherpack connector
If you’re making up a wiring harness for something that might require servicing down the road, it’s a very good idea to use a quick connector such as a Weatherpack. I use one for my roll control. In the event it needs servicing, it’s easier to unplug the solenoid rather than tracing the wires back to the switch and power source. (Image/Wayne Scraba)
Wiring harness in Chevy Nova
This is part of the slimmed down engine harness for my hot rod. There’s not much to it. Simple is good! (Image/Wayne Scraba)
Keywords
automotive electrical, Electrical Connectors, wiring harness
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Author: Wayne Scraba

Wayne Scraba is a diehard car guy and regular contributor to OnAllCylinders. He’s owned his own speed shop, built race cars, street rods, and custom motorcycles, and restored muscle cars. He’s authored five how-to books and written over 4,500 tech articles that have appeared in sixty different high performance automotive, motorcycle and aviation magazines worldwide.