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Wiring Harness Resto

Replacing old wiring

Most older domestic cars have two wiring harnesses: one for the headlights, taillights, and interior and another for the engine compartment. Over the years the wiring becomes dry and brittle due to extreme temperature changes and old age. Broken or split wires are a fire hazard and should be changed during a restoration, or just as a safety precaution. Several restoration suppliers carry replacement harnesses, and the kits that have correct factory connectors are the easiest to install.

The harness shown here is from a mid-Sixties musclecar. While the wire has no visible external damage, it's brittle when bent, so we chose to be safe now instead of sorry later. Below is an overview of the change-out; installing the new harness is self-explanatory when you compare its connectors to the original ones.

Domestic vehicles usually have two harness plug-ins on the firewall, often on the driver's side. One is the engine harness, and the other is for the lights and horns. These connections are simple push-in plugs with plastic snap-in retainers formed into the plug housing. By compressing these retainers' top and bottom with your fingers, you can remove the plug. The new replacement plug will only install one way so you can't make a mistake. Disconnect the harness plug and install the new harness, then follow the wiring—disconnecting the old and reconnecting the new—until the old harness is free and the new one is in place.

Engine Harness

The top right harness is the new replacement with all the correct factory connectors installed. It looks smaller, but that's only because it's soft and pliable, unlike the original. The original harness is now 36 years old and has taken a permanent shape. This engine harness only has a few connectors, so the job is a lot simpler than it looks. All the wires are the correct color for the application. A new engine or front lighting harness can be installed with normal hand tools in an afternoon.

Wiring Plugs

Here are the two new wiring plugs where they mate to the firewall. Inside the car, these plugs match with the wiring going to the fuse box, dash gauges and such. For a really authentic restoration, replacing the wiring is essential. Notice how nice the colored wires appear as they proceed to their final destinations. If you're doing a show-quality restoration and going for the gold, replacing the harnesses is an absolute requirement. The cost is usually around $100 per harness, but well worth the peace of mind.

Multi-Wire Plug

Several harness manufacturers supply the correct harness but include "universal" wires that aren't tailored to the specific application. This results in a lot of trimming and soldering of end connectors, and the finished product doesn't resemble the original factory wiring. If you're building a street rod or street machine and aren't interested in factory-correctness, this type of harness works well and is less expensive than the tailored variety. Here we see the correct multi-wire plug for the factory voltage regulator as well as miscellaneous plugs for the windshield wiper motor and windshield washer motor.

Correctness

Having the correct-length wiring saves a lot of time and energy. The correct-gauge wire is also extremely important, particularly for the heavier-voltage circuits such as this wire to the alternator. The correct rubber-boot insulator is visible here, as is the plug-in alternator connector and the green temperature-gauge wire on the intake-manifold sending unit. With the right connectors, changing a wiring harness is a "plug and play" event.